Java中的构造函数和抽象类

Question

I have an abstract class Entity.java and a class which extends this, Magician.java . Whenever I create a new Magician("Ged", 300) ; and then call System.out.println() it always prints null(0) , and I'm not sure why when it should print Ged(300). Here is the relevant code:

Entity fields/ constructor:

public abstract class Entity {

    protected String name;
    protected int lifePoints = 0;

    public Entity(String name, int lifePoints) {
        assert (lifePoints >= 0);
        this.name = name;
        this.lifePoints = lifePoints;
    }

    ...

}

Magician fields/ Constructor/ toString:

public class Magician extends Entity implements SpellCaster {

    public Magician(String name, int lifePoints) {
        super(name, lifePoints);
        // TODO Auto-generated constructor stub
    }

    protected String name;
    protected int lifePoints;

    ...

    public String toString() {
        return name + "(" + lifePoints + ")";
    }

}

Main class:

public static void main(String[]args) {

    Magician m1=new Magician("Ged",300);
    System.out.println(m1.toString());
}

Thanks in advance.

Answer 1

You are shadowing the super class Entity 's instance fields name and lifePoints in the sub class Magician .

And those by by default set to null and 0 respectively. Remove those instance fields declaration from sub class Magician .

public class Magician extends Entity implements SpellCaster {

   public Magician(String name, int lifePoints) {
    super(name, lifePoints);
    // TODO Auto-generated constructor stub
   }


    ...

    public String toString(){
       return name + "(" + lifePoints + ")";
    }

}

Answer 2

You are not initializing

protected String name;
protected int lifePoints;

of your Magician class

public Magician(String name, int lifePoints) {
    super(name, lifePoints); // initializing fields of parent(Entity) not of child(Magician)
    // TODO Auto-generated constructor stub
}

Also, protected should be used in Parent class, not in the child. Please revisit the basics of Java.