How to prune an object of some of its fields in Java?

Question

Suppose we have an object obj of type Object such that System.out.println(obj) produces {a=Some text, b=Some more text, c=Even more text}.

Now we want to make a new object obj2 that is just {a=Some text} (i.e., the fields b and c are pruned from obj). So we define a class A as follows:

class A {
    String a;
}

Then we initialize obj2 as follows:

A obj2 = (A) obj.

Unfortunately, I get a run time error when doing this.

Question: How do we define obj2 as outlined above?

Answer 1

Assume that you have three fields f1,f2,f3 in class A

Create a new class B with field f1

Declare a method in class A like this

public B getTrimmedObject()

Set required fields of B from A.

e.g. set f1 and return B Object

Other way : use composition over inheritance.B will become member of A and you can simply get B from A. In this case split fields between A and B and avoid duplication.

f1 will be part of B and B,f2,f3 will be part of A. A Constructor will set f1 value through B and f2,f3 are set with normal initialization

EDIT: example code:

public class A{
    private B b;
    private String f2;
    private String f3;

    public A(String f1,String f2,String f3){
        b = new B(f1);
        this.f2 = f2;
        this.f3 = f3;
        System.out.println("In A->B:"+getB().getF1()+":A:f2:"+f2+":A:f3:"+f3);
    }
    public B getB(){
        return this.b;
    }
    public static void main(String args[]){
        if (args.length < 3) {
            System.out.println("Usage: java A str1 str2 str3");
            return;
        }
        A a = new A(args[0],args[1],args[2]);

    }

}
class B{
    private String f1;
    public B(String f1){
        this.f1 = f1;
    }
    public String getF1(){
        return this.f1;
    }
}



java A apple banana camel

 In A->B:apple:A:f2:banana:A:f3:camel