BAEL-5610 Code for the clearing stringbuilder or stringbuffer article (#12378)

Co-authored-by: thibault.faure <thibault.faure@mimacom.com>

core-java-modules/core-java-string-apis/src/test/java/com/baeldung/clearstringbuilderorstringbuffer/ClearStringBuilderOrStringBufferUnitTest.java

@@ -0,0 +1,66 @@
+package com.baeldung.clearstringbuilderorstringbuffer;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ClearStringBuilderOrStringBufferUnitTest {
+
+    @Test
+    void whenSetLengthToZero_ThenStringBuilderIsCleared() {
+        StringBuilder stringBuilder = new StringBuilder();
+        stringBuilder.append("Hello World");
+        int initialCapacity = stringBuilder.capacity();
+        stringBuilder.setLength(0);
+        assertEquals("", stringBuilder.toString());
+        assertEquals(initialCapacity, stringBuilder.capacity());
+    }
+
+    @Test
+    void whenDeleteAll_ThenStringBuilderIsCleared() {
+        StringBuilder stringBuilder = new StringBuilder();
+        stringBuilder.append("Hello World");
+        int initialCapacity = stringBuilder.capacity();
+        stringBuilder.delete(0, stringBuilder.length());
+        assertEquals("", stringBuilder.toString());
+        assertEquals(initialCapacity, stringBuilder.capacity());
+    }
+
+    @Test
+    void whenSetLengthToZero_ThenStringBufferIsCleared() {
+        StringBuffer stringBuffer = new StringBuffer();
+        stringBuffer.append("Hello World");
+        int initialCapacity = stringBuffer.capacity();
+        stringBuffer.setLength(0);
+        assertEquals("", stringBuffer.toString());
+        assertEquals(initialCapacity, stringBuffer.capacity());
+    }
+
+    @Test
+    void whenDeleteAll_ThenStringBufferIsCleared() {
+        StringBuffer stringBuffer = new StringBuffer();
+        stringBuffer.append("Hello World");
+        int initialCapacity = stringBuffer.capacity();
+        stringBuffer.delete(0, stringBuffer.length());
+        assertEquals("", stringBuffer.toString());
+        assertEquals(initialCapacity, stringBuffer.capacity());
+    }
+
+    // Note: It did not make the cut to the article, but here is another way to reset a StringBuilder
+    @Test
+    void whenAssignedToNewStringBuilder_ThenStringBuilderIsCleared() {
+        StringBuilder stringBuilder = new StringBuilder();
+        stringBuilder.append("Hello World");
+        stringBuilder = new StringBuilder();
+        assertEquals("", stringBuilder.toString());
+    }
+
+    @Test
+    void whenAssignedToNewStringBuffer_ThenStringBufferIsCleared() {
+        StringBuffer stringBuffer = new StringBuffer();
+        stringBuffer.append("Hello World");
+        stringBuffer = new StringBuffer();
+        assertEquals("", stringBuffer.toString());
+    }
+
+}