请帮助我理解以下代码中的“分配”方法

Question

class Test { 

    int a, b; 

    Test(int a, int b) {    
        this.a = a; 
        this.b = b; 
    } 

    Test() {               
        a = 0; 
        b = 0; 
    } 

    void print() { 
        System.out.println("A=" + a + ", B=" + b); 
    } 

    void assign(Test ob) {  //How can a class(Test) be passed as an argument in a method?
        this.a = ob.a;      //What does ob.a do?
        this.b = ob.b;      //What does ob.b do?
    }                       //How can a object 'ob' be passed as an argument?
} 

.

class TestDem { 

    public static void main(String ar[]) { 
        Test ob1 = new Test(1, 2); 
        System.out.println("1st object"); 
        ob1.print(); 
        Test ob2 = new Test(); 
        System.out.println("2nd object"); 
        ob2.print(); 
        ob2.assign(ob1); //dont understand how this statement works with the 'assign' method 
        System.out.println("After assigning object 1 to object 2"); 
        System.out.println("1st object"); 
        ob1.print(); 
        System.out.println("2nd object"); 
        ob2.print(); 

    } 
} 

Output

1st object
A=1, B=2
2nd object
A=0, B=0
After assigning object 1 to object 2
1st object
A=1, B=2
2nd object 
A=1, B=2

Answer 1

void assign(Test ob) {  
    this.a = ob.a;      
    this.b = ob.b;      //What does ob.b do?
}

This method is similar to the constructor you have used: -

public Test(int a, int b) {
    this.a = a;
    this.b = b;
}

• Difference is that, in this method, you are getting the values of a and b from the reference passed rather than the parameter a, and b itself..
• Also note that it is not a constructor.. It is just assigning the value of passed reference to the current object.. Whereas constructor initializes the instance's state after it is created..

How can a class(Test) be passed as an argument in a method?

We are not passing the class Test here, rather, we are passing a reference to an object of class Test..

This method is doing nothing but creating a copy of the object pointed by that reference (ob in this case).. So, after this method executes, the current object(pointed by this ) will have the same value as the passed object..

What does ob.a do?

ob.a refers to the instance variable a for the reference ob of class Test , that you passed into this method..

So, if you have: -

Test ob2 = new Test(1, 2);
Test ob1 = new Test();
ob1.assign(ob2);

Then when this method executes: -

Since, this refers to the current reference, so this in method assign will refer to ob1 , since we are calling on this reference..

And, you will have both the reference ob2 in main method, and ob in the parameter of assign method pointing to the same object..

So, ob1.a(this.a) will be equal to ob2.a , and
ob1.b(this.b) will be equal to ob2.b

Answer 2

void assign(Test ob) {  //How can a class(Test) be passed as an argument in a method?
    this.a = ob.a;      //What does ob.a do?
    this.b = ob.b;      //What does ob.b do?
}                       //How can a object 'ob' be passed as an argument?

In Java (apart from primitive types like int, etc. let's not talk about them here) everything extends Object.

You defined a class Test that implictely extends Object . This class has two member variables: a and b , which are of type int .

If ob is of type Test , you can access its members using ob.a and ob.b (as a and b are public). And you can set their value using ob.a = anIntValue and ob.b = anIntValue .

That is what you do in the assign method: you pass another object of type Test and copy its values into your enclosing object ( this.a = value , this.b = value ).

So the result of your method is to modify your internal state by copying the state of an external object.

Answer 3

The method void assign(Test ob) takes an object ob of type Test . It then assigns the value of ob 's a value to the class that the method is being called on's a value, and then the same for the b values.

Perhaps if the method was renamed copy(Test ob) it would be clearer what the method does?