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Java Syntax and Runtime Error Notes

A diagnostic guide to Java enum switch labels, constant-expression requirements, and comparator contract violations.

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an enum switch case label must be the unqualified name of an enumeration constant

private void Test(ColorType type){
    switch (type){
        case ColorType.GREEN:
            break;
        case ColorType.RED:
            break;
        case ColorType.ORANGE:
            break;
        default:
            break;
    }
}

Fix

A case label must contain only the enum constant, without the class qualifier. The switch expression already determines the enum type.

private void Test(ColorType type){
    switch (type){
        case GREEN:
            break;
        case RED:
            break;
        case ORANGE:
            break;
        default:
            break;
    }
}

constant expression required

private void Test(int type){
    switch (type){
        case ColorType.GREEN.getCode():
            break;
        case ColorType.RED.getCode():
            break;
        case ColorType.ORANGE.getCode():
            break;
        default:
            break;
    }
}

Fix

The result of an enum method is not a compile-time constant, while a case label requires one. Convert the integer to an enum value first, then switch on the enum.

Comparison method violates its general contract!

Since JDK 7, a Comparable implementation must satisfy reflexivity, symmetry, and transitivity. This error commonly occurs when symmetry is violated—for example, when the comparator never returns equality.

For compatibility with code written for JDK 6 or earlier, call:

System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");

Alternatively, pass -Djava.util.Arrays.useLegacyMergeSort=true to the JVM. JDK 7 and later use TimSort by default.

Originally published on SegmentFault.