Zeichen: Das char-Keyword

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Du kennst bereits die Klasse String. Mit Strings lassen sich Zeichenketten speichern. Aber wie werden eigentlich die Zeichen in einem String gepeichert? Dafür gibt es das char-Keyword.

char Steht für "Character" (engl. für "Zeichen") und ist ebenso wie int, boolean, oder float ein Basistyp. chars lassen sich folgendermaßen verwenden:

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char lowerCaseA = 'a';
char upperCaseA = 'A';
char hashtag = '#';
char quotes = '"';
System.out.println("Ein kleines a: " + lowerCaseA);
 
 
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Achte dabei darauf, dass ein char im Gegensatz zu einem String immer von einzelnen Gänsefüßchen (') umgeben ist.

Intern werden chars ähnlich wie int als Ganzzahlen gespeichert. Daher können sie auch mit int verrechnet werden:

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char c = 'A';
System.out.println(c); // A
c += 2;
System.out.println(c); // C
c--;
System.out.println(c); // B
 
 
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Die Reihenfolge der Zeichen wird in einer Encoding Table definiert, die angibt, welches Zeichen welcher Ganzzahl entspricht. Wenn du wissen möchtest, welchen Code ein bestimmtes char hat, kannst du nach "ASCII table" im Internet suchen.

Vorsicht bei der Konkatenation von chars! Die Zeile

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System.out.println('a' + 'b');
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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produziert nicht ab auf der Konsole, sondern 195. Hier werden die Zeichen nämlich nicht zu einem String konkateniert, sondern der Ganzzahlwert von 'a' (97) mit dem Ganzzahlwert von 'b' (98) addiert und zu einem int umgewandelt.

Wenn du chars konkatenieren möchtest, musst du das linke zuerst zu einem String umwandeln, beispielsweise so:

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System.out.println("Ausgabe: " + 'a' + 'b');
 
 
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In diesem Fall wird der String von links nach rechts mit den chars zusammengefügt.

Übersicht Bisherige Basistypen

Basistyp	Art	Beispiele
`boolean`	Wahrheitswert	`false`, `true`
`int`	Ganzzahl	`-10`, `0`, `7`, `42`, `9999`
`char`	Zeichen	`'A'`, `'a'`, `'?'`, `'#'`, `'"'`
`double`	Fließkommazahl	`-1.125`, `0.0`, `3.14159`, `Double.NaN`, `Double.NEGATIVE_INFINITY`

In dieser Aufgabe ist eine Klasse CharacterTable gegeben, die in ihrem Attribut characters alle Zeichen mit einem Code zwischen 0 und einem festgelegten maximum speichert.

a) Implementiere den Konstruktor, der das characters Array mit genau so vielen Plätzen initialisiert, wie es Zeichen mit einem Code zwischen 0 (inklusiv) und maximum (exklusiv) gibt. Die Werte dürfen vorerst alle den char Standardwert behalten.

b) Implementiere die Methode init(), die das characters Array mit Zeichen befüllt. Jeder Eintrag soll genau das Zeichen enthalten, dessen Code dem Index im Array entspricht.

c) Implementiere zuletzt die Methode printRange(), die alle Zeichen vom Index from (inklusiv) bis zum index to (exklusiv) auf der Konsole ausgibt. Dabei soll der Index vorangestellt werden, sodass eine Ausgabe so aussehen könnte:

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table.printRange(65, 71);
 
 
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65: A
66: B
67: C
68: D
69: E
70: F

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Du siehst, dass die alphabetische Sortierung in der ASCII Tabelle beibehalten wurde. Wenn du mit dem Ausgabeintervall etwas herumspielst, kannst du dir auch anzeigen lassen, welche Sonderzeichen welchem Code entsprechen. Einige dieser Zeichen sind jedoch nicht anzeigbare Steuerzeichen, die durch ein Platzhalter-Zeichen auf der Konsole ausgegeben werden.

Häufige Fehlerquellen

Array zu klein: characters soll genau maximum Einträge haben
In init() int als Laufvariable verwendet. Verwende char, um mit Zeichen zu hantieren
to als inklusiven Index verwendet: Es sollen bei printRange() nur Zahlen bis to - 1 ausgegeben werden

Lösungsvorschlag Teilaufgabe a)

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// ...
    public CharacterTable(int maximum) {
        characters = new char[maximum];
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Teilaufgabe b)

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// ...
    public void init() {
        // Iteriere durch alle Zeichen von 0 bis (characters.length - 1)
        for (char c = 0; c < characters.length; c++) {
            // Setze das Zeichen an Stelle c auf den Wert von c
            characters[c] = c;
        }
    }
// ...
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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Lösungsvorschlag Teilaufgabe c)

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public class CharacterTable {
    private char[] characters;
    public CharacterTable(int maximum) {
        characters = new char[maximum];
    }
    public void init() {
        for (char c = 0; c < characters.length; c++) {
            characters[c] = c;
        }
    }
    public void printRange(int from, int to) {
        // Iteriere durch alle Elemente von from bis (to - 1)
        for (int i = from; i < to; i++) {
            System.out.println(i + ": " + characters[i]);
        }
    }
    public static void main(String[] args) {
        CharacterTable table = new CharacterTable(256);
        table.init();
        table.printRange(60, 100);
    }
}
 
 
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
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