immutability

I have got one class with various member variables. There is a constructor and there are getter-methods, but no setter-methods. In fact, this object should be immutable. public class Example { private ArrayList<String> list; } Now I noticed the following: when I get the variable list with a getter-method, I can add new values and so on - I can change the ArrayList . When I call the next time get() for this variable, the changed ArrayList is returned. How can this be? I didn't set it again, I just worked on it! With a String this behaviour isn't possible. So what is the difference here? Just

Does anyone know a library or some at least some research on creating and using persistent data structures in Java? I don't refer to persistence as long term storage but persistence in terms of immutability (see Wikipedia entry ). I'm currently exploring different ways to model an api for persistent structures. Using builders seems to be a interesting solution: // create persistent instance Person p = Builder.create(Person.class) .withName("Joe") .withAddress(Builder.create(Address.class) .withCity("paris") .build()) .build(); // change persistent instance, i.e. create a new one Person p2 =

I am using Redux. In my reducer I'm trying to remove a property from an object like this: const state = { a: '1', b: '2', c: { x: '42', y: '43' }, } And I want to have something like this without having to mutate the original state: const newState = { a: '1', b: '2', c: { x: '42', }, } I tried: let newState = Object.assign({}, state); delete newState.c.y but for some reasons, it deletes the property from both states. Could help me to do that? How about using destructuring assignment syntax? const original = { foo: 'bar', stack: 'overflow', }; // If the name of the property to remove is

I know that immutable objects always have the same state, the state in which they are actually created. Their invariants are establised by the constructor and since their state does not change after construction , those invariants always hold good and this is why they are safe to publish in a multi threaded environment. This is all fine but since we live in a dynamic world , where the state of program changes continuously , what benefits do such objects give us if we construct the state of our program through immutable objects? "what benefits do such objects give us" you already answered that.