Instance Hiding
Declaring an instance field with the same name as a visible superclass field is called hiding.
- This means you have a field in a superclass that the subclass can see, and the subclass declares a field with the same name.
Consider this example, where the Wizard class hides the GameCharacter's hitPoints field.
public class GameCharacter {
protected int hitPoints;
public GameCharacter(int hp) {
hitPoints = hp;
}
public void takeDamage(int damage) {
hitPoints = hitPoints - damage;
}
public int getHitPoints() {
return hitPoints;
}
// ...
}
public class Wizard extends GameCharacter {
protected int hitPoints; // hides GameCharacter hitPoints
public Wizard(int hp) {
super(hp);
}
// ...
public void healSelfWithMagic(int amount) {
hitPoints += amount; // accessing this class's field
}
}
This leads to a bug in the program.
* The Wizard tries to heal, but is still eliminated.
Wizard light = new Wizard(100);
// ...
light.takeDamage(50); // hitPoints = 50
light.healSelfWithMagic(10); // attempt to heal
light.takeDamage(50); // hitPoints = 0
if(light.getHitPoints() <= 0) {
System.out.println("Against the power of Mordor, there can be no victory.");
}
The Wizard light has two hitPoints fields, which leads to an unexpected end.
Try it: com.example.polymorphism.examples.game.GameApp
Field Shadowing¶
Field shadowing is where a local variable name in a method has the same name as a field.
* Our setters usually have field shadowing.
Shadowing vs. Hiding¶
- Shadowing happens in a single class.
- Hiding happens in a class hierarchy.
Practice Exercise¶
Shadowing is common, and can be handled with
this..Field hiding should not happen. Do not design your classes to hide fields.
Resource: https://docs.oracle.com/javase/tutorial/java/IandI/hidevariables.html