Konstruktoren

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Wie dir vielleicht schon aufgefallen ist, werden beim Erstellen von Objekten sehr häufig direkt im Anschluss Attribute dieses Objekts gesetzt. Um diesen Teil etwas zu verschönern gibt es einen besonderen Code-Block: Den Konstruktor

Ein Konstruktor nimmt Werte (Parameter / Argumente) entgegen, die direkt für die Instanziierung des Objekts verwendet werden können. In unserem Rechteck-Beispiel könnte das so aussehen:

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public class Rectangle {
    double posX;
    double posY;
    double width;
    double height;
    // Konstruktor
    Rectangle(double initialPosX, double initialPosY, /* Übergebene Parameter */
                double initialWidth, double initialHeight) {
        // Den Objektvariablen die Werte der lokalen Variablen zuweisen
        posX = initialPosX;
        posY = initialPosY;
        width = initialWidth;
        height = initialHeight;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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In diesem Beispiel werden dem Konstruktor also alle vier Attribute als lokale Variablen übergeben und anschließend im Objekt selbst gespeichert.

Wie du siehst, müssen hierbei die Benennungen der Parameter angepasst werden, weil es sonst Namenskonflikte gibt. Es gibt aber eine Möglichkeit, trotzdem identische Namen zu verwenden: Das this-Keyword:

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class Rectangle {
    double posX;
    double posY;
    double width;
    double height;
    // Konstruktor
    Rectangle(double posX, double posY, double width, double height) {
        this.posX = posX;
        this.posY = posY;
        this.width = width;
        this.height = height;
        /*     ^        ^
         *     |      Bezieht sich auf die übergebene lokale Variable height
         *     |
         *   Bezieht sich auf das Objektattribut height
         */
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Das this-Keyword ermöglicht es uns zu betonen, dass wir etwas aus dem eigenen Objekt meinen.

Der Compiler weiß also durch Verwendung von this, dass in Zeile 9 die in Zeile 2 deklarierte Objektvariable posX gemeint ist, und nicht der gleichnamige Parameter aus Zeile 8. Gleichzeitig ist auch klar, dass ohne Verwendung von this eben doch die übergebene lokale Variable gemeint ist, und nicht die Objektvariable.

Wenn wir nun ein neues Rectangle Objekt instanziieren, können wir direkt die Eigenschaften übergeben:

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// Erstelle ein Rechteck an Position (1|2) mit Breite 16 und Höhe 9
Rectangle rect = new Rectangle(1.0, 2.0, 16.0, 9.0);
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Erstelle einen Konstruktor für die in der letzten Aufgabe beschriebenen Klasse User. Dieser soll wie im obigen Beispiel lediglich alle Attribute der Klasse als Parameter entgegennehmen und dann die Objektattribute entsprechend setzen.

Erstelle anschließend in der main-Methode die folgenden User-Objekte:

Ted Shmosby, befreundet mit Barnacle, Stuart und Ranjit
Barnacle Stinson, befreundet mit Ted und Ranjit
Stuart Bowers, befreundet mit Ted
Ranjit Singh, befreundet mit Ted und Barnacle

Die ids kannst du frei wählen.

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Speichere anschließend die vier User in einem Array

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User[] users = {tedShmosby, barnacleStinson, stuartBowers, ranjitSingh};
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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und gib aus diesem Array die Namen aller User aus, die mehr als einen Freund haben.

Häufige Fehlerquellen

this falsch verwendet
Array-Größen falsch
error: cannot find symbol: Du hast eine Variable verwendet, bevor sie deklariert wurde. Erstelle zuerst die leeren Arrays, und befülle sie nachdem das User-Objekt instanziiert wurde.

Lösungsvorschlag Teilaufgabe a)

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public class User {
    int id;
    String firstName;
    String lastName;
    User[] friends;
    User(int id, String firstName, String lastName, User[] friends) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        this.friends = friends;
    }
    public static void main(String[] args) {
        User tedShmosby = new User(0, "Ted", "Shmosby", new User[3]);
        User barnacleStinson = new User(1, "Barnacle", "Stinson", new User[2]);
        User stuartBowers = new User(2, "Stuart", "Bowers", new User[1]);
        User ranjitSingh = new User(3, "Ranjit", "Singh", new User[2]);
        tedShmosby.friends[0] = barnacleStinson;
        tedShmosby.friends[1] = stuartBowers;
        tedShmosby.friends[2] = ranjitSingh;
        barnacleStinson.friends[0] = tedShmosby;
        barnacleStinson.friends[1] = ranjitSingh;
        stuartBowers.friends[0] = tedShmosby;
        ranjitSingh.friends[0] = tedShmosby;
        ranjitSingh.friends[1] = barnacleStinson;
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Teilaufgabe b)

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public class User {
    int id;
    String firstName;
    String lastName;
    User[] friends;
    User(int id, String firstName, String lastName, User[] friends) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        this.friends = friends;
    }
    public static void main(String[] args) {
        User tedShmosby = new User(0, "Ted", "Shmosby", new User[3]);
        User barnacleStinson = new User(1, "Barnacle", "Stinson", new User[2]);
        User stuartBowers = new User(2, "Stuart", "Bowers", new User[1]);
        User ranjitSingh = new User(3, "Ranjit", "Singh", new User[2]);
        tedShmosby.friends[0] = barnacleStinson;
        tedShmosby.friends[1] = stuartBowers;
        tedShmosby.friends[2] = ranjitSingh;
        barnacleStinson.friends[0] = tedShmosby;
        barnacleStinson.friends[1] = ranjitSingh;
        stuartBowers.friends[0] = tedShmosby;
        ranjitSingh.friends[0] = tedShmosby;
        ranjitSingh.friends[1] = barnacleStinson;
        User[] users = {tedShmosby, barnacleStinson, stuartBowers, ranjitSingh};
        for (int i = 0; i < users.length; i++) {
            User user = users[i];
            if (user.friends.length > 1) {
                System.out.println(user.firstName + " " + user.lastName
                    + " hat mehr als einen Freund.");
            }
        }
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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