Java Reserved Keywords — What They Are and Why You Can't Use Them

Direct Answer

Java keywords are reserved language words such as class, if, and return, so they cannot be ordinary identifiers. The exact list depends on the Java language version, and Java also has reserved literals (true, false, null) and restricted identifiers such as var, record, sealed, and yield with context-specific rules. Avoid quoting one timeless keyword count.

Code Example

public class KeywordDemo {
    private int count = 10;            // valid
    private boolean isActive = true;  // 'isActive' valid; 'true' is a literal

    void demo() {
        var orders = java.util.List.of("A", "B"); // 'var' as a keyword (type inference)
        // int class = 5;     // ERROR: keyword
        // const int x = 5;   // ERROR: 'const' reserved but unused — use 'final'
        System.out.println(count + orders.size());
    }
}

This compiles cleanly on Java 17. Uncomment either error line and the build breaks instantly — proof that the rule is enforced at compile time, long before the program ever runs. The var orders line shows the same word acting as a keyword, while the commented const line shows a word that is reserved yet does nothing.

Common Mistakes & Pitfalls

• Counting unlike categories together: keywords, contextual forms, restricted identifiers, and literals are not one category.
• Calling true, false, and null keywords: the JLS classifies them as literals.
• Using const for a Java constant: const is reserved but unused; use final for a variable that cannot be reassigned.
• Assuming a rule is timeless: newer Java versions add grammar forms.
• Treating contextual words as unrestricted everywhere: legality depends on their source position.