Arrays

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Bisher haben wir in Variablen immer nur einzelne Werte gespeichert. Mit Hilfe von Arrays lassen sich eine vorher festgelegte Anzahl an Werten in nur einer einzigen Referenzvariable speichern.

Arrays sind Felder, in denen man (nahezu) beliebig viele Werte speichern kann.

Arrays lassen sich folgendermaßen erstellen:

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int[] array; // Deklaration der Referenzvariable
array = new int[100]; // Erstellen und Zuweisen des tatsächlichen Array Objekts
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Oder kurz:

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int[] array = new int[100];
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Hierbei kann int natürlich auch ein anderer Typ sein.

Die eckigen Klammern [] stehen dafür, dass es sich nicht nur um ein int handelt, sondern um ein ganzes Array.
array ist wie bei anderen Variablen einfach nur ein frei wählbarer Name
Das new-Keyword sorgt für die tatsächliche Erzeugung des Array Objekts, bei der der Speicherplatz für die Elemente reserviert (alloziert) wird. Mehr dazu später bei objektorientierter Programmierung.
Die Zahl zwischen den eckigen Klammern auf der rechten Seite ist die Anzahl der Elemente im Array. In diesem Fall würde also Platz für 100 Elemente erstellt werden.

Bei Basistypen wird das Array mit Standardwerten befüllt.

Basistyp	Standardwert	Beispiel im Array
`boolean`	`false`	`new boolean[12]`: 12 Einträge mit Inhalt `false`
`int`	`0`	`new int[99]`: 99 Einträge mit Inhalt `0`
`double`	`0.0`	`new double[7776]`: 7776 Einträge mit Inhalt `0.0`

Eine Möglichkeit, Arrays direkt mit Initialwerten zu erstellen, ist folgende Schreibweise mit geschweiften Klammern:

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int[] squares = {1, 4, 9, 16, 25};
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Die Größe eines Arrays lässt sich über array.length ausgeben:

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int l = squares.length; // 5
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Um Werte im Array an einer bestimmten Position (Index) auszulesen oder zu verändern, werden ebenfalls eckige Klammern verwendet:

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int[] numbers = new int[10]; // 10 Werte, die alle 0 sind
int n = numbers[3]; // Wert an Index 3 auslesen
System.out.println(n); // 0
numbers[3] = 7; // Wert an Index 3 auf 7 setzen
System.out.println(numbers[3]); // 7
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Achtung! In Java (und so ziemlich jeder anderen seriösen Programmiersprache auch) beginnt man das Zählen bei 0 und nicht bei 1. Das bedeutet, dass das erste Element im Array an Index 0 steht:

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double[] values = {0.5, -0.2, 1.0, 0.3};
double firstValue = values[0];  //  0.5
double secondValue = values[1]; // -0.2
double thirdValue = values[2];  //  1.0
double fourthValue = values[3]; //  0.3
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Das bedeutet auch, dass sich das letzte Element eines Arrays immer über array[array.length - 1] ausgeben lässt, nicht über array[array.length]! Falsche Zugriffe auf ein Array werfen eine java.lang.ArrayIndexOutOfBoundsException.

Sobald ein Array erstellt ist, lässt sich dessen Größe nicht nachträglich ändern. Falls man genau das aber möchte, so muss man ein neues Array mit entsprechender Größe erstellen, und alle Werte von dem alten Array kopieren.

Erstelle ein int-Array und befülle es mit den Zahlen 1 bis 2000. Gib anschließend die ersten 5 und die letzten 5 Werte auf der Konsole aus.

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Häufige Fehlerquellen

Array-Größe falsch gesetzt: Es gibt genau 2000 Elemente
for-Schleife hat falsche Grenzen: Du musst von Index 0 bis index 1999 iterieren
Werte falsch gesetzt: Die kleinste Zahl im Array soll 1 sein, nicht 0
Beim Auslesen falsche Indizes: Du sollst alle Zahlen von Index 0 bis Index 4, und alle Zahlen von Index 1995 bis Index 1999 ausgeben

Lösungsvorschlag

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public class ArrayExample {
    public static void main(String[] args) {
        // Von 1 bis 2000 gibt es genau 2000 Zahlen
        int[] numbers = new int[2000];
        // Iteriere durch alle Indizes von 0 bis 1999
        for (int i = 0; i < numbers.length; i++) {
            /*
             * Der Inhalt muss immer 1 größer sein als der Index,
             * weil die kleinste Zahl 1 sein soll und nicht 0
             */
            numbers[i] = i + 1;
        }
        // Iteriere durch die ersten 5 Indizes (0 bis 4)
        for (int i = 0; i < 5; i++) {
            System.out.println(numbers[i]);
        }
        System.out.println("...");
        // Iteriere von Index 1995 bis Index 1999
        for (int i = numbers.length - 5; i < numbers.length; i++) {
            System.out.println(numbers[i]);
        }
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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