Private interface methods

Code Comparison

interface Logger {
    default void logInfo(String msg) {
        System.out.println(
            "[INFO] " + timestamp() + msg);
    }
    default void logWarn(String msg) {
        System.out.println(
            "[WARN] " + timestamp() + msg);
    }
}

interface Logger {
    private String format(String lvl, String msg) {
        return "[" + lvl + "] " + timestamp() + msg;
    }
    default void logInfo(String msg) {
        IO.println(format("INFO", msg));
    }
    default void logWarn(String msg) {
        IO.println(format("WARN", msg));
    }
}

Why the modern way wins

🧩

Code reuse

Share logic between default methods without duplication.

🔐

Encapsulation

Implementation details stay hidden from implementing classes.

🧹

DRY interfaces

No more copy-paste between default methods.

Old Approach

Duplicated Logic

Modern Approach

Private Methods

JDK Support

Private interface methods

Available

Widely available since JDK 9 (Sept 2017)

How it works

Java 9 allows private methods in interfaces, enabling you to share code between default methods without exposing implementation details to implementing classes.