CODEWITHSUNDEEP
java
geasssos
geasssos

Reputation: 429

Understanding object variable assignment behaviour

I am in some sort of weird problem in Java. I've nailed down the whole problem while debugging it. It happens at these 2 lines:

q=p;
q.addPair(2,3);

notes: p,q is a new class I've defined. In this class, it has a public function addPari(int, int).

Here is what I expected: copy p to q, then change q use q.addPair(), but leave p the same as before.

I thought this could work, but somehow, it turns out q.addPair(2,3) will change both p and q. Anyone can help me about that?

Upvotes: 0

Views: 96

Answers (3)

Michael Ford
Michael Ford

Reputation: 861

Java assigns by reference, so you would need to do a deep copy in order to copy one object into variable of the same type but then have two seperate objects.

Take a look at this: http://www.jusfortechies.com/java/core-java/deepcopy_and_shallowcopy.php

Upvotes: 0

flex36
flex36

Reputation: 146

With q=p; you don't 'copy' p to q, but you instead create a reference for q that points to the instance of p. Now the 2 variables point to the same object.

Upvotes: 0

Simeon Visser
Simeon Visser

Reputation: 122366

q=p;

does not actually copy. It means modifying q will also modify p as they're the same instance.

If you want q to be a new object, you need to use new:

q = new MyObject(p);

In other words, you're using a copy constructor to create a new copy of p. If your class doesn't have a copy constructor you'll need to create one: it needs to be able to create a new object from an existing one. It can do this by copying over the appropriate values from the given instance p.

Upvotes: 5

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