1, Use of enumeration classes

2, How to define enumeration class

Method 1: before jdk5.0, customize enumeration class

public class SeasonTest {
    public static void main(String[] args) {
        Season spring = Season.SPRING;
        System.out.println(spring);
    }
}
//Custom enumeration class
class Season{
    //1. Declare the properties of the Season object: private final decoration
    private final String seasonName;
    private final String seasonDesc;

    //2. Privatize the constructor of the class and assign a value to the object property
    private Season(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    //3. Provide multiple objects of the current enumeration class: public static final
    public static final Season SPRING = new Season("spring","In the warm spring, flowers are coming out with a rush");
    public static final Season SUMMER = new Season("summer","Scorching summer");
    public static final Season AUTUMN = new Season("Autumn","Fresh autumn weather");
    public static final Season WINTER = new Season("winter","A world of ice and snow");

    //4. Other claims 1: get the properties of enumeration objects
    public String getSeasonName() {
        return seasonName;
    }

    public String getSeasonDesc() {
        return seasonDesc;
    }
    //4. Other claims 1: toString()
    @Override
    public String toString() {
        return "Season{" +
                "seasonName='" + seasonName + '\'' +
                ", seasonDesc='" + seasonDesc + '\'' +
                '}';
    }
}

Mode 2: jdk5.0, enum keyword can be used to define enumeration class

public class SeasonTest1 {
    public static void main(String[] args) {
        Season1 summer = Season1.SUMMER;
        //toString(): returns the name of an enumeration class object
        System.out.println(summer.toString());
//        System.out.println(Season1.class.getSuperclass());
        System.out.println("****************");
        //values(): returns an array of all enumeration class objects
        Season1[] values = Season1.values();
        for(int i = 0;i < values.length;i++){
            System.out.println(values[i]);
            values[i].show();
        }
        System.out.println("****************");
        Thread.State[] values1 = Thread.State.values();
        for (int i = 0; i < values1.length; i++) {
            System.out.println(values1[i]);
        }
        //valueOf(String objName): returns the object in the enumeration class whose object name is objName.
        Season1 winter = Season1.valueOf("WINTER");
        //If there is no enumeration class object of objName, throw an exception: IllegalArgumentException
//        Season1 winter = Season1.valueOf("WINTER1");
        System.out.println(winter);
        winter.show();
    }
}

interface Info{
    void show();
}

//Enumerating classes using the enum keyword
enum Season1 implements Info{
    //1. Provide the object of the current enumeration class, separate multiple objects with "," and end with ";"
    SPRING("spring","In the warm spring, flowers are coming out with a rush"){
        @Override
        public void show() {
            System.out.println("Where is spring?");
        }
    },
    SUMMER("summer","Scorching summer"){
        @Override
        public void show() {
            System.out.println("Ningxia");
        }
    },
    AUTUMN("Autumn","Fresh autumn weather"){
        @Override
        public void show() {
            System.out.println("Don't come back in autumn");
        }
    },
    WINTER("winter","A world of ice and snow"){
        @Override
        public void show() {
            System.out.println("About in winter");
        }
    };

    //2. Declare the properties of the Season object: private final decoration
    private final String seasonName;
    private final String seasonDesc;

    //2. Privatize the constructor of the class and assign a value to the object property
    private Season1(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    //4. Other claims 1: get the properties of enumeration objects
    public String getSeasonName() {
        return seasonName;
    }

    public String getSeasonDesc() {
        return seasonDesc;
    }
//    //4. Other claims 1: toString()
//
//    @Override
//    public String toString() {
//        return "Season1{" +
//                "seasonName='" + seasonName + '\'' +
//                ", seasonDesc='" + seasonDesc + '\'' +
//                '}';
//    }

//    @Override
//    public void show() {
//        System.out.println("it's a season");
//    }
}

3, Common methods in Enum class:

4, Implementation of the interface using enum class defined by enum keyword

5, Use of annotations

1. Understand Annotation:

2. Use example of announcement

3. How to customize annotation: refer to @ SuppressWarnings definition

4. Four meta annotations provided by JDK

5. Get annotation information through reflection - systematic explanation when reflecting content

6. New features of annotation in JDK 8: repeatable annotation and type annotation

6.1 repeatable notes:

6.2 type notes:

public class AnnotationTest {
    public static void main(String[] args) {
        Person p = new Student();
        p.walk();
        Date date = new Date(2020, 10, 11);
        System.out.println(date);
        @SuppressWarnings("unused")
        int num = 10;
//        System.out.println(num);
        @SuppressWarnings({ "unused", "rawtypes" })
        ArrayList list = new ArrayList();
    }

    @Test
    public void testGetAnnotation(){
        Class clazz = Student.class;
        Annotation[] annotations = clazz.getAnnotations();
        for(int i = 0;i < annotations.length;i++){
            System.out.println(annotations[i]);
        }
    }
}

//Before jdk 8:
//@MyAnnotations({@MyAnnotation(value="hi"),@MyAnnotation(value="hi")})
@MyAnnotation(value="hi")
@MyAnnotation(value="abc")
class Person{
    private String name;
    private int age;

    public Person() {
    }
    @MyAnnotation
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @MyAnnotation
    public void walk(){
        System.out.println("People walking");
    }
    public void eat(){
        System.out.println("People eat");
    }
}

interface Info{
    void show();
}

class Student extends Person implements Info{
    @Override
    public void walk() {
        System.out.println("Students walk");
    }
    public void show() {
    }
}

class Generic<@MyAnnotation T>{
    public void show() throws @MyAnnotation RuntimeException{
        ArrayList<@MyAnnotation String> list = new ArrayList<>();
        int num = (@MyAnnotation int) 10L;
    }
}

@Inherited
@Repeatable(MyAnnotations.class)
@Retention(RetentionPolicy.RUNTIME)
@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE,TYPE_PARAMETER,TYPE_USE})
public @interface MyAnnotation {

    String value() default "hello";
}

@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
public @interface MyAnnotations {

    MyAnnotation[] value();
}