嵌套类

提示

• Nested Classes

嵌套类

优点

• 只对一个类有用
• 更具可读性和可维护性

嵌套类

class OuterClass {
    // ...

    // 非静态类
    // 只存在在实例中
    class InnerClass {
        // ...
    }

    // 静态类
    // 不能直接访问实例字段
    static class StaticNestedClass {
        // ...
    }
}

实例化

OuterClass outer = new OuterClass();
OuterClass.InnerClass inner = outer.new InnerClass();

shadowing

public class ShadowTest {

    public int x = 0;

    class FirstLevel {

        // 覆盖了 外层类的 x
        public int x = 1;

        void methodInFirstLevel(int x) {
            // 此处 x 覆盖了 外层类的 x
            System.out.println("x = " + x);

            System.out.println("this.x = " + this.x);

            System.out.println("ShadowTest.this.x = " + ShadowTest.this.x);
        }
    }

    public static void main(String... args) {
        ShadowTest st = new ShadowTest();
        ShadowTest.FirstLevel fl = st.new FirstLevel();
        fl.methodInFirstLevel(23);
    }
}

序列化

信息

不建议序列化内部类，包括局部类和匿名类。非静态内部类会自动持有外部类的引用，序列化内部类时，会序列化整个外部类的实例或其部分。

局部类

危险

待补充

• 可在任意块中定义局部类

public class LocalClassExample {

    static String regularExpression = "[^0-9]";

    public static void validatePhoneNumber(
        String phoneNumber1, String phoneNumber2) {

        final int numberLength = 10;

        /**
         * 局部类
         */
        class PhoneNumber {

            String formattedPhoneNumber = null;

            PhoneNumber(String phoneNumber){
                // numberLength = 7;
                String currentNumber = phoneNumber.replaceAll(
                  regularExpression, "");
                if (currentNumber.length() == numberLength)
                    formattedPhoneNumber = currentNumber;
                else
                    formattedPhoneNumber = null;
            }

            public String getNumber() {
                return formattedPhoneNumber;
            }
        }

        PhoneNumber myNumber1 = new PhoneNumber(phoneNumber1);
        PhoneNumber myNumber2 = new PhoneNumber(phoneNumber2);

        if (myNumber1.getNumber() == null)
            System.out.println("First number is invalid");
        else
            System.out.println("First number is " + myNumber1.getNumber());
        if (myNumber2.getNumber() == null)
            System.out.println("Second number is invalid");
        else
            System.out.println("Second number is " + myNumber2.getNumber());

    }

    public static void main(String... args) {
        validatePhoneNumber("123-456-7890", "456-7890");
    }
}

匿名类

危险

待补充

public class HelloWorldAnonymousClasses {

    interface HelloWorld {
        public void greet();
        public void greetSomeone(String someone);
    }

    public void sayHello() {

        class EnglishGreeting implements HelloWorld {
            String name = "world";
            public void greet() {
                greetSomeone("world");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Hello " + name);
            }
        }

        HelloWorld englishGreeting = new EnglishGreeting();

        // 初始化匿名类
        HelloWorld frenchGreeting = new HelloWorld() {
            String name = "tout le monde";
            public void greet() {
                greetSomeone("tout le monde");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Salut " + name);
            }
        };

        HelloWorld spanishGreeting = new HelloWorld() {
            String name = "mundo";
            public void greet() {
                greetSomeone("mundo");
            }
            public void greetSomeone(String someone) {
                name = someone;
                System.out.println("Hola, " + name);
            }
        };
        englishGreeting.greet();
        frenchGreeting.greetSomeone("Fred");
        spanishGreeting.greet();
    }

    public static void main(String... args) {
        HelloWorldAnonymousClasses myApp =
            new HelloWorldAnonymousClasses();
        myApp.sayHello();
    }
}