变量在执行操作后不改变其值

Question

I have the following piece of code:

 int[] a = {1,2,3,4};
 int temp = a[0];
 a[0] = 2;
 System.out.println(temp); //1

Why is it that the variable temp does not change its value to 2 after I execute the command on line 3?

Answer 1

In Java, everything is assigned by value.

int[] a = {1,2,3,4};

In the above line, you create an array that stores 4 numbers.

int temp = a[0];

Here, you assign the value stored in a[0] to the variable temp .

a[0] = 2;

Here you reassigned the value in a[0] , so there is something new that a[0] references to.

System.out.println(temp);

Prints the value stored in the temp variable.

---

The variables in Java aren't pointers. They store values. Those values are either the value itself (for primitives; eg 1,100, false) or the reference to an object itself. When you use the assignment operator ( = ) you say, that you would like the variable on the left to store the value which is on the right.

It can be either the value of a primitive or value of the reference to an object which is stored on the heap.

The difference between primitives and objects doesn't matter in this case actually. We could simplify the code to:

a = {1,2,3} // we somehow declare four placeholders for values.
            // a, a[0], a[1], a[2]
temp = a[0] // here we say, that we want temp to have the same value as a[0]
            // a[0] doesn't care
a[0] = 10   // here we say, that we want a[0] to have the same value as 10
            // temp doesn't care

To achieve the behavior that would't suprise OP we would need the following class:

class Foo {
    int i;
    Foo(int i) {
        this.i = i;
    }
}

and

Foo[] a = new Foo[]{new Foo(1), new Foo(2)};
Foo temp = a[0];
a[0].i = 100;
System.out.println(temp.i); //100

Answer 2

The value temp is assigned with a[0] that at the time of the assignment is 1

Then you change the value a[0] to 2. This does not change the value temp

To change temp you should reassign the new value with temp = a[0]