Interfaces

阅读Java的官方Doc，总结如下。

What is Interface

An interface is a reference type, similar to a class, that can contain only

1. constants (implicitly public, static, final)
2. method signatures (no method body, no braces)
3. default methods (has method body)
4. static methods (has method body)
5. nested types

Keypoints

Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.

A class can implement multi interfaces.

An interface can extend multi interface.

All abstract, default, and static methods in an interface are implicitly public, so you can omit the public modifier.

Default Methods

A pretty clear application scenario here.

Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.

When you extend an interface that contains a default method, you can do the following:

1. Not mention the default method at all, which lets your extended interface inherit the default method.
2. Redeclare the default method, which makes it abstract.
3. Redefine the default method, which overrides it.

Special Case

Look at following codes. ContentVisitor is an interface.

private final ContentVisitor visitor = new ContentVisitor() {
    public void onStartDocument() {
        throw new IllegalStateException();
    }

Did we create an instance of interface?

No. We can't instantiate an interface. Actually, what these codes do is to define an anonymous class that implements the interface, and instantiate that class.

It's the same with following codes.

private final class AnonymousContentVisitor implements ContentVisitor {
    public void onStartDocument() {
        throw new IllegalStateException();
    }
}

private final ContentVisitor visitor = new AnonymousContentVisitor();