Is it possible to assign a java object with only the attributes that are available?-CodePudding

I have a java pojo class as below

public class ClassA {
    private String a;
    private String b;
    private String c;

    public String getA() {
        return a;
    }

    public void setA(String a) {
        this.a = a;
    }

    public String getB() {
        return b;
    }

    public void setB(String b) {
        this.b = b;
    }

    public String getC() {
        return c;
    }

    public void setC(String c) {
        this.c = c;
    }
}

And I am using it in another class as below

public class Test {

    public static void main(String[] args) {
        ClassA ca1 = new ClassA();
        ca1.setA("One");
        
        ClassA ca2 = new ClassA();
        ca2 = ca1;
        
        System.out.println(ca2);
        
    }
}

The output is: ClassA@53bd815b[a=One,b=<null>,c=<null>]

Is it possible that when I assign ca1 to ca2, ca2 will have only the attribute "a" and not have the other attributes, "b" and "c" as no values are assigned to them?

CodePudding user response：

Your code:

ClassA ca2 = new ClassA();
ca2 = ca1;

… makes no sense. You instantiate a new object of class ClassA. You assign a reference to that object to be stored in a reference variable named ca2. Then you immediately assign another reference to a different object to that same reference variable ca2.

So there was no point to the first of those two lines. Logically, you could replace those two lines with the following, and end up with the same effect.

ClassA ca2 = ca1;

You asked:

is it possible that when I assign ca1 to ca2, ca2 will have only the attribute "a" and not have the other attributes, "b" and "c" as no values are assigned to them.

If you are asking if the second ClassA object can somehow absorb values from the first, the Answer is “No”.

Your two instances of ClassA are separate and distinct from one another. Each holds its own state (member field values). That state is not altered by you assigning a reference to either into a reference variable.

In your Question’s code, you end up with two objects in memory:

One has one member field assigned a value, and the other two yields are null. That object has two reference variables pointing to it.
The other object has all three of its member fields null, with none of the three ever assigned a value. This object has no references left pointing to it. Having no references means this object is effectively “forgotten”, or lost. This object is now a candidate for the garbage collection. After garbage collection, the memory used by this object is freed up to be used for other purposes.

The key concept here is that ca1 and ca2 are not themselves objects. They are reference variables, also known as pointers. They know where to find a particular object. They can each point to different objects, or they can both point to the same object.

See the illustration I made in another Answer of mine on a similar Question.

When you declare ClassA ca2, you are not saying that ca2 is a ClassA, you are saying ca2 will track the location in memory of an object of that class and only that class (and its subclasses).

As for the output of toString being called implicitly by System.out.println, you should override toString yourself to generate whatever output you choose.