default, static & private interface methods

default methods — API evolution without breaking changes

Java 8 introduced default methods: interface methods with a full body, marked default. A class that implements the interface inherits the default implementation and may override it:

public interface Logger {
    void log(String msg);              // abstract — must implement

    default void logInfo(String msg) { // default — inherited or overridable
        log("[INFO] " + msg);
    }
}

public class ConsoleLogger implements Logger {
    @Override
    public void log(String msg) { System.out.println(msg); }
    // logInfo is inherited — no need to implement it
}

Why they exist: before Java 8, adding a method to a published interface broke every existing implementation. Default methods let library authors add new methods without forcing every user to update.

static interface methods

Declared with static, called via the interface name — not inherited by implementing classes:

public interface MathOp {
    static int clamp(int v, int lo, int hi) {
        return Math.max(lo, Math.min(hi, v));
    }
}

int x = MathOp.clamp(150, 0, 100);  // 100

private interface methods (Java 9+)

Private methods (with or without static) allow code to be shared between default methods without exposing it as part of the interface's API:

public interface Validator {
    default boolean isValid(String s) { return !isEmpty(s); }
    default boolean isBlank(String s)  { return isEmpty(s); }
    private boolean isEmpty(String s)  { return s == null || s.isEmpty(); }
}