Vererbung

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Keine Sorge, beim Thema Vererbung geht es weder um Todesfälle noch um Debatten über soziale Gerechtigkeit. Worum geht es dann?

Stell dir vor, dass ein Computerspiel programmiert werden soll. Schau dir dazu die folgenden beiden Klassen an:

Alien.java

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public class Alien {
    private float posX;
    private float posY;
    private int health;
    public Alien(float posX, float posY, int health) {
        this.posX = posX;
        this.posY = posY;
        this.health = health;
    }
    public void attack(Sheep s) {
        // ... Greife ein bestimmtes Schaf an
    }
    public float getPosX() {
        return posX;
    }
    public float getPosY() {
        return posY;
    }
    public int getHealth() {
        return health;
    }
    // Optional noch Setter
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Sheep.java

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public class Sheep {
    private float posX;
    private float posY;
    private int health;
    private Color color;
    public Alien(float posX, float posY, int health, Color color) {
        this.posX = posX;
        this.posY = posY;
        this.health = health;
        this.color = color;
    }
    public float getPosX() {
        return posX;
    }
    public float getPosY() {
        return posY;
    }
    public int getHealth() {
        return health;
    }
    public Color getColor() {
        return color;
    }
    // Optional noch Setter
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Du siehst, dass die Attribute und Methoden der beiden Klassen teilweise übereinstimmen. Das liegt daran, dass beide Klassen irgendeine Art von Kreatur repräsentieren und damit eine Ähnlichkeit aufweisen.

Wollte man nun beispielsweise bei allen Kreaturen ein Attribut oder eine Methode hinzufügen, verändern, oder entfernen, so müsste man dies in allen zugehörigen Klassen tun. Du weißt was jetzt kommt: "Das ist unübersichtlich und geht besser."

Das `extends` Keyword

Lass uns eine dritte Klasse Creature erstellen, die alle Attribute und Methoden enthält, die Alien und Sheep teilen:

Creature.java

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public class Creature {
    private float posX;
    private float posY;
    private int health;
    public Creature(float posX, float posY, int health) {
        this.posX = posX;
        this.posY = posY;
        this.health = health;
    }
    public float getPosX() {
        return posX;
    }
    public float getPosY() {
        return posY;
    }
    public int getHealth() {
        return health;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Nun lassen wir Alien und Sheep von Creature erben.

Wenn eine Klasse B von einer Klasse A erbt, dann übernimmt B implizit alle Attribute und Methoden von A. Man kann dann also beispielsweise eine Methode der Oberklasse A auch über eine Instanz der Unterklasse B aufrufen, obwohl diese Methode nirgendwo in B vorkommt.

Vererbung lässt sich in Java über das Keyword extends realisieren. Dieses wird einfach in die Signatur der Klasse B aufgenommen:

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public class B extends A { // B erbt alle Attribute und Methoden von A
    // ...
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Grafisch lässt sich dieser Zusammenhang nun in einem UML Klassendiagramm folgendermaßen darstellen:

(Getter & Setter kann man hier weglassen.)

Die Zusammenhänge sind nun folgende:

Jedes Alien ist eine Creature
Jedes Sheep ist eine Creature
Kein Alien ist ein Sheep
Kein Sheep ist ein Alien
Jede Creature ist entweder ein Alien oder ein Sheep oder keines der beiden

Du siehst, dass Alien und Sheep nur noch die Attribute bzw. Methoden enthalten, die nicht sowieso schon in Creature enthalten sind:

Alien.java

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public class Alien extends Creature { // Alien erbt von Creature
    public Alien(float posX, float posY, int health) {
        super(posX, posY, health);
    }
    public void attack(Sheep s) {
        // ... Greife ein bestimmtes Schaf an
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Sheep.java

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public class Sheep extends Creature { // Sheep erbt von Creature
    private Color color;
    public Sheep(float posX, float posY, int health, Color color) {
        super(posX, posY, health);
        this.color = color;
    }
    public Color getColor() {
        return color;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Dir ist sicher das super Keyword aufgefallen.

Mit super ist immer die Oberklasse gemeint. Beispielsweise ruft die Zeile

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super(a, b, c);
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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den Konstruktor der Klasse auf, von der die aktuelle Klasse erbt, wobei die Argumente a, b und c übergeben werden.

super kann auch verwendet werden, um explizit ein Attribut oder eine Methode aus der Oberklasse zu referenzieren, ähnlich wie this für die eigene Klasse funktioniert:

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super.methodeXyz(); // Ruft methodeXyz() in der Oberklasse auf
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Eine Sache noch

Der private Modifier verhindert ja, dass von außerhalb einer Klasse auf ein Attribut oder eine Methode zugegriffen werden kann. Das gilt auch, wenn die aufrufende Klasse von dieser erbt. Häufig will man aber von einer Unterklasse auf ein Attribut einer Oberklasse zugreifen, ohne einen Getter verwenden zu müssen. Hiefür gibt es den protected Modifier:

Mit dem protected Modifier gekennzeichnete Attribute und Methoden sind nicht nur von Klassen im selben Package erreichbar, sondern auch von allen eigenen Unterklassen:

A.java

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public class A {
    private int x;
    protected int y;
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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B.java

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public class B extends A {
    public void blah() {
        System.out.println(x); // Compilerfehler
        System.out.println(y); // Funktioniert
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Felder, die protected sind, werden im UML Klassendiagramm mit einem # markiert.

Damit kennst du nun alle Access Modifiers in Java:

Modifier	`private`	package (kein Modifier)	`protected`	`public`
Zugriff aus eigener Klasse	✔	✔	✔	✔
Zugriff aus eigenem Package (Außerhalb der eigenen Klasse)	❌	✔	✔	✔
Zugriff aus Unterklassen (Außerhalb des eigenen Packages)	❌	❌	✔	✔
Zugriff aus anderen Packages (Und keine Unterklasse)	❌	❌	❌	✔

Erinnerst du dich an die Klasse Rectangle aus dem Einführungsthema zu objektorientierter Programmierung? Wir werden die Idee von geometrischen Formen als Klassen aufgreifen und um einige weitere Formen erweitern.

Deine Aufgabe ist es, in deiner IDE ein Projekt mit der folgenden Klassenhierarchie zu erstellen:

Erstelle alle 5 Klassen mit korrekter Verwendung von extends
Erstelle alle Attribute. posX und posY sollen protected sein, alle anderen private
Ergänze passende Konstruktoren in allen Klassen (denke an super()!)
Füge alle Getter & Setter hinzu
Implementiere die Methode move() in der Klasse Shape, die die X- und Y-Position der Form um (dx | dy) verändert

Bitte überprüfe anschließend deine Lösung genau!

Häufige Fehlerquellen

Modifier vertauscht oder vergessen: private / protected / public
Argumente im Konstruktor falsch: Du benötigst zusätzlich zu den eigenen Attributen vorher noch die der Oberklasse!
super() Aufrufe vergessen oder falsch: Es müssen immer die Argumente übergeben werden, die der Konstruktor der Oberklasse benötigt
Bei Square width und height übergeben, obwohl ein Quadrat nur eine Seitenlänge hat
Bei move() die Position gesetzt, anstatt zu addieren
Getter / Setter vergessen

Lösungsvorschlag Shape.java

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public class Shape {
    protected double posX;
    protected double posY;
    public Shape(double posX, double posY) {
        this.posX = posX;
        this.posY = posY;
    }
    public void move(double dx, double dy) {
        posX += dx;
        posY += dy;
    }
    public double getPosX() {
        return posX;
    }
    public void setPosX(double posX) {
        this.posX = posX;
    }
    public double getPosY() {
        return posY;
    }
    public void setPosY(double posY) {
        this.posY = posY;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Rectangle.java

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public class Rectangle extends Shape {
    private double width;
    private double height;
    public Rectangle(double posX, double posY, double width, double height) {
        super(posX, posY);
        this.width = width;
        this.height = height;
    }
    public double getWidth() {
        return width;
    }
    public void setWidth(double width) {
        this.width = width;
    }
    public double getHeight() {
        return height;
    }
    public void setHeight(double height) {
        this.height = height;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Square.java

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public class Square extends Rectangle {
    public Square(double posX, double posY, double size) {
        super(posX, posY, size, size); // Quadrat ist genauso breit wie hoch
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Circle.java

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public class Circle extends Shape {
    private double radius;
    public Circle(double posX, double posY, double radius) {
        super(posX, posY);
        this.radius = radius;
    }
    public double getRadius() {
        return radius;
    }
    public void setRadius(double radius) {
        this.radius = radius;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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