Assigning in Java?

Question

Say I set int A = int B. When I change A after, it will not change the value of B. But when I set a SomeClass A = SomeClass B, and I change A\'s contents (like a.cost), it c

Answer 1

In java when you assign Object to Object its assign by reference. Int is non an Object in java so when you assign int to int its assign by value.

Answer 2

A is a reference to the object. So if you change the object internal state, it will be reflected to every other variable pointing to it.

If you re-assign A, then B will not change:

Foo a = new Foo();
Foo b = a;
a.bar = "bar"; // this is reflected in b
a = new Foo(); // b stays pointing to the previous Foo
a.bar = "baaar"; // b stays with a value of bar="bar"

(Java is pass by value. Check this article about it.)

Answer 3

In Java, your variables can be split into two categories: Objects, and everything else (int, long, byte, etc).

A primitive type (int, long, etc), holds whatever value you assign it. An object variable, by contrast, holds a reference to an object somewhere. So if you assign one object variable to another, you have copied the reference, both A and B point to the same object.

NOTE: Strings in Java are actually objects, not primitives, which beginners often assume.

Hope this helps

Answer 4

I thought Java is assigned by value, not reference?

What does "assigned by value" mean? Are you maybe confusing it with "pass by value/reference"?

At any rate, if you handle a class instance in Java, you are actually handling a reference to that class (much like a pointer in C/C++). Your assignment only copies the reference, so both A and B refer to the same instance, i.e. the data is shared, hence the result.

Answer 5

Yes, it does - but the value of A is a reference, not a copy of the object itself.

I like to give the following analogy...

Suppose two people both have my address: that's like two variables of type House in Java. Now one of them comes and paints my door red. The second person will still see the red door if they visit:

House jonsHouse = new House(); // Even the variable jonsHouse is only a reference

House firstAddressCopy = jonsHouse; // Just a copy of the reference
House secondAddressCopy = jonsHouse; // Just a copy of the reference

firstAddressCopy.paintDoor(Color.Red);

Color color = secondAddressCopy.getDoorColor(); // Now color will be red

Basically, remember a few rules and things will become clear:

• The value of an expression in Java is never an object - only ever a reference or a primitive value
• (Corollary of first point) A variable never holds an object - only ever a reference or a primitive value
• Assignment (and argument passing) always copies the value, whether that value is a reference or a primitive value