diff --git a/backend.native/tests/build.gradle b/backend.native/tests/build.gradle
index 28a4aef367b..ecec21959fd 100644
--- a/backend.native/tests/build.gradle
+++ b/backend.native/tests/build.gradle
@@ -314,6 +314,11 @@ task tostring1(type: RunKonanTest) {
     source = "runtime/basic/tostring1.kt"
 }
 
+task tostring2(type: RunKonanTest) {
+    goldValue = "H e l l o \nHello\n"
+    source = "runtime/basic/tostring2.kt"
+}
+
 task array0(type: RunKonanTest) {
     goldValue = "5\n6\n7\n8\n9\n10\n11\n12\n13\n"
     source = "runtime/basic/array0.kt"
@@ -444,6 +449,11 @@ task moderately_large_array(type: RunKonanTest) {
     source = "runtime/collections/moderately_large_array.kt"
 }
 
+task string_builder0(type: RunKonanTest) {
+    goldValue = "OK\n"
+    source = "runtime/text/string_builder0.kt"
+}
+
 task catch1(type: RunKonanTest) {
     goldValue = "Before\nCaught Throwable\nDone\n"
     source = "runtime/exceptions/catch1.kt"
diff --git a/backend.native/tests/runtime/basic/tostring2.kt b/backend.native/tests/runtime/basic/tostring2.kt
new file mode 100644
index 00000000000..aa17c92b735
--- /dev/null
+++ b/backend.native/tests/runtime/basic/tostring2.kt
@@ -0,0 +1,10 @@
+fun main(args : Array<String>) {
+    val hello = "Hello"
+    val array = toCharArray(hello)
+    for (ch in array) {
+        print(ch)
+        print(" ")
+    }
+    println()
+    println(fromCharArray(array, 0, array.size))
+}
\ No newline at end of file
diff --git a/backend.native/tests/runtime/collections/array_list1.kt b/backend.native/tests/runtime/collections/array_list1.kt
index 96037257306..ffc53ddb329 100644
--- a/backend.native/tests/runtime/collections/array_list1.kt
+++ b/backend.native/tests/runtime/collections/array_list1.kt
@@ -182,6 +182,12 @@ fun testToString() {
     assertTrue(a.toString() == makeList123().toString())
 }
 
+fun testToString2() {
+    val a = makeList123()
+    assertEquals(a.toString(), "[1, 2, 3]")
+}
+
+
 fun testSubList() {
     val a0 = makeList01234()
     val a = a0.subList(1, 4)
@@ -271,11 +277,12 @@ fun testIteratorAdd() {
         val next = it.next()
         if (i++ % 2 == 0)
             it.add("-" + next)
-        it.next()
     }
     //assertEquals(listOf("1", "2", "-2", "3", "4", "-4", "5"), a)
 }
 
+
+
 fun main(args : Array<String>) {
     testBasic()
     testIterator()
@@ -286,6 +293,7 @@ fun main(args : Array<String>) {
     testEquals()
     testHashCode()
     testToString()
+    testToString2()
     testSubList()
     testResize()
     testSubListContains()
diff --git a/backend.native/tests/runtime/text/string_builder0.kt b/backend.native/tests/runtime/text/string_builder0.kt
new file mode 100644
index 00000000000..5af19eee715
--- /dev/null
+++ b/backend.native/tests/runtime/text/string_builder0.kt
@@ -0,0 +1,46 @@
+fun assertTrue(cond: Boolean) {
+    if (!cond)
+        println("FAIL")
+}
+
+fun assertFalse(cond: Boolean) {
+    if (cond)
+        println("FAIL")
+}
+
+fun assertEquals(value1: String, value2: String) {
+    if (value1 != value2)
+        println("FAIL: '" + value1 + "' != '" + value2 + "'")
+}
+
+fun assertEquals(value1: Int, value2: Int) {
+    if (value1 != value2)
+        println("FAIL" + value1.toString() + " != " + value2.toString())
+}
+
+fun testBasic() {
+    val sb = StringBuilder()
+    assertEquals(0, sb.length)
+    assertEquals("", sb.toString())
+    sb.append(1)
+    assertEquals(1, sb.length)
+    assertEquals("1", sb.toString())
+    sb.append(", ")
+    assertEquals(3, sb.length)
+    assertEquals("1, ", sb.toString())
+    sb.append(true)
+    assertEquals(7, sb.length)
+    assertEquals("1, true", sb.toString())
+    sb.append(12345678L)
+    assertEquals(15, sb.length)
+    assertEquals("1, true12345678", sb.toString())
+
+    sb.length = 0
+    assertEquals(0, sb.length)
+    assertEquals("", sb.toString())
+}
+
+fun main(args : Array<String>) {
+    testBasic()
+    println("OK")
+}
\ No newline at end of file
diff --git a/runtime/src/main/cpp/Arrays.cpp b/runtime/src/main/cpp/Arrays.cpp
index 383d0443d3c..a661c8ae9aa 100644
--- a/runtime/src/main/cpp/Arrays.cpp
+++ b/runtime/src/main/cpp/Arrays.cpp
@@ -115,6 +115,17 @@ ArrayHeader* Kotlin_CharArray_clone(const ArrayHeader* array) {
   return result;
 }
 
+ArrayHeader* Kotlin_CharArray_copyOf(const ArrayHeader* array, KInt newSize) {
+  ArrayHeader* result = ArrayContainer(
+      theCharArrayTypeInfo, newSize).GetPlace();
+  KInt toCopy = array->count_ < newSize ?  array->count_ : newSize;
+  memcpy(
+      PrimitiveArrayAddressOfElementAt<KChar>(result, 0),
+      PrimitiveArrayAddressOfElementAt<KChar>(array, 0),
+      toCopy * sizeof(KChar));
+  return result;
+}
+
 KInt Kotlin_CharArray_getArrayLength(const ArrayHeader* array) {
   return array->count_;
 }
diff --git a/runtime/src/main/cpp/Natives.cpp b/runtime/src/main/cpp/Natives.cpp
index 5962aeaea0c..8775183d8be 100644
--- a/runtime/src/main/cpp/Natives.cpp
+++ b/runtime/src/main/cpp/Natives.cpp
@@ -79,18 +79,48 @@ KInt Kotlin_String_getStringLength(KString thiz) {
   return thiz->count_;
 }
 
-KString Kotlin_String_fromUtf8Array(const ArrayHeader* array) {
+KString Kotlin_String_fromUtf8Array(
+    const ArrayHeader* array, KInt start, KInt size) {
   RuntimeAssert(array->type_info() == theByteArrayTypeInfo, "Must use a byte array");
+  if (start < 0 || start + size > array->count_) {
+    ThrowArrayIndexOutOfBoundsException();
+  }
   // TODO: support full UTF-8.
-  ArrayHeader* result = ArrayContainer(
-      theStringTypeInfo, array->count_).GetPlace();
+  ArrayHeader* result = ArrayContainer(theStringTypeInfo, size).GetPlace();
   memcpy(
       ByteArrayAddressOfElementAt(result, 0),
-      ByteArrayAddressOfElementAt(array, 0),
-      ArrayDataSizeBytes(array));
+      ByteArrayAddressOfElementAt(array, start),
+      size);
   return result;
 }
 
+KString Kotlin_String_fromCharArray(
+    const ArrayHeader* array, KInt start, KInt size) {
+  RuntimeAssert(array->type_info() == theCharArrayTypeInfo, "Must use a byte array");
+  if (start < 0 || start + size > array->count_) {
+    ThrowArrayIndexOutOfBoundsException();
+  }
+  // TODO: support full UTF-8.
+  ArrayHeader* result = ArrayContainer(theStringTypeInfo, size).GetPlace();
+  for (KInt index = 0; index < size; ++index) {
+    *ByteArrayAddressOfElementAt(result, index) =
+        *PrimitiveArrayAddressOfElementAt<KChar>(array, start + index);
+  }
+  return result;
+}
+
+ArrayHeader* Kotlin_String_toCharArray(KString string) {
+  // TODO: support full UTF-8.
+  ArrayHeader* result = ArrayContainer(
+      theCharArrayTypeInfo, string->count_).GetPlace();
+  for (int index = 0; index < string->count_; ++index) {
+    *PrimitiveArrayAddressOfElementAt<KChar>(result, index) =
+        *ByteArrayAddressOfElementAt(string, index);
+  }
+  return result;
+}
+
+
 KString Kotlin_String_plusImpl(KString thiz, KString other) {
   // TODO: support UTF-8
   RuntimeAssert(thiz != nullptr, "this cannot be null");
@@ -113,7 +143,7 @@ KString Kotlin_String_plusImpl(KString thiz, KString other) {
 
 KBoolean Kotlin_String_equals(KString thiz, KConstRef other) {
   if (other == nullptr || other->type_info() != theStringTypeInfo) return 0;
-  const ArrayHeader* otherString = reinterpret_cast<const ArrayHeader*>(other);
+  KString otherString = reinterpret_cast<KString>(other);
   return thiz->count_ == otherString->count_ &&
       memcmp(ByteArrayAddressOfElementAt(thiz, 0),
              ByteArrayAddressOfElementAt(otherString, 0),
diff --git a/runtime/src/main/cpp/Natives.h b/runtime/src/main/cpp/Natives.h
index 5f678554b9a..c19d7e055fa 100644
--- a/runtime/src/main/cpp/Natives.h
+++ b/runtime/src/main/cpp/Natives.h
@@ -80,7 +80,8 @@ KInt Kotlin_String_hashCode(KString thiz);
 KBoolean Kotlin_String_equals(KString thiz, KConstRef other);
 KInt Kotlin_String_compareTo(KString thiz, KString other);
 KChar Kotlin_String_get(KString thiz, KInt index);
-KString Kotlin_String_fromUtf8Array(const ArrayHeader* array);
+KString Kotlin_String_fromUtf8Array(const ArrayHeader* array, KInt start, KInt size);
+KString Kotlin_String_fromCharArray(const ArrayHeader* array, KInt start, KInt size);
 KString Kotlin_String_plusImpl(KString thiz, KString other);
 KInt Kotlin_String_getStringLength(KString thiz);
 KString Kotlin_String_subSequence(KString thiz, KInt startIndex, KInt endIndex);
diff --git a/runtime/src/main/kotlin/kotlin/Arrays.kt b/runtime/src/main/kotlin/kotlin/Arrays.kt
index 8ce4170bd00..9b38edbffc9 100644
--- a/runtime/src/main/kotlin/kotlin/Arrays.kt
+++ b/runtime/src/main/kotlin/kotlin/Arrays.kt
@@ -66,6 +66,9 @@ public final class CharArray : Cloneable {
     @SymbolName("Kotlin_CharArray_clone")
     external public override fun clone(): Any
 
+    @SymbolName("Kotlin_CharArray_copyOf")
+    external public fun copyOf(newSize: Int): CharArray
+
     @SymbolName("Kotlin_CharArray_getArrayLength")
     external private fun getArrayLength(): Int
 
diff --git a/runtime/src/main/kotlin/kotlin/String.kt b/runtime/src/main/kotlin/kotlin/String.kt
index 4609ed0c770..021c2874f12 100644
--- a/runtime/src/main/kotlin/kotlin/String.kt
+++ b/runtime/src/main/kotlin/kotlin/String.kt
@@ -1,7 +1,5 @@
 package kotlin
 
-@SymbolName("Kotlin_String_fromUtf8Array")
-external fun fromUtf8Array(array: ByteArray) : String
 
 // TODO: enable, once global variables implemented.
 // @SymbolName("theEmptyString")
diff --git a/runtime/src/main/kotlin/kotlin/collections/ArrayList.kt b/runtime/src/main/kotlin/kotlin/collections/ArrayList.kt
index f829871cab7..af554fd91bd 100644
--- a/runtime/src/main/kotlin/kotlin/collections/ArrayList.kt
+++ b/runtime/src/main/kotlin/kotlin/collections/ArrayList.kt
@@ -167,15 +167,16 @@ class ArrayList<E> private constructor(
     }
 
     override fun toString(): String {
-        var result = "["
+        val sb = StringBuilder(2 + length * 3)
+        sb.append("[")
         var i = 0
         while (i < length) {
-            if (i > 0) result += ", "
-            result += array[offset + i].toString()
+            if (i > 0) sb.append(", ")
+            sb.append(array[offset + i])
             i++
         }
-        result += "]"
-        return result
+        sb.append("]")
+        return sb.toString()
     }
 
     // ---------------------------- private ----------------------------
diff --git a/runtime/src/main/kotlin/kotlin/text/StringBuilder.kt b/runtime/src/main/kotlin/kotlin/text/StringBuilder.kt
new file mode 100644
index 00000000000..fbc2dcb04ed
--- /dev/null
+++ b/runtime/src/main/kotlin/kotlin/text/StringBuilder.kt
@@ -0,0 +1,103 @@
+package kotlin.text
+
+@SymbolName("Kotlin_String_fromUtf8Array")
+external fun fromUtf8Array(array: ByteArray, start: Int, size: Int) : String
+
+// TODO: make it somewhat private?
+@SymbolName("Kotlin_String_fromCharArray")
+external fun fromCharArray(array: CharArray, start: Int, size: Int) : String
+
+@SymbolName("Kotlin_String_toCharArray")
+external fun toCharArray(string: String) : CharArray
+
+
+class StringBuilder private constructor (
+        private var array: CharArray
+) : CharSequence {
+    constructor() : this(10)
+
+    constructor(capacity: Int) : this(CharArray(capacity))
+
+    constructor(string: String) : this(toCharArray(string)) {
+        length = array.size
+    }
+
+    override var length: Int = 0
+        set(capacity) {
+            ensureCapacity(capacity)
+            field = capacity
+        }
+
+    override fun get(index: Int): Char {
+        checkIndex(index)
+        return array[index]
+    }
+
+    override fun subSequence(startIndex: Int, endIndex: Int): CharSequence = substring(startIndex, endIndex)
+
+    override fun toString(): String = fromCharArray(array, 0, length)
+
+    fun substring(startIndex: Int, endIndex: Int): String {
+        checkInsertIndex(startIndex)
+        checkInsertIndexFrom(endIndex, startIndex)
+        return fromCharArray(array, startIndex, endIndex - startIndex)
+    }
+
+    fun trimToSize() {
+        if (length < array.size)
+            array = array.copyOf(length)
+    }
+
+    fun ensureCapacity(capacity: Int) {
+        if (capacity > array.size) {
+            var newSize = array.size * 3 / 2
+            if (capacity > newSize)
+                newSize = capacity
+            array = array.copyOf(newSize)
+        }
+    }
+
+    fun append(it: Char) {
+        ensureExtraCapacity(1)
+        array[length++] = it
+    }
+
+    fun append(it: CharArray) {
+        ensureExtraCapacity(it.size)
+        for (c in it)
+            array[length++] = c
+    }
+
+    fun append(it: String) {
+        ensureExtraCapacity(it.length)
+        for (c in toCharArray(it))
+            array[length++] = c
+    }
+
+    fun append(it: Boolean) = append(it.toString())
+    fun append(it: Byte) = append(it.toString())
+    fun append(it: Short) = append(it.toString())
+    fun append(it: Int) = append(it.toString())
+    fun append(it: Long) = append(it.toString())
+    fun append(it: Float) = append(it.toString())
+    fun append(it: Double) = append(it.toString())
+    fun append(it: Any?) = append(it.toString())
+
+    // ---------------------------- private ----------------------------
+
+    private fun ensureExtraCapacity(n: Int) {
+        ensureCapacity(length + n)
+    }
+
+    private fun checkIndex(index: Int) {
+        if (index < 0 || index >= length) throw IndexOutOfBoundsException()
+    }
+
+    private fun checkInsertIndex(index: Int) {
+        if (index < 0 || index > length) throw IndexOutOfBoundsException()
+    }
+
+    private fun checkInsertIndexFrom(index: Int, fromIndex: Int) {
+        if (index < fromIndex || index > length) throw IndexOutOfBoundsException()
+    }
+}
\ No newline at end of file