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Javaのエラーいくつか

2021/05/23に公開約2,900字

以下のコードで起こるエラーをいくつか解説していきます。

  • 3 / 0
  • String s = null; s.length();
  • int[] a = new int[1]; a[1] = 1;
  • Map m = Map.of("a", 1); m.remove("a");
  • Integer.parseInt("123.33");

3 / 0

java.lang.ArithmeticException

public class Main {
    public static void main(String args[]) {
            // System.out.println("x");
            int a = 3 / 0;
    }
}

-> Exception in thread "main" java.lang.ArithmeticException: / by zero at Main.main(Main.java:4)

String s = null; s.length();

java.lang.NullPointerException

public class Main {
    public static void main(String args[]) {
            // System.out.println("x");
            String s = null; s.length();
    }
}

-> xception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "<local1>" is null at Main.main(Main.java:4)

int[] a = new int[1]; a[1] = 1;

java.lang.ArithmeticException

public class Main {
    public static void main(String args[]) {
            // System.out.println("x");
            // String s = null; s.length();
            int[] a =  new int[1];
            a[1] = 1;
    }
}

-> Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1 at Main.main(Main.java:6)

Map m = Map.of("a", 1); m.remove("a");

java.lang.UnsupportedOperationException

import java.util.Map;

public class Main {    
    public static void main(String args[]) {
            // System.out.println("x");
            // String s = null; s.length();
            // int[] a =  new int[1];
            // a[1] = 1;
            Map m = Map.of("a", 1); m.remove("a");
    }
}
Exception in thread "main" java.lang.UnsupportedOperationException
        at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:127)
        at java.base/java.util.ImmutableCollections$AbstractImmutableMap.remove(ImmutableCollections.java:910)
        at Main.main(Main.java:9)

Integer.parseInt("123.33");

java.lang.NumberFormatException

public class Main {    
    public static void main(String args[]) {
            // System.out.println("x");
            // String s = null; s.length();
            // int[] a =  new int[1];
            // a[1] = 1;
            // Map m = Map.of("a", 1); m.remove("a");
            Integer.parseInt("123.33");
    }
}
Exception in thread "main" java.lang.NumberFormatException: For input string: "123.33"
        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:68)
        at java.base/java.lang.Integer.parseInt(Integer.java:652)
        at java.base/java.lang.Integer.parseInt(Integer.java:770)
        at Main.main(Main.java:11)

おまけ: ConcurrentModificationException

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