Java计算的意外结果

Question

As a result of the following snipet, i got "1 0 1", but i don't know why, im expecting "1 1 1" as a result. please can anyone explain to me how things go arround

public class Qcb90 {
int a;
int b;
public void f() {
    a = 0;
    b = 0;
    int[] c = { 0 };
    g(b, c);
    System.out.println(a + " " + b + " " + c[0] + " ");
}
public void g(int b, int[] c) {
    a = 1;
    b = 1;
    c[0] = 1;
}
public static void main(String[] args) {
    Qcb90 obj = new Qcb90();
    obj.f();
}
}

Answer 1

Change

b = 1;

to

this.b = 1;

The way you have it now, you are changing the parameter (local) variable not the class member variable.

Answer 2

It is because int is not a reference object, for example it is not the object which is created by the word new , so inside the method when you pass b to the method, a new variable will be created for this, and it can just be valid in this method. If an Object that is created by the word new , then it will got affect if it changed by any other method.

Answer 3

The parameter named b in function g(int b, int[] c) hides the class member variable b , so you are setting a local parameter called b to 1 in g(int b, int[] c) . This doesn't affect the member variable at all, and the new value is discarded after g exits.

However, the local parameter c is a copy of the pointer to the memory that was allocated in f , so you can modify the contents memory since both copies of the pointer (the copy passed in as a parameter to g and also the original in f ) point to the same block of memory.

Answer 4

  public class Qcb90 {
    int a;
    int b;
    public void f() {
        a = 0;
        b = 0;
        int[] c = { 0 };
        g(b, c);
    // so here b is your instance variable
        System.out.println(a + " " + b + " " + c[0] + " ");
    }
    public void g(int b, int[] c) {
        a = 1;
        //b = 1;  this b is a parameter of your method 
this.b=1; //now run your program


        c[0] = 1;
    }
    public static void main(String[] args) {
        Qcb90 obj = new Qcb90();
        obj.f();
    }
    }

If you want to print b value you need to write this.b inside g()