BAEL-6627: How to start a thread using an anonymous class (#14314)

Co-authored-by: Tapan Avasthi <tavasthi@Tapans-MacBook-Air.local>

core-java-modules/core-java-concurrency-simple/src/test/java/com/baeldung/concurrent/anonymousthread/AnonymousThreadUnitTest.java

@@ -0,0 +1,112 @@
+package com.baeldung.concurrent.anonymousthread;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.PrintStream;
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.After;
+import org.junit.AfterClass;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class AnonymousThreadUnitTest {
+    static ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+    static PrintStream printStream = new PrintStream(outputStream);
+    static List<String> expectedNumbers = new ArrayList<>();
+
+    @BeforeClass
+    public static void setupClass() {
+        for (int i = 1; i <= 10; i++) {
+            expectedNumbers.add(String.valueOf(i));
+        }
+    }
+
+    @Before
+    public void setupMethod() {
+        System.setOut(printStream);
+    }
+
+    @After
+    public void cleanUpMethod() {
+        outputStream.reset();
+        System.setOut(System.out);
+    }
+
+    @AfterClass
+    public static void cleanupClass() throws IOException {
+        expectedNumbers.clear();
+        outputStream.close();
+        printStream.close();
+    }
+
+    @Test
+    public void givenAnonymousThreadSubclassOverridingRunMethod_whenStart_thenRunAsExpected() {
+        Thread anonymousThread = new Thread() {
+            @Override
+            public void run() {
+                printNumbersFrom1To10();
+            }
+        };
+        anonymousThread.start();
+        waitForThreadToFinish();
+        anonymousThread.interrupt();
+
+        String[] printedNumbers = getPrintedNumbers();
+        assertEquals(expectedNumbers, List.of(printedNumbers));
+    }
+
+    @Test
+    public void givenAnonymousThreadWithLambdaExpression_whenStart_thenRunAsExpected() {
+        Thread anonymousThread = new Thread(() -> {
+            printNumbersFrom1To10();
+        });
+
+        anonymousThread.start();
+        waitForThreadToFinish();
+        anonymousThread.interrupt();
+
+        String[] printedNumbers = getPrintedNumbers();
+        assertEquals(expectedNumbers, List.of(printedNumbers));
+    }
+
+    @Test
+    public void givenAnonymousThreadWithRunnableObject_whenStart_thenRunAsExpected() {
+        Thread anonymousThread = new Thread(new Runnable() {
+            @Override
+            public void run() {
+                printNumbersFrom1To10();
+            }
+        });
+        anonymousThread.start();
+        waitForThreadToFinish();
+        anonymousThread.interrupt();
+
+        String[] printedNumbers = getPrintedNumbers();
+        assertEquals(expectedNumbers, List.of(printedNumbers));
+    }
+
+    private void printNumbersFrom1To10() {
+        for (int i = 1; i <= 10; i++) {
+            System.out.println(i);
+        }
+    }
+
+    private void waitForThreadToFinish() {
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException e) {
+            // ignore
+        }
+    }
+
+    private String[] getPrintedNumbers() {
+        String printedOutput = outputStream.toString()
+            .trim();
+        return printedOutput.split(System.lineSeparator());
+    }
+}