How to create a persistent object in Java?

Question

How to create a link to a permanent object?

final Myclass obj = new Myclass();

doesn’t work, he says that obj (link) should not be rewritten, but we can still change the related object.

I want to ensure that the object itself does not change after construction.

Answer 1

Just make it immutable (e.g. Stringis). Or wrap it in another object that restricts access to the mutators of the object in question (for example, Collections.unmodifiableList()gophers).

Answer 2

You mix two things: the final and the unchanging.

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Answer 3

. Java , . , . BalusC, , .

Answer 4

, , Java. /, .

- ReadOnly .

: http://en.wikipedia.org/wiki/Const-correctness#final_in_Java

Answer 5

Java , "", , "getter", . , , :

public class MyClass {
  String something;
  int somethingElse;

  // The class can only be modified by the constructor
  public MyClass(String something, int somethingElse) {
    this.something = something;
    this.somethingElse = somethingElse;
  }

  // Access "something".  Note that it is a String, which is immutable.
  public String getSomething() {
    return something;
  }

  // Access "somethingElse".  Note that it is an int, which is immutable.
  public int getSomethingElse() {
     return somethingElse;
  }
}

Answer 6

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final MyClass obj = new Myclass();

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Answer 7

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final MyClass obj = new MyClass();

Answer 8

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Answer 9

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• get is
• ( void )

Yes, getter methods can mutate an object. But if your code (or the code you use) does this, you have some big problems, please ask for help :)

the code:

class ImmutableWrapper
    public static <T> T wrap(T thing) {
        return (T) Proxy.newProxyInstance(thing.getClass().getClassLoader(), new Class[]{thing.getClass()}, OnlyGettersInvocationHandler.instance);
    }

    private static class OnlyGettersInvocationHandler implements InvocationHandler {
        public static InvocationHandler instance;
        @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            final String name = method.getName();
            if ((args == null || args.length == 0)
                    && (name.startsWith("get") || name.startsWith("is")
                    && !method.getReturnType().equals(Void.class))) {
                return method.invoke(proxy, args);
            } else {
                throw new UnsupportedOperationException("immutable object: " + proxy + ", cannot call " + name);
            }
        }
    }
}

SomeClass myThing = ... create and populate some object ...
SomeClass myImmutableThing = ImmutableWrapper.wrap(myThing);
myImmutableThing.setValue('foo');   // throws Exception
myImmutableThing.whatever();        // throws Exception
myImmutableThing.getSomething();    // returns something
myImmutableThing.isHappy();         // returns something