[BAEL-7043] mutable strings (#15112)

Co-authored-by: Bhaskar <bhaskar.dastidar@freshworks.com>

core-java-modules/core-java-string-operations-7/src/main/java/com/baeldung/mutablestrings/CharsetUsageExample.java

@@ -0,0 +1,34 @@
+package com.baeldung.mutablestrings;
+
+import java.nio.ByteBuffer;
+import java.nio.CharBuffer;
+import java.nio.charset.CharacterCodingException;
+import java.nio.charset.Charset;
+import java.nio.charset.CharsetDecoder;
+import java.nio.charset.CharsetEncoder;
+import java.nio.charset.CoderResult;
+
+public class CharsetUsageExample {
+
+    public ByteBuffer encodeString(String inputString) {
+        Charset charset = Charset.forName("UTF-8");
+        CharsetEncoder encoder = charset.newEncoder();
+
+        CharBuffer charBuffer = CharBuffer.wrap(inputString);
+        ByteBuffer byteBuffer = ByteBuffer.allocate(50);
+
+        encoder.encode(charBuffer, byteBuffer, true); // true indicates the end of input
+        byteBuffer.flip();
+        return byteBuffer;
+    }
+
+    public String decodeString(ByteBuffer byteBuffer) {
+        Charset charset = Charset.forName("UTF-8");
+        CharsetDecoder decoder = charset.newDecoder();
+        CharBuffer decodedCharBuffer = CharBuffer.allocate(50);
+        decoder.decode(byteBuffer, decodedCharBuffer, true);
+        decodedCharBuffer.flip();
+
+        return decodedCharBuffer.toString();
+    }
+}

core-java-modules/core-java-string-operations-7/src/main/java/com/baeldung/mutablestrings/MutableStringUsingCharset.java

@@ -0,0 +1,65 @@
+package com.baeldung.mutablestrings;
+
+import java.nio.ByteBuffer;
+import java.nio.CharBuffer;
+import java.nio.charset.Charset;
+import java.nio.charset.CharsetDecoder;
+import java.nio.charset.CharsetEncoder;
+import java.nio.charset.CoderResult;
+import java.util.concurrent.atomic.AtomicReference;
+
+public class MutableStringUsingCharset {
+
+    private final AtomicReference<CharBuffer> cbRef = new AtomicReference<>();
+    private final Charset myCharset = new Charset("mycharset", null) {
+        @Override
+        public boolean contains(Charset cs) {
+            return false;
+        }
+
+        @Override
+        public CharsetDecoder newDecoder() {
+            return new CharsetDecoder(this, 1.0f, 1.0f) {
+                @Override
+                protected CoderResult decodeLoop(ByteBuffer in, CharBuffer out) {
+                    cbRef.set(out);
+                    while (in.remaining() > 0) {
+                        out.append((char) in.get());
+                    }
+                    return CoderResult.UNDERFLOW;
+                }
+            };
+        }
+
+        @Override
+        public CharsetEncoder newEncoder() {
+            CharsetEncoder cd = new CharsetEncoder(this, 1.0f, 1.0f) {
+                @Override
+                protected CoderResult encodeLoop(CharBuffer in, ByteBuffer out) {
+                    while (in.hasRemaining()) {
+                        if (!out.hasRemaining()) {
+                            return CoderResult.OVERFLOW;
+                        }
+                        char currentChar = in.get();
+                        if (currentChar > 127) {
+                            return CoderResult.unmappableForLength(1);
+                        }
+                        out.put((byte) currentChar);
+                    }
+                    return CoderResult.UNDERFLOW;
+                }
+            };
+            return cd;
+        }
+    };
+
+    public String createModifiableString(String s) {
+        return new String(s.getBytes(), myCharset);
+    }
+
+    public void modifyString() {
+        CharBuffer cb = cbRef.get();
+        cb.position(0);
+        cb.put("xyz");
+    }
+}

core-java-modules/core-java-string-operations-7/src/main/java/com/baeldung/mutablestrings/MutableStrings.java

@@ -0,0 +1,24 @@
+package com.baeldung.mutablestrings;
+
+import java.lang.reflect.Field;
+import java.nio.charset.Charset;
+
+import com.google.errorprone.annotations.DoNotCall;
+
+public class MutableStrings {
+
+    /**
+     * This involves using Reflection to change String fields and it is not encouraged to use this in programs.
+     * @throws NoSuchFieldException
+     * @throws IllegalAccessException
+     */
+    @DoNotCall
+    public void mutableUsingReflection() throws NoSuchFieldException, IllegalAccessException {
+        String myString = "Hello World";
+        String otherString = new String("Hello World");
+        Field f = String.class.getDeclaredField("value");
+        f.setAccessible(true);
+        f.set(myString, "Hi World".toCharArray());
+        System.out.println(otherString);
+    }
+}

core-java-modules/core-java-string-operations-7/src/test/java/com/baeldung/mutablestring/CharsetUsageUnitTest.java

@@ -0,0 +1,17 @@
+package com.baeldung.mutablestring;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.mutablestrings.CharsetUsageExample;
+
+public class CharsetUsageUnitTest {
+
+    @Test
+    public void givenCharset_whenStringIsEncodedAndDecoded_thenGivesCorrectResult() {
+        CharsetUsageExample ch = new CharsetUsageExample();
+        String inputString = "hello दुनिया";
+        String result = ch.decodeString(ch.encodeString(inputString));
+        Assertions.assertEquals(inputString, result);
+    }
+}

core-java-modules/core-java-string-operations-7/src/test/java/com/baeldung/mutablestring/MutableStringUsingCharsetUnitTest.java

@@ -0,0 +1,24 @@
+package com.baeldung.mutablestring;
+
+import org.junit.Assert;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.mutablestrings.MutableStringUsingCharset;
+
+public class MutableStringUsingCharsetUnitTest {
+    @Test
+    @Disabled
+    /**
+     * This test is disabled as it works well for Java 8 and below
+     */
+    public void givenCustomCharSet_whenStringUpdated_StringGetsMutated() throws Exception {
+        MutableStringUsingCharset ms = new MutableStringUsingCharset();
+        String s = ms.createModifiableString("Hello");
+        Assert.assertEquals("Hello", s);
+        ms.modifyString();
+        Assert.assertEquals("something", s);
+    }
+
+}
+