java 内部类

一个类内部包含另一个类，被包含的类则称为内部类。例如汽车类中包含发动机类，发动机类可称为内部类。内部类可分为：成员内部类和匿名内部类

成员内部类

/**
 * 汽车类中包含发动机内部类
 */
public class Car {

    private Engine engine;

    public Engine getEngine() {
        return engine;
    }

    public void setEngine(Engine engine) {
        this.engine = engine;
    }

    public void carStart() {
        engine.start();
    }

    public void carStop() {
        engine.stop();
    }

    //成员内部类
    public class Engine {
        private String name;

        private String manufacturer;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getManufacturer() {
            return manufacturer;
        }

        public void setManufacturer(String manufacturer) {
            this.manufacturer = manufacturer;
        }

        private void start() {
            System.out.println("发动机发动");
        }

        private void stop() {
            System.out.println("发动机停止");
        }
    }
}

public class CarTest {
    public static void main(String[] args) {

        Car car = new Car();
        //外部类名称.内部类名称 对象名= new 外部类名称().内部类名称()
        Car.Engine engine = new Car().new Engine();
        engine.setName("EA888");
        engine.setManufacturer("大众");

        car.setEngine(engine);
        car.carStart();
        car.carStop();

    }
}

局部内部类

只有当前所属的方法才能使用，在方法外部不能使用

public class LocalCar {

    public void start() {

        //在成员方法中定义局部内部类
        class Engine {
            private String name;

            public String getName() {
                return name;
            }

            public void setName(String name) {
                this.name = name;
            }

            public void start() {
                System.out.println(this.name + "发动机启动");
            }
        }

        Engine engine = new Engine();
        engine.setName("EA888");
        engine.start();
    }

    public void stop() {
        //错误，stop方法无法访问在start方法中定义的局部内部类
        //Engine engine = new Engine();
    }
}

局部内部类访问局部变量的限制

public class LocalCar {

    public void start() {

        int length = 50;

        //在成员方法中定义局部内部类
        class Engine {

            public void start() {
                //在java 8之前，局部内部类如果要访问方法的局部变量，则该局部变量必须加上final关键字修饰
                //在java 8开始，只要局部变量不变，final关键字可以省略
                System.out.println(length);
                //局部变量存在于栈内存中，在方法运行结束之后会立刻消失，
                //但是局部对象在堆内存中持续存在，直到垃圾回收消失，所以局部内部类访问局部变量，该局部变量不能发生变化
                //length=60; 该语句错误，因为局部变量length实际是以final关键字修饰的
                System.out.println("发动机启动");
            }

        }

        Engine engine = new Engine();
        engine.start();
    }

    public void stop() {
        //错误，stop方法无法访问在start方法中定义的局部内部类
        //Engine engine = new Engine();
    }
}

匿名内部类

如果接口的实现类（或者定义父类的子类）只需要用到唯一的一次，则可以省略该实现类的定义，而直接使用匿名内部类。且匿名内部类也可以用lambda表达式代替。

public interface AnonymousInterface {
    void method();
}

public class AnonymousClass {

    AnonymousInterface anonymousInterface = new AnonymousInterface() {
        @Override
        public void method() {
            System.out.println("匿名内部类实现方法");
        }
    };
}
//以上代码用lambda表达式代替如下:
public class AnonymousClass {
    AnonymousInterface anonymousInterface = () -> System.out.println("匿名内部类实现方法");
}

匿名内部类的应用：接口作为成员变量的类型

public interface School {

    void goSchool();
}

public class AnonymousMember {

    private String name;

    private int age;

    private School school;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public School getSchool() {
        return school;
    }

    public void setSchool(School school) {
        this.school = school;
    }
}

public class AnonymousMemberTest {
    public static void main(String[] args) {
        AnonymousMember anonymousMember = new AnonymousMember();
        anonymousMember.setName("小明");
        anonymousMember.setAge(10);
        School school = new School() {
            @Override
            public void goSchool() {
                System.out.println("上常青藤小学");
            }
        };
        anonymousMember.setSchool(school);

    }
}