What are default and static methods in Java interfaces?

Java 8 added default and static methods to interfaces, fundamentally changing the relationship between interfaces and abstract classes.

interface Validator {
    // Abstract method — implementors must provide
    boolean isValid(T value);

    // Default method — provided implementation, can be overridden
    default Validator and(Validator other) {
        return value -> this.isValid(value) && other.isValid(value);
    }

    default Validator or(Validator other) {
        return value -> this.isValid(value) || other.isValid(value);
    }

    default Validator negate() {
        return value -> !this.isValid(value);
    }

    // Static factory method — belongs to the interface, not instances
    static  Validator of(Validator validator) {
        return Objects.requireNonNull(validator);
    }
}

// Usage
Validator notBlank  = s -> !s.isBlank();
Validator notTooLong = s -> s.length() <= 100;
Validator combined  = notBlank.and(notTooLong);

System.out.println(combined.isValid("hello")); // true
System.out.println(combined.isValid(""));      // false

// Diamond problem resolution
interface A { default String hello() { return "A"; } }
interface B extends A { default String hello() { return "B"; } }
class C implements A, B {
    // Must override — most-specific interface wins (B),
    // but compiler forces explicit resolution
    @Override public String hello() { return B.super.hello(); }
}

Default methods enable interface evolution: adding a new method to an interface in a library no longer breaks all existing implementors, because the default provides backward compatibility. This is how Java 8 added methods like Collection.forEach(), List.sort(), and Map.getOrDefault() without breaking existing code.

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