Java对象分配行为不一致？

Question

According to this answer https://stackoverflow.com/a/12020435/562222 , assigning an object to another is just copying references, but let's see this code snippet:

public class TestJava {

    public static void main(String[] args) throws IOException {

        {
            Integer x;
            Integer y = 223432;
            x = y;
            x += 23;
            System.out.println(x);
            System.out.println(y);
        }
        {
            Integer[] x;
            Integer[] y = {1,2, 3, 4, 5};
            x = y;
            x[0] += 10;
            printArray(x);
            printArray(y);
        }
    }

    public static <T> void printArray(T[] inputArray) {
        for(T element : inputArray) {
            System.out.printf("%s ", element);
        }
        System.out.println();
    }

}

Running it gives:

223455
223432
11 2 3 4 5 
11 2 3 4 5 

Answer 1

The behavior is consistent. This line:

x += 23;

actually assigns a different Integer object to x ; it does not modify the value represented by x before that statement (which was, in fact, identical to object y ). Behind the scenes, the compiler is unboxing x and then boxing the result of adding 23, as if the code were written:

x = Integer.valueOf(x.intValue() + 23);

You can see exactly this if you examine the bytecode that is generated when you compile (just run javap -c TestJava after compiling).

What's going on in the second piece is that this line:

x[0] += 10;

also assigns a new object to x[0] . But since x and y refer to the same array, this also changes y[0] to be the new object.

Answer 2

Integer is immutable, so you cannot modify its state once created. When you do this:

Integer x;
Integer y = 223432;
x = y;
x += 23;

The line x += 23 is assigning a new Integer value to x variable.

Arrays, on the other hand, are mutable, so when you change the state of the array eg changing one of the elements in the array, the other is affected as well.

Answer 3

When you do this x += 23; , you actually create another object, and you made x point on this new object. Therefore, x and y are 2 different objects.