Allow setting seed only once, as it is set from superclass constructor

Add tests for Java<->Kotlin Random wrappers.

#KT-29520 Fixed

libraries/stdlib/jvm/src/kotlin/random/PlatformRandom.kt

@@ -75,7 +75,13 @@ private class KotlinRandom(val impl: Random) : java.util.Random() {
         impl.nextBytes(bytes)
     }
 
+    private var seedInitialized: Boolean = false
     override fun setSeed(seed: Long) {
-        throw UnsupportedOperationException("Setting seed is not supported.")
+        if (!seedInitialized) {
+            // ignore seed value from constructor
+            seedInitialized = true
+        } else {
+            throw UnsupportedOperationException("Setting seed is not supported.")
+        }
     }
 }

libraries/stdlib/jvm/test/random/RandomWrappersTest.kt

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package test.random
+
+import kotlin.random.*
+import kotlin.test.*
+
+class RandomWrappersTest {
+    @Test
+    fun kotlinRandomAsJavaRandom() {
+        val expect = Random(42)
+        val actual = Random(42)
+
+        val actualJava = actual.asJavaRandom()
+
+        repeat(10) {
+            assertEquals(expect.nextInt(), actualJava.nextInt())
+            assertEquals(expect.nextInt(100), actualJava.nextInt(100))
+            assertEquals(expect.nextLong(), actualJava.nextLong())
+            assertEquals(expect.nextDouble(), actualJava.nextDouble())
+            assertEquals(expect.nextFloat(), actualJava.nextFloat())
+            assertEquals(expect.nextBoolean(), actualJava.nextBoolean())
+        }
+
+        assertSame(actual, actualJava.asKotlinRandom())
+
+        assertFailsWith<UnsupportedOperationException> { actualJava.setSeed(1L) }
+
+        val defaultAsJava = Random.asJavaRandom()
+        assertFailsWith<UnsupportedOperationException> { defaultAsJava.setSeed(1L) }
+    }
+
+    @Test
+    fun javaRandomAsKotlinRandom() {
+        val expect = java.util.Random(42L)
+        val actual = java.util.Random(42L)
+
+        val actualKotlin = actual.asKotlinRandom()
+
+        repeat(10) {
+            assertEquals(expect.nextInt(), actualKotlin.nextInt())
+            assertEquals(expect.nextInt(100), actualKotlin.nextInt(100))
+            assertEquals(expect.nextLong(), actualKotlin.nextLong())
+            assertEquals(expect.nextDouble(), actualKotlin.nextDouble())
+            assertEquals(expect.nextFloat(), actualKotlin.nextFloat())
+            assertEquals(expect.nextBoolean(), actualKotlin.nextBoolean())
+        }
+
+        assertSame(actual, actualKotlin.asJavaRandom())
+    }
+}