Methoden

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Bisher hast du die komplette Programm-Logik immer so gut wie komplett in die main-Methode geschrieben. Damit ist jetzt Schluss! Ich präsentiere dir hiermit Methoden:

Eine Methode ist ein Programm-Stück, das (wie der Konstruktor) Parameter entgegen nimmt und irgendetwas in deinem Objekt macht. Anschließend wird ein Wert zurück gegeben (returned).

Methoden sind also quasi Daten-Fabriken, die gewisse Inputs bekommen und daraus neue Outputs erzeugen. Außerdem kann eine Methode auch den Zustand von Objekten verändern.

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int square(int x) {
    int result = x * x;
    return result;
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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In diesem Beispiel nimmt die Methode square einen Parameter x entgegen und erzeugt eine neue Zahl, die dem Quadrat von x entspricht. return ist hierbei das Keyword, das die Ausführung der Methode beendet und den Wert (hier result) zurückgibt. Nachdem return ausgeführt wurde, können keine weiteren Statements innerhalb der Methode abgearbeitet werden. Das int ganz oben links in Zeile 1 ist der Typ des Rückgabe-Werts. Diese Methode produziert also ein int.

Um eine Methode aufzurufen, verwendet man folgende Schreibweise mit in Klammern übergebenen Parametern:

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int threeSquared = square(3); // 9
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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square ähnelt sehr einer mathematischen Funktion, die einen Wert entgegen nimmt und einen neuen erzeugt, ohne dabei Nebeneffekte auf Zustände von Objekten zu haben. Deshalb werden solche Methoden auch oft Funktion genannt.

Wie beschrieben können Methoden aber auch den Zustand eines Objekts verändern:

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class Rectangle {
    double posX;
    double posY;
    double width;
    double height;
    Rectangle(double posX, double posY, double width, double height) {
        this.posX = posX;
        this.posY = posY;
        this.width = width;
        this.height = height;
    }
    double increaseWidth(double amount) {
        width += amount; // Auf this.width wird amount addiert
        return width; // Die neue Breite des Rechtecks wird zurückgegeben
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Die Methode increaseWidth verändert die Breite des Rechtecks um den Wert amount und gibt die neue Größe zurück.

Methoden können auch mehrere Parameter haben:

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double increaseSize(double amountX, double amountY) {
    width += amountX;
    height += amountY;
    return width * height; // Diese Methode gibt den neuen Flächeninhalt zurück
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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...Oder keine Parameter haben:

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boolean isVertical() {
    return height > width;
    // Gibt genau dann true zurück, falls das Rechteck höher ist als breit
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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...Oder keinen Rückgabewert haben. In diesem Fall wird der return-Type als void (engl. "Leere", "Nichts") angegeben:

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void resetPosition() { // Kein Rückgabewert, keine Parameter
    posX = 0.0;
    posY = 0.0;
    // Rechteck wurde zum Ursprung verschoben
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Hiermit ist also ein weiteres Rätsel der main-Methode gelüftet: void steht dafür, dass main lediglich eine Prozedur ist, die zwar Befehle ausführt, aber keinen Rückgabewert erzeugt.

Während eine Methode 0 bis beliebig viele Parameter haben kann, gibt es immer entweder keinen oder genau einen Rückgabewert. Es ist in Java unmöglich, mehrere Werte zurück zu geben. (Dieses vermeintliche Problem lässt sich aber auf unterschiedliche Arten umgehen.)

Methoden lassen sich in einem UML Klassendiagramm im unteren Block auflisten:

Implementiere folgende Programmteile basierend auf der vorherigen Aufgabe:

Ein Konstruktor, der die drei Parameter id, firstName, und lastName entgegennimmt und die Objektattribute entsprechend setzt. Das vierte Attribut (friends) soll immer mit einem neuen leeren Array initialisiert werden.
Eine Methode String fullName(), die den Vor- und Nachnamen des Users mit Leerzeichen getrennt zurück gibt.
Eine Methode boolean hasMoreFriendsThan(User user), die genau dann true zurück gibt, wenn das eigene friends Array größer ist, als user.friends
Eine Methode boolean hasFriend(User user), die genau dann true zurück gibt, wenn user in friends enthalten ist
Eine Methode addFriend(User user), die keinen Rückgabewert hat, sondern friends durch ein neues Array ersetzt, das alle Elemente aus friends enthält, und zusätzlich als letztes Element user. addFriend() soll also das existierende friends Array um einen weiteren User erweitern.

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Teste anschließend deine Eingaben, indem du über die addFriend() Methode den einzelnen User Objekten Freunde hinzufügst und dann die restlichen Methoden mit beispielhaften Eingaben ausführst.

Häufige Fehlerquellen

error: ';' expected bzw. cannot find symbol: Klammern bei Funktion vergessen
NullPointerException: Im Konstruktor friends nicht initialisiert
error: not a statement: return vergesssen
return-Typ falsch
Auf falsches User Objekt bezogen: Achte immer darauf, ob du gerade den User der Klasse meinst (Objektattribut), oder den übergebenen User (lokale Variable).
for-Schleife(n) falsch
Indizes bzw. größe von Arrays falsch

Lösungsvorschlag Konstruktor

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// ...
    User(int id, String firstName, String lastName) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        // Initialisiere friends mit einem leeren Array
        friends = new User[0];
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag fullName()

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// ...
    String fullName() {
        return firstName + " " + lastName;
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag hasMoreFriendsThan()

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// ...
    boolean hasMoreFriendsThan(User user) {
        // true, falls die Bedingung wahr ist, sonst false
        return friends.length > user.friends.length;
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag hasFriend()

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// ...
    boolean hasFriend(User user) {
        // Iteriere durch alle Elemente in friends
        for (int i = 0; i < friends.length; i++) {
            if (friends[i] == user) {
                /*
                 * Falls das aktuelle Element der gesuchte User ist
                 * wird die Methode abgebrochen und true zurückgegeben
                 */
                return true;
            }
        }
        // Falls user nicht gefunden wurde wird false zurückgegeben
        return false;
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag addFriend()

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// ...
    void addFriend(User user) {
        // Erstelle neues Array mit mehr Platz als in friends
        User[] friendsCopy = new User[friends.length + 1];
        // Kopiere alle Elemente aus friends in das neue Array
        for (int i = 0; i < friends.length; i++) {
            friendsCopy[i] = friends[i];
        }
        // Kopiere user an den letzten Index
        friendsCopy[friendsCopy.length - 1] = user;
        // Ersetze das Objektattribut mit dem neuen Array
        friends = friendsCopy;
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Gesamt

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public class User {
    int id;
    String firstName;
    String lastName;
    User[] friends;
    User(int id, String firstName, String lastName) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        friends = new User[0];
    }
    String fullName() {
        return firstName + " " + lastName;
    }
    boolean hasMoreFriendsThan(User user) {
        return friends.length > user.friends.length;
    }
    boolean hasFriend(User user) {
        for (int i = 0; i < friends.length; i++) {
            if (friends[i] == user) {
                return true;
            }
        }
        return false;
    }
    void addFriend(User user) {
        User[] friendsCopy = new User[friends.length + 1];
        for (int i = 0; i < friends.length; i++) {
            friendsCopy[i] = friends[i];
        }
        friendsCopy[friendsCopy.length - 1] = user;
        friends = friendsCopy;
    }
    public static void main(String[] args) {
        User tedShmosby = new User(0, "Ted", "Shmosby");
        User barnacleStinson = new User(1, "Barnacle", "Stinson");
        User stuartBowers = new User(2, "Stuart", "Bowers");
        User ranjitSingh = new User(3, "Ranjit", "Singh");
        tedShmosby.addFriend(barnacleStinson);
        tedShmosby.addFriend(stuartBowers);
        tedShmosby.addFriend(ranjitSingh);
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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