Das final Keyword

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Bisher haben wir nur mit Variablen hantiert, deren Werte sich immer wieder ändern lassen. Was aber, wenn du eine Konstante erstellen willst?

Mit dem final Keyword lassen sich Variablen als konstant deklarieren. Eine final Variable kann, nachdem sie einmal gesetzt wurde, nicht mehr verändert werden.

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final int x = 5;
x = 42; // error: cannot assign a value to final variable x
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Nicht nur lokale Variablen innerhalb einer Methode, sondern auch Objektvariablen können final sein:

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public class Example {
    private final int x;
    public Example(int x) {
        this.x = x; // x wird ein einziges Mal gesetzt und danach nie wieder verändert
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Achtung!

Wenn eine Referenzvariable final ist, bedeutet das nicht, dass die Attribute des dahinterliegenden Objekts konstant sind, sondern lediglich dass sich die Referenz selbst nicht mehr ändern kann:

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final User user = new User("Hans", "Müller", 42);
user.setFirstName("Peter"); /* Kein Fehler, weil nur Attribute
                               von user verändert werden */
user = new User("Peter", "Müller", 42); /* Fehler, weil Referenz
                                           selbst verändert wird */
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Gleiches Verhalten für Arrays:

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final int[] numbers = {1, 2, 3};
numbers[2] = 7; // Kein Fehler
numbers = {1, 2, 7}; // Fehler
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Auch Argumente von Methoden können final sein:

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public void f(final double x) {
    x = 2.0; // Fehler
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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`final` Klassen und Methoden

Das final Keyword hat noch weitere Bedeutungen, wenn es in einer Klassen- oder Methodensignatur verwendet wird:

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public final class A {
    // ...
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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In diesem Fall bedeutet final, dass keine Klasse von A erben kann. Ein Beispiel kennst du hierfür bereits: Die Klasse String ist final, sie lässt sich also nicht erweitern.

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public final void doSomething() {
    // ...
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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In diesem Fall bedeutet final, dass die Methode doSomething() nicht in Unterklassen überschrieben werden kann.

Welche der unten stehenden Aufrufe sind zulässig, und welche erzeugen einen Fehler? Gehe jeweils davon aus, dass alle Aufrufe zuvor mit ausgeführt werden, sofern diese selbst keinen Fehler erzeugt haben.

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Lösungsvorschlag

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/*
 * Aufruf 1: Zulässig. Die Objektvariable a ist zwar final, aber noch nicht
 *           initialisiert. Daher darf ihr ein einziges Mal im Konstruktor ein
 *           Wert zugewiesen werden. Dass der zugewiesene Wert null ist ist
 *           irrelevant, es könnte auch eine Referenz auf jedes beliebige
 *           tatsächliche Objekt der Klasse A sein.
 *
 * Aufruf 2: Fehler. Die konstante Objektvariable b hat bereits den Wert 3.
 *
 * Aufruf 3: Fehler. Die Referenz der Objektvariable c kann nicht verändert
 *           werden, weil die Variable final ist.
 *
 * Aufruf 4: Zulässig. Das Array c selbst ist zwar final, aber die Werte
 *           innerhalb des Arrays lassen sich trotzdem verändern.
 *
 * Aufruf 5: Fehler. Das Argument d ist final.
 *
 * Aufruf 6: Zulässig. Das Argument e ist nicht final.
 *
 * Aufruf 7: Zulässig. (Analog zu 4)
 *
 * Aufruf 8: Zulässig. f[0] ist ein Objekt der Klasse A, das ein finales
 *           Objektattribut c des typs int[] hat. Da nicht die Referenz
 *           selbst verändert wird, sondern nur der Inhalt des Arrays,
 *           entsteht kein Fehler.
 *
 * Aufruf 9: Fehler. f[0] ist ein Objekt der Klasse A, das ein finales
 *           Objektattribut b des typs int hat. Die primitive Variable
 *           kann nicht verändert werden, weil sie final ist.
 */
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Das final Keyword

Achtung!

final Klassen und Methoden

`final` Klassen und Methoden