Default Methods in Java Interfaces

Java 8 needed to add forEach to every collection without breaking millions of classes nobody could edit.

A default method is a method in an interface with a body, marked default. Classes that implement the interface inherit it and may override it. It exists so an interface can gain a method without breaking the classes that already implement it.

Java 8 wanted to add forEach to Iterable, and through it to every List. Adding a method to an interface used to break every class implementing it, and there were millions of them in code nobody at Oracle could edit. A default method arrives with a body, so existing classes keep compiling:

java

interface Archivable {
    String archiveKey();
    LocalDate archivedOn();

    default boolean isExpired() {
        return archivedOn().isBefore(LocalDate.now().minusYears(7));
    }
}

The default body may only use the interface's own methods, because an interface still has no fields to read. That is the line between a default method and an abstract class, a partly written class that can hold fields, and it has not moved.

The four kinds of method an interface may declare
Kind	Has a body	Overridable	Since
Abstract	No	Must be implemented	Java 1
`default`	Yes	Yes	Java 8
`static`	Yes	No: called on the interface name	Java 8
`private`	Yes	No: a helper for the two above	Java 9

So an interface can now carry behaviour without carrying data, which is what lets it grow without breaking its implementations. Use it for that, a default most classes want, rather than as a way to avoid writing an abstract class.

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