diff --git a/idea/testData/completion/basic/java/NoDuplicationForRuntimeClass.kt b/idea/testData/completion/basic/java/NoDuplicationForRuntimeClass.kt
index 4b0ee96eb83..e2b811d0958 100644
--- a/idea/testData/completion/basic/java/NoDuplicationForRuntimeClass.kt
+++ b/idea/testData/completion/basic/java/NoDuplicationForRuntimeClass.kt
@@ -1,5 +1,5 @@
-val x = LinesStream<caret>
+val x = LinesSequence<caret>
 
 // INVOCATION_COUNT: 2
-// EXIST: { lookupString:"LinesStream", tailText:" (kotlin.io)" }
+// EXIST: { lookupString:"LinesSequence", tailText:" (kotlin.io)" }
 // NUMBER: 1
diff --git a/libraries/stdlib/src/kotlin/io/IOStreams.kt b/libraries/stdlib/src/kotlin/io/IOStreams.kt
index 89ea6900817..55c941cedb5 100644
--- a/libraries/stdlib/src/kotlin/io/IOStreams.kt
+++ b/libraries/stdlib/src/kotlin/io/IOStreams.kt
@@ -62,12 +62,6 @@ public fun InputStream.reader(charset: String): InputStreamReader = InputStreamR
 /** Creates a buffered reader on this input stream using the specified [charset]. */
 public fun InputStream.bufferedReader(charset: String): BufferedReader = reader(charset).buffered()
 
-/** Creates a reader on this input stream using the specified [decoder]. */
-public fun InputStream.reader(decoder: CharsetDecoder): InputStreamReader = InputStreamReader(this, decoder)
-
-/** Creates a reader on this input stream using the specified [decoder]. */
-public fun InputStream.bufferedReader(decoder: CharsetDecoder): BufferedReader = reader(decoder).buffered()
-
 /**
  * Creates a buffered output stream wrapping this stream.
  * @param bufferSize the buffer size to use.
@@ -78,11 +72,14 @@ public fun OutputStream.buffered(bufferSize: Int = defaultBufferSize): BufferedO
 /** Creates a writer on this output stream using UTF-8 or the specified [charset]. */
 public fun OutputStream.writer(charset: Charset = Charsets.UTF_8): OutputStreamWriter = OutputStreamWriter(this, charset)
 
+/** Creates a buffered writer on this output stream using UTF-8 or the specified [charset]. */
+public fun OutputStream.bufferedWriter(charset: Charset = Charsets.UTF_8): BufferedWriter = writer(charset).buffered()
+
 /** Creates a writer on this output stream using the specified [charset]. */
 public fun OutputStream.writer(charset: String): OutputStreamWriter = OutputStreamWriter(this, charset)
 
-/** Creates a writer on this output stream using the specified [encoder]. */
-public fun OutputStream.writer(encoder: CharsetEncoder): OutputStreamWriter = OutputStreamWriter(this, encoder)
+/** Creates a buffered writer on this output stream using the specified [charset]. */
+public fun OutputStream.bufferedWriter(charset: String): BufferedWriter = writer(charset).buffered()
 
 /**
  * Copies this stream to the given output stream, returning the number of bytes copied
@@ -112,16 +109,3 @@ public fun InputStream.readBytes(estimatedSize: Int = defaultBufferSize): ByteAr
     return buffer.toByteArray()
 }
 
-/**
- * Constructs a new FileInputStream of this file and returns it as a result.
- */
-public fun File.inputStream(): InputStream {
-    return FileInputStream(this)
-}
-
-/**
- * Constructs a new FileOutputStream of this file and returns it as a result.
- */
-public fun File.outputStream(): OutputStream {
-    return FileOutputStream(this)
-}
diff --git a/libraries/stdlib/src/kotlin/io/ReadWrite.kt b/libraries/stdlib/src/kotlin/io/ReadWrite.kt
index 1c4bbe5a1af..d29e71d14b9 100644
--- a/libraries/stdlib/src/kotlin/io/ReadWrite.kt
+++ b/libraries/stdlib/src/kotlin/io/ReadWrite.kt
@@ -33,49 +33,34 @@ public fun File.bufferedWriter(bufferSize: Int = defaultBufferSize): BufferedWri
 /**
  * Returns a new [PrintWriter] for writing the content of this file.
  */
-public fun File.printWriter(): PrintWriter = PrintWriter(writer().buffered())
+public fun File.printWriter(): PrintWriter = PrintWriter(bufferedWriter())
 
 /**
- * Reads the entire content of this file as a byte array.
+ * Gets the entire content of this file as a byte array.
  *
  * This method is not recommended on huge files. It has an internal limitation of 2 GB byte array size
  *
  * @return the entire content of this file as a byte array
  */
-public fun File.readBytes(): ByteArray {
-    return FileInputStream(this).use { it.readBytes(length().toInt()) }
-}
+public fun File.readBytes(): ByteArray = FileInputStream(this).use { it.readBytes(length().toInt()) }
 
 /**
- * Replaces the content of this file with [data].
+ * Sets the content of this file as an [array] of bytes.
+ * If this file already exists, it becomes overwritten.
  *
- * @param data byte array to write into this file
+ * @param array byte array to write into this file
  */
-deprecated("Use replaceBytes instead")
-public fun File.writeBytes(data: ByteArray): Unit {
-    return FileOutputStream(this).use { it.write(data) }
-}
+public fun File.writeBytes(array: ByteArray): Unit = FileOutputStream(this).use { it.write(array) }
 
 /**
- * Replaces the content of this file with [data].
+ * Appends an [array] of bytes to the content of this file.
  *
- * @param data byte array to write into this file
+ * @param array byte array to append to this file
  */
-public fun File.replaceBytes(data: ByteArray): Unit {
-    return FileOutputStream(this).use { it.write(data) }
-}
+public fun File.appendBytes(array: ByteArray): Unit = FileOutputStream(this, true).use { it.write(array) }
 
 /**
- * Appends [data] to the content of this file.
- *
- * @param data byte array to append to this file
- */
-public fun File.appendBytes(data: ByteArray): Unit {
-    return FileOutputStream(this, true).use { it.write(data) }
-}
-
-/**
- * Reads the entire content of this file as a String using specified [charset].
+ * Gets the entire content of this file as a String using specified [charset].
  *
  * This method is not recommended on huge files. It has an internal limitation of 2 GB file size
  *
@@ -85,7 +70,7 @@ public fun File.appendBytes(data: ByteArray): Unit {
 public fun File.readText(charset: String): String = readBytes().toString(charset)
 
 /**
- * Reads the entire content of this file as a String using UTF-8 or specified [charset].
+ * Gets the entire content of this file as a String using UTF-8 or specified [charset].
  *
  * This method is not recommended on huge files. It has an internal limitation of 2 GB file size
  *
@@ -95,47 +80,22 @@ public fun File.readText(charset: String): String = readBytes().toString(charset
 public fun File.readText(charset: Charset = Charsets.UTF_8): String = readBytes().toString(charset)
 
 /**
- * Replaces the content of this file with [text] encoded using the specified [charset].
+ * Sets the content of this file as [text] encoded using the specified [charset].
+ * If this file exists, it becomes overwritten.
  *
  * @param text text to write into file
  * @param charset character set to use
  */
-deprecated("Use replaceText instead")
-public fun File.writeText(text: String, charset: String): Unit {
-    replaceBytes(text.toByteArray(charset))
-}
+public fun File.writeText(text: String, charset: String): Unit = writeBytes(text.toByteArray(charset))
 
 /**
- * Replaces the content of this file with [text] encoded using the specified [charset].
+ * Sets the content of this file as [text] encoded using UTF-8 or specified [charset].
+ * If this file exists, it becomes overwritten.
  *
  * @param text text to write into file
  * @param charset character set to use
  */
-deprecated("There is a same function with default parameter value")
-public fun File.replaceText(text: String, charset: String): Unit {
-    replaceBytes(text.toByteArray(charset))
-}
-
-/**
- * Replaces the content of this file with [text] encoded using UTF-8 or specified [charset].
- *
- * @param text text to write into file
- * @param charset character set to use
- */
-deprecated("Use replaceText instead")
-public fun File.writeText(text: String, charset: Charset = Charsets.UTF_8): Unit {
-    replaceBytes(text.toByteArray(charset))
-}
-
-/**
- * Replaces the content of this file with [text] encoded using UTF-8 or specified [charset].
- *
- * @param text text to write into file
- * @param charset character set to use
- */
-public fun File.replaceText(text: String, charset: Charset = Charsets.UTF_8): Unit {
-    replaceBytes(text.toByteArray(charset))
-}
+public fun File.writeText(text: String, charset: Charset = Charsets.UTF_8): Unit = writeBytes(text.toByteArray(charset))
 
 /**
  * Appends [text] to the content of this file using UTF-8 or the specified [charset].
@@ -143,9 +103,7 @@ public fun File.replaceText(text: String, charset: Charset = Charsets.UTF_8): Un
  * @param text text to append to file
  * @param charset character set to use
  */
-public fun File.appendText(text: String, charset: Charset = Charsets.UTF_8): Unit {
-    appendBytes(text.toByteArray(charset))
-}
+public fun File.appendText(text: String, charset: Charset = Charsets.UTF_8): Unit = appendBytes(text.toByteArray(charset))
 
 /**
  * Appends [text] to the content of the file using the specified [charset].
@@ -153,31 +111,29 @@ public fun File.appendText(text: String, charset: Charset = Charsets.UTF_8): Uni
  * @param text text to append to file
  * @param charset character set to use
  */
-public fun File.appendText(text: String, charset: String): Unit {
-    appendBytes(text.toByteArray(charset))
-}
+public fun File.appendText(text: String, charset: String): Unit = appendBytes(text.toByteArray(charset))
 
 /**
- * Reads file by byte blocks and calls [closure] for each block read.
+ * Reads file by byte blocks and calls [operation] for each block read.
  * Block has default size which is implementation-dependent.
- * This functions passes the byte array and amount of bytes in the array to the [closure] function.
+ * This functions passes the byte array and amount of bytes in the array to the [operation] function.
  *
  * You can use this function for huge files.
  *
- * @param closure function to proceed file blocks
+ * @param operation function to process file blocks
  */
-public fun File.forEachBlock(closure: (ByteArray, Int) -> Unit): Unit = forEachBlock(closure, defaultBlockSize)
+public fun File.forEachBlock(operation: (ByteArray, Int) -> Unit): Unit = forEachBlock(operation, defaultBlockSize)
 
 /**
- * Reads file by byte blocks and calls [closure] for each block read.
- * This functions passes the byte array and amount of bytes in the array to the [closure] function.
+ * Reads file by byte blocks and calls [operation] for each block read.
+ * This functions passes the byte array and amount of bytes in the array to the [operation] function.
  *
  * You can use this function for huge files.
  *
- * @param closure function to proceed file blocks
+ * @param operation function to process file blocks
  * @param blockSize size of a block, replaced by 512 if it's less, 4096 by default
  */
-public fun File.forEachBlock(closure: (ByteArray, Int) -> Unit, blockSize: Int): Unit {
+public fun File.forEachBlock(operation: (ByteArray, Int) -> Unit, blockSize: Int): Unit {
     val arr = ByteArray(if (blockSize < minimumBlockSize) minimumBlockSize else blockSize)
     val fis = FileInputStream(this)
 
@@ -187,7 +143,7 @@ public fun File.forEachBlock(closure: (ByteArray, Int) -> Unit, blockSize: Int):
             if (size <= 0) {
                 break
             } else {
-                closure(arr, size)
+                operation(arr, size)
             }
         } while (true)
     } finally {
@@ -196,28 +152,28 @@ public fun File.forEachBlock(closure: (ByteArray, Int) -> Unit, blockSize: Int):
 }
 
 /**
- * Reads this file line by line using the specified [charset] and calls [closure] for each line.
+ * Reads this file line by line using the specified [charset] and calls [operation] for each line.
  * Default charset is UTF-8.
  *
  * You may use this function on huge files.
  *
  * @param charset character set to use
- * @param closure function to proceed file lines
+ * @param operation function to process file lines
  */
-public fun File.forEachLine(charset: Charset = Charsets.UTF_8, closure: (line: String) -> Unit): Unit {
+public fun File.forEachLine(charset: Charset = Charsets.UTF_8, operation: (line: String) -> Unit): Unit {
     // Note: close is called at forEachLine
-    BufferedReader(InputStreamReader(FileInputStream(this), charset)).forEachLine(closure)
+    BufferedReader(InputStreamReader(FileInputStream(this), charset)).forEachLine(operation)
 }
 
 /**
- * Reads this file line by line using the specified [charset] and calls [closure] for each line.
+ * Reads this file line by line using the specified [charset] and calls [operation] for each line.
  *
  * You may use this function on huge files.
  *
  * @param charset character set to use
- * @param closure function to proceed file lines
+ * @param operation function to process file lines
  */
-public fun File.forEachLine(charset: String, closure: (line: String) -> Unit): Unit = forEachLine(Charset.forName(charset), closure)
+public fun File.forEachLine(charset: String, operation: (line: String) -> Unit): Unit = forEachLine(Charset.forName(charset), operation)
 
 /**
  * Reads the file content as a list of lines, using the specified [charset].
@@ -229,6 +185,20 @@ public fun File.forEachLine(charset: String, closure: (line: String) -> Unit): U
  */
 public fun File.readLines(charset: String): List<String> = readLines(Charset.forName(charset))
 
+/**
+ * Constructs a new FileInputStream of this file and returns it as a result.
+ */
+public fun File.inputStream(): InputStream {
+    return FileInputStream(this)
+}
+
+/**
+ * Constructs a new FileOutputStream of this file and returns it as a result.
+ */
+public fun File.outputStream(): OutputStream {
+    return FileOutputStream(this)
+}
+
 /**
  * Reads the file content as a list of lines. By default uses UTF-8 charset.
  *
@@ -255,7 +225,7 @@ public fun Writer.buffered(bufferSize: Int = defaultBufferSize): BufferedWriter
  * Iterates through each line of this reader, calls [block] for each line read
  * and closes the [Reader] when it's completed
  *
- * @param block function to proceed file lines
+ * @param block function to process file lines
  */
 public fun Reader.forEachLine(block: (String) -> Unit): Unit = useLines { it.forEach(block) }
 
@@ -278,12 +248,12 @@ public inline fun <T> Reader.useLines(block: (Sequence<String>) -> T): T =
  *
  * @return a sequence of corresponding file lines
  */
-public fun BufferedReader.lines(): Sequence<String> = LinesStream(this)
+public fun BufferedReader.lines(): Sequence<String> = LinesSequence(this)
 
 deprecated("Use lines() function which returns Sequence<String>")
 public fun BufferedReader.lineIterator(): Iterator<String> = lines().iterator()
 
-private class LinesStream(private val reader: BufferedReader) : Sequence<String> {
+private class LinesSequence(private val reader: BufferedReader) : Sequence<String> {
     override public fun iterator(): Iterator<String> {
         return object : Iterator<String> {
             private var nextValue: String? = null
@@ -312,7 +282,7 @@ private class LinesStream(private val reader: BufferedReader) : Sequence<String>
 /**
  * Reads this reader completely as a String
  *
- * *Note*:  It is the caller's responsibility to close this resource.
+ * *Note*:  It is the caller's responsibility to close this reader.
  *
  * @return the string with corresponding file content
  */
@@ -364,19 +334,19 @@ public fun URL.readText(charset: String): String = readBytes().toString(charset)
 public fun URL.readText(charset: Charset = Charsets.UTF_8): String = readBytes().toString(charset)
 
 /**
- * Reads the entire content of the URL as bytes
+ * Reads the entire content of the URL as byte array
  *
  * This method is not recommended on huge files.
  *
  * @return a byte array with this URL entire content
  */
-public fun URL.readBytes(): ByteArray = openStream().use<InputStream, ByteArray>{ it.readBytes() }
+public fun URL.readBytes(): ByteArray = openStream().use { it.readBytes() }
 
 /**
  * Executes the given [block] function on this resource and then closes it down correctly whether an exception
  * is thrown or not
  *
- * @param block a function to proceed this closable resource
+ * @param block a function to process this closable resource
  * @return the result of [block] function on this closable resource
  */
 public inline fun <T : Closeable, R> T.use(block: (T) -> R): R {
diff --git a/libraries/stdlib/src/kotlin/io/files/FilePathComponents.kt b/libraries/stdlib/src/kotlin/io/files/FilePathComponents.kt
index 695f1d32173..16ce99396f0 100644
--- a/libraries/stdlib/src/kotlin/io/files/FilePathComponents.kt
+++ b/libraries/stdlib/src/kotlin/io/files/FilePathComponents.kt
@@ -1,8 +1,22 @@
 package kotlin.io
 
 import java.io.File
+import java.util.Collections
 import java.util.NoSuchElementException
 
+/**
+ * Estimation of a root name by a given file name.
+ *
+ * This implementation is able to find /, Drive:/, Drive: or
+ * //network.name/root as possible root names.
+ * / denotes File.separator here so \ can be used instead.
+ * All other possible roots cannot be identified by this implementation.
+ * It's also not guaranteed (but possible) that function will be able to detect a root
+ * which is incorrect for current OS. For instance, in Unix function cannot detect
+ * network root names like //network.name/root, but can detect Windows roots like C:/.
+ *
+ * @return string representing the root for this file, or empty string is this file name is relative
+ */
 private fun String.getRootName(): String {
     // Note: separators should be already replaced to system ones
     var first = indexOf(File.separatorChar, 0)
@@ -36,9 +50,17 @@ private fun String.getRootName(): String {
 }
 
 /**
- * Returns a string representation of root component of this abstract name, like / from /home/user, or C:\ from C:\file.tmp,
- * or //my.host/home for //my.host/home/user,
- * or empty string if this name is relative, like bar/gav
+ * Estimation of a root name for this file.
+ *
+ * This implementation is able to find /, Drive:/, Drive: or
+ * //network.name/root as possible root names.
+ * / denotes File.separator here so \ can be used instead.
+ * All other possible roots cannot be identified by this implementation.
+ * It's also not guaranteed (but possible) that function will be able to detect a root
+ * which is incorrect for current OS. For instance, in Unix function cannot detect
+ * network root names like //network.name/root, but can detect Windows roots like C:/.
+ *
+ * @return string representing the root for this file, or empty string is this file name is relative
  */
 public val File.rootName: String
     get() = separatorsToSystem().getRootName()
@@ -55,23 +77,13 @@ public val File.root: File?
     }
 
 public data class FilePathComponents(public val rootName: String, public val fileList: List<File>) {
-    public val size: Int = fileList.size()
+    public fun size(): Int = fileList.size()
 
     public fun subPath(beginIndex: Int, endIndex: Int): File {
-        if (beginIndex < 0 || beginIndex >= endIndex || endIndex > size)
+        if (beginIndex < 0 || beginIndex > endIndex || endIndex > size())
             throw IllegalArgumentException()
 
-        var res = ""
-        var first = true
-        for (elem in fileList.subList(beginIndex, endIndex)) {
-            if (!first) {
-                res += File.separatorChar
-            } else {
-                first = false
-            }
-            res += elem.toString()
-        }
-        return File(res)
+        return File(fileList.subList(beginIndex, endIndex).joinToString(File.separator))
     }
 }
 
@@ -82,6 +94,7 @@ public fun File.filePathComponents(): FilePathComponents {
     // if: a special case when we have only root component
     // Split not only by / or \, but also by //, ///, \\, \\\, etc.
     val list = if (rootName.length() > 0 && subPath.isEmpty()) listOf() else
+        // Looks awful but we split just by /+ or \+ depending on OS
         subPath.split("""\Q${File.separatorChar}\E+""").toList().map { it -> File(it) }
     return FilePathComponents(rootName, list)
 }
@@ -94,6 +107,6 @@ public fun File.filePathComponents(): FilePathComponents {
  * number count-1 belongs to a component farthest from the root
  * @throws IllegalArgumentException if [beginIndex] is negative,
 * or [endIndex] is greater than existing number of components,
-* or [beginIndex] is greater than or equals to [endIndex]
+* or [beginIndex] is greater than [endIndex]
  */
 public fun File.subPath(beginIndex: Int, endIndex: Int): File = filePathComponents().subPath(beginIndex, endIndex)
diff --git a/libraries/stdlib/src/kotlin/io/files/FileTreeWalk.kt b/libraries/stdlib/src/kotlin/io/files/FileTreeWalk.kt
index 0a7d1a9013e..f7d77124e85 100644
--- a/libraries/stdlib/src/kotlin/io/files/FileTreeWalk.kt
+++ b/libraries/stdlib/src/kotlin/io/files/FileTreeWalk.kt
@@ -8,9 +8,9 @@ import java.util.NoSuchElementException
 import java.util.Stack
 
 /**
- * Created by Mikhail.Glukhikh on 13/03/2015.
+ * Created by Mikhail Glukhikh on 13/03/2015.
  *
- * An alternative version of file walker
+ * An implementation of file walker
  */
 
 /**
@@ -31,6 +31,15 @@ public enum class FileWalkDirection {
  * It allows to iterate through all files inside [start] directory.
  * If [start] is just a file, walker iterates only it.
  * If [start] does not exist, walker does not do any iterations at all.
+ *
+ * @param start directory to walk into
+ * @param direction selects top-down or bottom-up order (in other words, parents first or children first).
+ * @param enter is called on any entered directory before its files are visited and before it is visited itself
+ * @param leave is called on any left directory after its files are visited and after it is visited itself
+ * @param fail is called on a directory when it's impossible to get its file list
+ * @param filter is called just before visiting a file, and if false is returned, file is not visited
+ * @param maxDepth is maximum walking depth, it must be positive. With a value of 1,
+ * walker visits [start] and all its children, with a value of 2 also grandchildren, etc.
  */
 public class FileTreeWalk(private val start: File,
                           private val direction: FileWalkDirection = FileWalkDirection.TOP_DOWN,
@@ -41,15 +50,18 @@ public class FileTreeWalk(private val start: File,
                           private val maxDepth: Int = Int.MAX_VALUE
 ) : Sequence<File> {
 
+    /** Abstract class that encapsulates file visiting in some order, beginning from a given [rootDir] */
     private abstract class DirectoryState(public val rootDir: File) {
         init {
             if (!rootDir.isDirectory())
                 throw IllegalArgumentException("Directory is needed")
         }
 
+        /** Call of this function proceeds to a next file for visiting and returns it */
         abstract public fun step(): File?
     }
 
+    /** Visiting in bottom-up order */
     private inner class BottomUpDirectoryState(rootDir: File) : DirectoryState(rootDir) {
 
         private var rootVisited = false
@@ -60,6 +72,7 @@ public class FileTreeWalk(private val start: File,
 
         private var failed = false
 
+        /** First all children, then root directory */
         override public fun step(): File? {
             if (!failed && fileList == null) {
                 enter(rootDir)
@@ -84,6 +97,7 @@ public class FileTreeWalk(private val start: File,
         }
     }
 
+    /** Visiting in top-down order */
     private inner class TopDownDirectoryState(rootDir: File) : DirectoryState(rootDir) {
 
         private var rootVisited = false
@@ -92,6 +106,7 @@ public class FileTreeWalk(private val start: File,
 
         private var fileIndex = 0
 
+        /** First root directory, then all children */
         override public fun step(): File? {
             if (!rootVisited) {
                 // First visit root
@@ -167,10 +182,10 @@ public class FileTreeWalk(private val start: File,
             if (!filter(file))
                 return gotoNext()
             if (file == topState.rootDir || !file.isDirectory() || state.size() >= maxDepth) {
-                // Proceed a root directory or a simple file
+                // Proceed to a root directory or a simple file
                 return file
             } else {
-                // Proceed a sub-directory
+                // Proceed to a sub-directory
                 pushState(file)
                 return gotoNext()
             }
@@ -178,42 +193,42 @@ public class FileTreeWalk(private val start: File,
     }
 
     /**
-     * Sets enter directory function.
-     * Enter function is called BEFORE the corresponding directory and its files are visited
+     * Sets enter directory [function].
+     * Enter [function] is called BEFORE the corresponding directory and its files are visited
      */
-    public fun enter(f: (File) -> Unit): FileTreeWalk {
-        return FileTreeWalk(start, direction, f, leave, fail, filter, maxDepth)
+    public fun enter(function: (File) -> Unit): FileTreeWalk {
+        return FileTreeWalk(start, direction, function, leave, fail, filter, maxDepth)
     }
 
     /**
-     * Sets leave directory function.
-     * Leave function is called AFTER the corresponding directory and its files are visited
+     * Sets leave directory [function].
+     * Leave [function] is called AFTER the corresponding directory and its files are visited
      */
-    public fun leave(f: (File) -> Unit): FileTreeWalk {
-        return FileTreeWalk(start, direction, enter, f, fail, filter, maxDepth)
+    public fun leave(function: (File) -> Unit): FileTreeWalk {
+        return FileTreeWalk(start, direction, enter, function, fail, filter, maxDepth)
     }
 
     /**
-     * Set fail entering directory function.
-     * Fail function is called when walker is unable to get list of directory files.
+     * Set fail entering directory [function].
+     * Fail [function] is called when walker is unable to get list of directory files.
      * Enter and leave functions are called even in this case.
      */
-    public fun fail(f: (File, IOException) -> Unit): FileTreeWalk {
-        return FileTreeWalk(start, direction, enter, leave, f, filter, maxDepth)
+    public fun fail(function: (File, IOException) -> Unit): FileTreeWalk {
+        return FileTreeWalk(start, direction, enter, leave, function, filter, maxDepth)
     }
 
     /**
-     * Sets filter function.
-     * Filter function is called before visiting files and entering directories.
+     * Sets filter [predicate].
+     * Filter [predicate] function is called before visiting files and entering directories.
      * If it returns false, file is not visited, directory is not entered and all its content is also not visited.
      * If it returns true, everything goes in the regular way
      */
-    public fun filter(f: (File) -> Boolean): FileTreeWalk {
-        return FileTreeWalk(start, direction, enter, leave, fail, f, maxDepth)
+    public fun filter(predicate: (File) -> Boolean): FileTreeWalk {
+        return FileTreeWalk(start, direction, enter, leave, fail, predicate, maxDepth)
     }
 
     /**
-     * Sets maximum depth of walk. Int.MAX_VALUE is used for unlimited.
+     * Sets maximum [depth] of walk. Int.MAX_VALUE is used for unlimited.
      * Negative and zero values are not allowed
      */
     public fun maxDepth(depth: Int): FileTreeWalk {
@@ -222,6 +237,7 @@ public class FileTreeWalk(private val start: File,
         return FileTreeWalk(start, direction, enter, leave, fail, filter, depth)
     }
 
+    /** An iterator associated with this walker */
     private val it = object : Iterator<File> {
         override public fun hasNext(): Boolean {
             if (nextFile == null)
@@ -242,6 +258,7 @@ public class FileTreeWalk(private val start: File,
         }
     }
 
+    /** Returns an associated file iterator */
     override public fun iterator(): Iterator<File> {
         return it
     }
@@ -271,9 +288,9 @@ public fun File.walkBottomUp(): FileTreeWalk = walk(FileWalkDirection.BOTTOM_UP)
  * Recursively process this file and all children with the given block.
  * Note that if this file doesn't exist, then the block will be executed on it anyway.
  *
- * @param block the function to call on each file
+ * @param function the function to call on each file
  */
 deprecated("It's recommended to use walkTopDown() / walkBottomUp()")
-public fun File.recurse(block: (File) -> Unit): Unit {
-    walkTopDown().forEach { block(it) }
+public fun File.recurse(function: (File) -> Unit): Unit {
+    walkTopDown().forEach { function(it) }
 }
diff --git a/libraries/stdlib/src/kotlin/io/files/Utils.kt b/libraries/stdlib/src/kotlin/io/files/Utils.kt
index 40c7483a019..2bfc236a600 100644
--- a/libraries/stdlib/src/kotlin/io/files/Utils.kt
+++ b/libraries/stdlib/src/kotlin/io/files/Utils.kt
@@ -22,7 +22,7 @@ public fun createTempDir(prefix: String = "tmp", suffix: String? = null, directo
     if (dir.mkdir()) {
         return dir
     } else {
-        throw IOException("Unable to create temporary directory")
+        throw IOException("Unable to create temporary directory $dir")
     }
 }
 
@@ -76,9 +76,7 @@ public val File.path: String
  * Returns the extension of this file (not including the dot), or an empty string if it doesn't have one.
  */
 public val File.extension: String
-    get() {
-        return name.substringAfterLast('.', "")
-    }
+    get() = name.substringAfterLast('.', "")
 
 /**
  * Replaces all separators in the string used to separate directories with system ones and returns the resulting string.
@@ -143,28 +141,28 @@ public fun File.relativeTo(base: File): String {
     // Check roots
     val components = filePathComponents()
     val baseComponents = base.filePathComponents()
-    if (components.rootName != base.rootName)
-        throw IllegalArgumentException("this and base files have different roots")
+    if (components.rootName != baseComponents.rootName)
+        throw IllegalArgumentException("this and base files have different roots: ${components.rootName} and ${baseComponents.rootName}")
     var i = 0
-    while (i < components.size && i < baseComponents.size && components.fileList[i] == baseComponents.fileList[i])
+    while (i < components.size() && i < baseComponents.size() && components.fileList[i] == baseComponents.fileList[i])
         i++
     val sameCount = i
-    val baseCount = baseComponents.size
+    val baseCount = baseComponents.size()
     // Add all ..
-    var res = ""
-    for (j in sameCount + 1..baseCount - 1)
-        res += (".." + File.separator)
+    val res = StringBuilder()
+    for (j in sameCount..baseCount - 2)
+        res.append("..").append(File.separator)
     // If .. is the last element, no separator should present
     if (baseCount > sameCount) {
-        res += if (sameCount < components.size) ".." + File.separator else ".."
+        res.append(if (sameCount < components.size()) ".." + File.separator else "..")
     }
     // Add remaining this components
-    if (sameCount < components.size - 1)
-        res += (components.subPath(sameCount, components.size - 1).toString() + File.separator)
+    if (sameCount < components.size() - 1)
+        res.append(components.subPath(sameCount, components.size() - 1)).append(File.separator)
     // The last one should be without separator
-    if (sameCount < components.size)
-        res += components.subPath(components.size - 1, components.size).toString()
-    return res
+    if (sameCount < components.size())
+        res.append(components.subPath(components.size() - 1, components.size()))
+    return res.toString()
 }
 
 /**
@@ -191,8 +189,8 @@ public fun File.relativePath(descendant: File): String {
  * Copies this file to the given output [dst], returning the number of bytes copied.
  *
  * If some directories on a way to the [dst] are missing, then they will be created.
- * If the [dst] file already exists, then this function will fail unless [overwrite] argument is set to true and
- * the [dst] file is not a non-empty directory.
+ * If the [dst] file already exists, then this function will fail unless [overwrite] argument is set to true.
+ * Otherwise this file overwrites [dst] if it's a file to, or is written into [dst] if it's a directory.
  *
  * Note: this function fails if you call it on a directory.
  * If you want to copy directories, use 'copyRecursively' function instead.
@@ -208,7 +206,7 @@ public fun File.copyTo(dst: File, overwrite: Boolean = false, bufferSize: Int =
     if (!exists()) {
         throw NoSuchFileException(file = this, reason = "The source file doesn't exist")
     } else if (isDirectory()) {
-        throw IllegalArgumentException("Use copyRecursively to copy a directory")
+        throw IllegalArgumentException("Use copyRecursively to copy a directory $this")
     } else if (dst.exists()) {
         if (!overwrite) {
             throw FileAlreadyExistsException(file = this,
@@ -243,6 +241,7 @@ public enum class OnErrorAction {
     TERMINATE
 }
 
+/** Private exception class, used to terminate recursive copying */
 private class TerminateException(file: File) : FileSystemException(file) {}
 
 /**
@@ -265,13 +264,14 @@ private class TerminateException(file: File) : FileSystemException(file) {}
  */
 public fun File.copyRecursively(dst: File,
                                 onError: (File, IOException) -> OnErrorAction =
-                                { file, e -> throw e }
+                                { file, exception -> throw exception }
 ): Boolean {
     if (!exists()) {
         return onError(this, NoSuchFileException(file = this, reason = "The source file doesn't exist")) !=
                 OnErrorAction.TERMINATE
     }
     try {
+        // We cannot break for loop from inside a lambda, so we have to use an exception here
         for (src in walkTopDown().fail { f, e -> if (onError(f, e) == OnErrorAction.TERMINATE) throw TerminateException(f) }) {
             if (!src.exists()) {
                 if (onError(src, NoSuchFileException(file = src, reason = "The source file doesn't exist")) ==
@@ -307,86 +307,68 @@ public fun File.copyRecursively(dst: File,
  *
  * @return true if the file or directory is successfully deleted, false otherwise.
  */
-public fun File.deleteRecursively(): Boolean {
-    var result = exists()
-    walkBottomUp().forEach { if (!it.delete()) result = false }
-    return result
-}
+public fun File.deleteRecursively(): Boolean = walkBottomUp().fold(exists(), { res, it -> res && it.delete() })
 
 /**
  * Returns an array of files and directories in the directory that match the specified [filter]
  * or null if this file does not denote a directory.
  */
-public fun File.listFiles(filter: (file: File) -> Boolean): Array<File>? = listFiles(
-        object : FileFilter {
-            override fun accept(file: File) = filter(file)
-        }
-)
+public fun File.listFiles(filter: (file: File) -> Boolean): Array<File>? = listFiles(FileFilter(filter))
 
 /**
- * Determines whether this file belongs to the same root as [o]
- * and starts with all components of [o] in the same order.
- * So if [o] has N components, first N components of `this` must be the same as in [o].
- * For relative [o], `this` can belong to any root.
+ * Determines whether this file belongs to the same root as [other]
+ * and starts with all components of [other] in the same order.
+ * So if [other] has N components, first N components of `this` must be the same as in [other].
+ * For relative [other], `this` can belong to any root.
  *
- * @return true if this path starts with [o] path, false otherwise
+ * @return true if this path starts with [other] path, false otherwise
  */
-public fun File.startsWith(o: File): Boolean {
+public fun File.startsWith(other: File): Boolean {
     val components = filePathComponents()
-    val otherComponents = o.filePathComponents()
+    val otherComponents = other.filePathComponents()
     if (components.rootName != otherComponents.rootName && otherComponents.rootName != "")
         return false
-    if (components.size < otherComponents.size)
-        return false
-    for (i in 0..otherComponents.size - 1) {
-        if (components.fileList[i] != otherComponents.fileList[i])
-            return false
-    }
-    return true
+    return if (components.size() < otherComponents.size()) false
+    else components.fileList.subList(0, otherComponents.size()).equals(otherComponents.fileList)
 }
 
 /**
- * Determines whether this file belongs to the same root as [o]
- * and starts with all components of [o] in the same order.
- * So if [o] has N components, first N components of `this` must be the same as in [o].
- * For relative [o], `this` can belong to any root.
+ * Determines whether this file belongs to the same root as [other]
+ * and starts with all components of [other] in the same order.
+ * So if [other] has N components, first N components of `this` must be the same as in [other].
+ * For relative [other], `this` can belong to any root.
  *
- * @return true if this path starts with [o] path, false otherwise
+ * @return true if this path starts with [other] path, false otherwise
  */
-public fun File.startsWith(o: String): Boolean = startsWith(File(o))
+public fun File.startsWith(other: String): Boolean = startsWith(File(other))
 
 /**
- * Determines whether this file belongs to the same root as [o]
- * and ends with all components of [o] in the same order.
- * So if [o] has N components, last N components of `this` must be the same as in [o].
- * For relative [o], `this` can belong to any root.
+ * Determines whether this file belongs to the same root as [other]
+ * and ends with all components of [other] in the same order.
+ * So if [other] has N components, last N components of `this` must be the same as in [other].
+ * For relative [other], `this` can belong to any root.
  *
- * @return true if this path ends with [o] path, false otherwise
+ * @return true if this path ends with [other] path, false otherwise
  */
-public fun File.endsWith(o: File): Boolean {
+public fun File.endsWith(other: File): Boolean {
     val components = filePathComponents()
-    val otherComponents = o.filePathComponents()
+    val otherComponents = other.filePathComponents()
     if (components.rootName != otherComponents.rootName && otherComponents.rootName != "")
         return false
-    val shift = components.size - otherComponents.size
-    if (shift < 0)
-        return false
-    for (i in 0..otherComponents.size - 1) {
-        if (components.fileList[i + shift] != otherComponents.fileList[i])
-            return false
-    }
-    return true
+    val shift = components.size() - otherComponents.size()
+    return if (shift < 0) false
+    else components.fileList.subList(shift, components.size()).equals(otherComponents.fileList)
 }
 
 /**
- * Determines whether this file belongs to the same root as [o]
- * and ends with all components of [o] in the same order.
- * So if [o] has N components, last N components of `this` must be the same as in [o].
- * For relative [o], `this` can belong to any root.
+ * Determines whether this file belongs to the same root as [other]
+ * and ends with all components of [other] in the same order.
+ * So if [other] has N components, last N components of `this` must be the same as in [other].
+ * For relative [other], `this` can belong to any root.
  *
- * @return true if this path ends with [o] path, false otherwise
+ * @return true if this path ends with [other] path, false otherwise
  */
-public fun File.endsWith(o: String): Boolean = endsWith(File(o))
+public fun File.endsWith(other: String): Boolean = endsWith(File(other))
 
 /**
  * Removes all . and resolves all possible .. in this file name.
@@ -397,77 +379,62 @@ public fun File.endsWith(o: String): Boolean = endsWith(File(o))
 public fun File.normalize(): File {
     val components = filePathComponents()
     val rootName = components.rootName
-    val list: MutableList<String> = components.fileList.filter { it.toString() != "." }.map { it.toString() }.toLinkedList()
-    var first = 0
-    var dots = list.subList(first, list.size()).indexOf("..")
-    while (dots != -1) {
-        if (dots > 0) {
-            list.remove(dots + first)
-            list.remove(dots + first - 1)
-        } else {
-            first++
+    val list: MutableList<String> = ArrayList()
+    for (file in components.fileList) {
+        val name = file.toString()
+        when (name) {
+            "." -> {
+            }
+            ".." -> if (!list.isEmpty() && list.get(list.size() - 1) != "..") list.remove(list.size() - 1) else list.add(name)
+            else -> list.add(name)
         }
-        dots = list.subList(first, list.size()).indexOf("..")
     }
-    var res = rootName
-    var addSeparator = rootName != "" && !rootName.endsWith(File.separatorChar)
-    for (elem in list) {
-        if (addSeparator)
-            res += File.separatorChar
-        res += elem
-        addSeparator = true
-    }
-    return File(res)
+    return File(list.joinToString(File.separator, rootName))
 }
 
 /**
- * Adds relative [o] to this, considering this as a directory.
- * If [o] has a root, [o] is returned back.
+ * Adds [relative] file to this, considering this as a directory.
+ * If [relative] has a root, [relative] is returned back.
  * For instance, File("/foo/bar").resolve(File("gav")) is File("/foo/bar/gav").
  * This function is complementary with (File.relativeTo),
  * so f.resolve(g.relativeTo(f)) == g should be always true except for different roots case.
  *
- * @return concatenated this and [o] paths, or just [o] if it's absolute
+ * @return concatenated this and [relative] paths, or just [relative] if it's absolute
  */
-public fun File.resolve(o: File): File {
-    if (o.root != null)
-        return o
+public fun File.resolve(relative: File): File {
+    if (relative.root != null)
+        return relative
     val ourName = toString()
-    return if (ourName.endsWith(File.separatorChar)) File(ourName + o) else File(ourName + File.separatorChar + o)
+    return if (ourName.endsWith(File.separatorChar)) File(ourName + relative) else File(ourName + File.separatorChar + relative)
 }
 
 /**
- * Adds relative [o] to this, considering this as a directory.
- * If [o] has a root, [o] is returned back.
+ * Adds [relative] name to this, considering this as a directory.
+ * If [relative] has a root, [relative] is returned back.
  * For instance, File("/foo/bar").resolve("gav") is File("/foo/bar/gav")
  *
- * @return concatenated this and [o] paths, or just [o] if it's absolute
+ * @return concatenated this and [relative] paths, or just [relative] if it's absolute
  */
-public fun File.resolve(o: String): File = resolve(File(o))
+public fun File.resolve(relative: String): File = resolve(File(relative))
 
 /**
- * Adds relative [o] to this parent directory.
- * If [o] has a root or this has no parent directory, [o] is returned back.
+ * Adds [relative] file to this parent directory.
+ * If [relative] has a root or this has no parent directory, [relative] is returned back.
  * For instance, File("/foo/bar").resolveSibling(File("gav")) is File("/foo/gav")
  *
- * @return concatenated this.parent and [o] paths, or just [o] if it's absolute or this has no parent
+ * @return concatenated this.parent and [relative] paths, or just [relative] if it's absolute or this has no parent
  */
-public fun File.resolveSibling(o: File): File {
+public fun File.resolveSibling(relative: File): File {
     val components = filePathComponents()
     val rootName = components.rootName
-    val parentFile = when (components.size) {
-        0 -> null
-        1 -> File(rootName)
-        else -> File(rootName + components.subPath(0, components.size - 1).path)
-    }
-    return if (parentFile != null) parentFile.resolve(o) else o
+    return if (components.size() == 0) relative else File(rootName).resolve(components.subPath(0, components.size() - 1)).resolve(relative)
 }
 
 /**
- * Adds relative [o] to this parent directory.
- * If [o] has a root or this has no parent directory, [o] is returned back
+ * Adds [relative] name to this parent directory.
+ * If [relative] has a root or this has no parent directory, [relative] is returned back
  * For instance, File("/foo/bar").resolveSibling("gav") is File("/foo/gav")
  *
- * @return concatenated this.parent and [o] paths, or just [o] if it's absolute or this has no parent
+ * @return concatenated this.parent and [relative] paths, or just [relative] if it's absolute or this has no parent
  */
-public fun File.resolveSibling(o: String): File = resolveSibling(File(o))
+public fun File.resolveSibling(relative: String): File = resolveSibling(File(relative))
diff --git a/libraries/stdlib/test/io/Files.kt b/libraries/stdlib/test/io/Files.kt
index 0e4b93e42c3..6e1ff70c135 100644
--- a/libraries/stdlib/test/io/Files.kt
+++ b/libraries/stdlib/test/io/Files.kt
@@ -733,21 +733,22 @@ class FilesTest {
     test fun testCopyTo() {
         val srcFile = createTempFile()
         val dstFile = createTempFile()
-        srcFile.replaceText("Hello, World!")
+        srcFile.writeText("Hello, World!", "UTF8")
         try {
             srcFile.copyTo(dstFile)
             assert(false)
         } catch (e: FileAlreadyExistsException) {
+            println(e.getMessage())
         }
 
         var len = srcFile.copyTo(dstFile, overwrite = true)
         assertEquals(13L, len)
-        assertEquals(srcFile.readText(), dstFile.readText())
+        assertEquals(srcFile.readText(), dstFile.readText("UTF8"))
 
         assert(dstFile.delete())
         len = srcFile.copyTo(dstFile)
         assertEquals(13L, len)
-        assertEquals(srcFile.readText(), dstFile.readText())
+        assertEquals(srcFile.readText("UTF8"), dstFile.readText())
 
         assert(dstFile.delete())
         dstFile.mkdir()
@@ -811,8 +812,8 @@ class FilesTest {
             createTempDir(prefix = "d1_", directory = subDir1)
             val file1 = createTempFile(prefix = "f1_", directory = src)
             val file2 = createTempFile(prefix = "f2_", directory = subDir1)
-            file1.replaceText("hello")
-            file2.replaceText("wazzup")
+            file1.writeText("hello")
+            file2.writeText("wazzup")
             createTempDir(prefix = "d1_", directory = subDir2)
 
             assert(src.copyRecursively(dst))
diff --git a/libraries/stdlib/test/io/ReadWrite.kt b/libraries/stdlib/test/io/ReadWrite.kt
index da19e384eda..1d9d5ee03a3 100644
--- a/libraries/stdlib/test/io/ReadWrite.kt
+++ b/libraries/stdlib/test/io/ReadWrite.kt
@@ -15,10 +15,12 @@ fun sample(): Reader = StringReader("Hello\nWorld");
 class ReadWriteTest {
     test fun testAppendText() {
         val file = File.createTempFile("temp", System.nanoTime().toString())
-        file.replaceText("Hello\n")
-        file.appendText("World")
+        file.writeText("Hello\n", "UTF8")
+        file.appendText("World\n", "UTF8")
+        file.appendText("Again")
 
-        assertEquals("Hello\nWorld", file.readText())
+        assertEquals("Hello\nWorld\nAgain", file.readText())
+        assertEquals(listOf("Hello", "World", "Again"), file.readLines("UTF8"))
         file.deleteOnExit()
     }
 
@@ -71,34 +73,34 @@ class ReadWriteTest {
         writer.close()
 
         //file.replaceText("Hello\nWorld")
-        file.forEachBlock {(arr: ByteArray, size: Int) ->
+        file.forEachBlock { arr: ByteArray, size: Int ->
             assertTrue(size >= 11 && size <= 12, size.toString())
             assertTrue(arr.contains('W'.toByte()))
         }
         val list = ArrayList<String>()
-        file.forEachLine {
+        file.forEachLine("UTF8", {
             list.add(it)
-        }
+        })
         assertEquals(arrayListOf("Hello", "World"), list)
         val text = file.inputStream().reader().readText()
         assertTrue(text.contains("Hello"))
         assertTrue(text.contains("World"))
 
-        file.replaceText("")
+        file.writeText("")
         var c = 0
         file.forEachLine { c++ }
         assertEquals(0, c)
 
-        file.replaceText(" ")
+        file.writeText(" ")
         file.forEachLine { c++ }
         assertEquals(1, c)
 
-        file.replaceText(" \n")
+        file.writeText(" \n")
         c = 0
         file.forEachLine { c++ }
         assertEquals(1, c)
 
-        file.replaceText(" \n ")
+        file.writeText(" \n ")
         c = 0
         file.forEachLine { c++ }
         assertEquals(2, c)
@@ -167,5 +169,7 @@ class ReadWriteTest {
         val url = URL("http://kotlinlang.org")
         val text = url.readText()
         assertFalse(text.isEmpty())
+        val text2 = url.readText("UTF8")
+        assertFalse(text2.isEmpty())
     }
 }