Private interface methods

Definition

Since Java 9 an interface can have private methods. They let several default methods share code inside the interface without exposing that code to anyone else.

Example code

Two default methods sharing one private helper

interface Logger {
    default void info(String message)  { System.out.println(format("INFO", message)); }
    default void error(String message) { System.out.println(format("ERROR", message)); }

    private String format(String level, String message) {      // hidden helper, Java 9
        return "[" + level + "] " + message;
    }
}

class Demo {
    public static void main(String[] args) {
        Logger logger = new Logger() { };
        logger.info("started");
        logger.error("something failed");
    }
}

// Output:
// [INFO] started
// [ERROR] something failed