Java：创建一个将从父对象继承父属性的子对象

Question

I am new to Inheritance properties.

We created a Child object "obnewChild" which inherits the Parent properties from the Parent Object, "obParent" .

The main program will use the Child object "obnewChild" and manipulate the fields inherited from the Parent as well as its own fields.

At the end - the Output would be the Parent Object, "obParent" back in Json format.. with the updated values of the fields from the program run.

(From a Program perspective - The Child object has transient fields - that are created for each Parent Object - used in the main program during its run. The expectation is that any changes to the Child object will also show up in the Parent Object .. from which it was inherited)

The test program is shown below. It is not working in the current format.

Can the above be done elegantly? What would be a good way to execute this in Java.

Thanks for your help.

public class Inheritance {
    public static void main(String[] args) {
        Parent obParent = new Parent();
        Child obChild = new Child();
        //================
        obParent.setVariables();
        obParent.parentMethod();
        //================
        obChild.parentMethod();
        obChild.childMethod();
        obChild.childParentMethod();
        //================
        Child obnewChild = (Child) obParent; //Error
    }
}
public class Parent {
    String varA = "A";
    String varB = "B";
    public void parentMethod(){
        System.out.println(varA + " " + varB);
    }
    public void setVariables(){
        this.varA = "X";
        this.varB = "Y";
    }
}
public class Child extends Parent {
    String varC = "C";
    String varD = "D";
    public void childMethod(){
        System.out.println( varC + " " + varD);
    }
    public void childParentMethod(){
        System.out.println(super.varA + " " + super.varB + " " + this.varC + " " + this.varD);
    }
}

//********************************** // New Program

public class Inheritance {

public static void main(String[] args) {

    //================  

    Parent obParent = new Parent();
    obParent.setVariables();
    obParent.printParent();

    //================  

    Child obnewChild = new Child(obParent);
    obnewChild.printChildAndParent();

    obnewChild.setVariables();
    obnewChild.printChildAndParent();

    //================

    obParent.printParent();
    //================


  }

}

public class Parent {

    String varA = "A";
    String varB = "B";

    public void printParent(){

    System.out.println(varA + " " + varB);  
}

public void setVariables(){

    this.varA = "X";
    this.varB = "Y";    

  }

}

public class Child  {

    Parent obParent = new Parent();

    String varC = "C";
    String varD = "D";

    public Child(){

    }

    public Child(Parent obParent){

        System.out.println( obParent.varA + " " + obParent.varB);

        this.obParent = obParent;

    }

    public void setVariables(){

        this.obParent.varA = "XYZ";
        this.obParent.varB = "ABC";
    }

    public void printChild(){

        System.out.println( varC + " " + varD); 

    }

    public void printChildAndParent(){

        System.out.println(obParent.varA + " " + obParent.varB + " " + this.varC + " " + this.varD);

      }

}

Output:

X Y
X Y
X Y C D
XYZ ABC C D
XYZ ABC

In the seconds case.. I change the Child Object fields, the same changes show up when I print the Parent Object.

Answer 1

You get a ClassCastException because the object that you are trying to cast to a Child is not actually a child. However, you should be able to see that the child can manipulate the fields it inherits from the parent if you add some lines after

    //================
    obChild.parentMethod();
    obChild.childMethod();
    obChild.childParentMethod();
    //================

so that it reads

    //================
    obChild.parentMethod();
    obChild.childMethod();
    obChild.childParentMethod();
    obChild.setVariables();  // set the values of fields inherited from the parent
    obChild.childParentMethod(); // print and observe the values have changed
    //================

If you want to cast in this situation, you can cast -- or actually just assign -- the Child object to a Parent object.

If you want to avoid an error at the line where you are getting an error, you could also change the construction of obParent so that it actually is a Child object. If you had written

Parent obParent = new Child();

the line where you are getting an error would succeed.