Java extends example

Question

i have a java beginner question: Parent.print() prints "hallo" in the console, but also Child.print() prints "hallo". I thought it has to print "child". How can i solve this?

public class Parent {

    private String output = "hallo";

    public void print() {
        System.out.println(output);
    }

}

public class Child extends Parent {

   private String output = "child";

}

Answer 1

Currently you've got two separate variables, and the code in Parent only knows about Parent.output. You need to set the value of Parent.output to "child". For example:

public class Parent {

  private String output = "hallo";

  protected void setOutput(String output) {
    this.output = output;
  }

  public void print() {
    System.out.println(output );
  }
}

public class Child extends Parent {
  public Child() {
    setOutput("child");
  }
}

An alternative approach would be to give the Parent class a constructor which took the desired output:

public class Parent {
  private String output;

  public Parent(String output) {
    this.output = output;
  }

  public Parent() {
    this("hallo");
  }

  public void print() {
    System.out.println(output );
  }
}

public class Child extends Parent {
  public Child() {
    super("child");
  }
}

It really depends on what you want to do.

Answer 2

Child doesn't have access to Parent's output instance variable because it is private. What you need to do is make it protected and in the constructor of Child set output to "child".

In other words, the two output variables are different.

You could also do this if you change output to be protected in Parent:

public void print(){
    output = "child"
    super.print();
}