Creating a java.lang.String with the Charset constructor, one can inject your own Charset, which brings your own CharsetDecoder. The CharsetDecoder gets a reference to a CharBuffer object in the decodeLoop method. The CharBuffer wraps the char[] of the original String object. Since the CharsetDecoder has a reference to it, you can change the underlying char[] using the CharBuffer, thus you have a mutable String.
public class MutableStringTest {
// http://stackoverflow.com/questions/11146255/how-to-create-mutable-java-lang-string#11146288
@Test
public void testMutableString() throws Exception {
final String s = createModifiableString();
System.out.println(s);
modify(s);
System.out.println(s);
}
private final AtomicReference<CharBuffer> cbRef = new AtomicReference<CharBuffer>();
private String createModifiableString() {
Charset charset = new Charset("foo", null) {
@Override
public boolean contains(Charset cs) {
return false;
}
@Override
public CharsetDecoder newDecoder() {
CharsetDecoder cd = 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;
}
};
return cd;
}
@Override
public CharsetEncoder newEncoder() {
return null;
}
};
return new String("abc".getBytes(), charset);
}
private void modify(String s) {
CharBuffer charBuffer = cbRef.get();
charBuffer.position(0);
charBuffer.put("xyz");
}
}
Running the code prints
abc
zzz
I don’t know how to correctly implement decodeLoop(), but i don’t care right now 🙂