1 enumeration class

1.1 user defined enumeration class

1. The constructor needs to be privatized
2. There is no need to provide a set method because the value of the enumeration object is read-only
3. The enumerating objects / attributes are decorated with final + static to realize the underlying optimization
4. Enumeration object names are usually capitalized
5. Enumeration objects can have multiple attributes according to requirements

package com.enum_;



public class Enumeration01 {
    public static void main(String[] args) {
        System.out.println(Season.SPRING);
        System.out.println(Season.SUMMER);
        System.out.println(Season.AUTUMN);
        System.out.println(Season.WINTER);

    }
}

class Season{
   private String name;
   private String describe;
   //

   public final static Season SPRING = new Season("spring","warm");
   public final static Season SUMMER = new Season("summer","scorching hot");
   public final static Season AUTUMN= new Season("autumn","pleasantly cool");
   public final static Season WINTER = new Season("winter","cold");

    private Season(String name, String describe) {
        this.name = name;
        this.describe = describe;
    }

    public String getName() {
        return name;
    }

    public String getDescribe() {
        return describe;
    }

    @Override
    public String toString() {
        return "Season{" +
                "name='" + name + '\'' +
                ", describe='" + describe + '\'' +
                '}';
    }
}

1.2 enum keyword

• It is equivalent to simplifying the creation of objects into object names (parameters), which must be separated by commas, and the definitions should be placed at the beginning.
• If there are no parameters, it can be simplified to direct definition of object name. For example:

enum Gender{
    BOY,GIRL;
}

package com.enum_;

public class Enumeration02 {
    public static void main(String[] args) {
        System.out.println(Season2.SPRING);
        System.out.println(Season2.SUMMER);
        System.out.println(Season2.AUTUMN);
        System.out.println(Season2.WINTER);

    }
}

enum  Season2{
    SPRING("spring","warm"),
    SUMMER("summer","scorching hot"),
    AUTUMN("autumn","pleasantly cool"),
    WINTER("winter","cold");

    private String name;
    private String describe;




    private Season2(String name, String describe) {
        this.name = name;
        this.describe = describe;
    }

    public String getName() {
        return name;
    }

    public String getDescribe() {
        return describe;
    }

    @Override
    public String toString() {
        return "Season2{" +
                "name='" + name + '\'' +
                ", describe='" + describe + '\'' +
                '}';
    }
}

1.3 various methods in enum class

• Name: return object name
• ordinal: returns the location number of the object (starting from 0)
• values: returns all constants (an array) in the current class
• Valueof (converts a string to an enumeration object, but requires that the string must be an existing constant name

Season value = Season.valueOf("SPRING");

• toString: return object name

package com.enum_;



public class EnumExercise {
    public static void main(String[] args) {
        Gender boy = Gender.BOY;
        Gender boy2 = Gender.BOY;
        System.out.println(boy);
        System.out.println(boy == boy2);
        System.out.println("==============enum Method test of class==========");
        System.out.println("name()function:" + boy.name());
        System.out.println("ordinal()function:" +boy.ordinal());


        Gender[] genders = Gender.values();
        for(Gender gender:genders){
            System.out.println("values()function:" +gender);
        }
        Gender girl = Gender.valueOf("GIRL");
        System.out.println("valueOf()function:" +girl);
    }


}

enum Gender{
    BOY,GIRL;
}

2 annotation

• Basic Annotation
  1. @Override: it is a method that overrides the parent class. This annotation can only be used for methods
  2. @Deprecated: used to indicate that a program element is obsolete
  3. @SuppressWarnings: suppress compiler warnings

2.1 @Override

• Using @ Override indicates that the method overrides the method of the parent class
• The compiler will check whether it is really rewritten. If it is not rewritten, the compiler will report an error
  -@Definition of Override

@Target(ElementType.METHOD)//Description can only modify methods. Target is the annotation that modifies annotations, which is called meta annotation
@Retention(RetentionPolicy.SOURCE)
public @interface Override {//@interface indicates that it is an annotation class
}

2.2 @Deprecated

• Used to indicate that a program element is obsolete, which does not mean it cannot be used
• You can modify methods, classes, fields, packages, and parameters

2.3 @SuppressWarnings

• If you don't want to see compiler related warnings, you can use this annotation with related parameters

2.4 yuan notes

• A comment that modifies an annotation

2.4.1 @Retention

• Three values of Retention
  1. RententionPolicy.SOURCE: after the compiler uses it, it directly discards the annotation of this strategy
  2. RententionPolicy.CLASS: the compiler records the annotations in the class file. When running, the JVM will not keep them
  3. RententionPolicy.RUNTIME: the compiler records the annotation in the class file. When running, the JVM will keep it, and the program can get the annotation through reflection

2.4.2 @Target

• Used to specify which program elements can be modified by the modified annotation

2.4.3 @Documented

• It is used to specify that the annotation modified by the annotation will be extracted into a document by javadoc tool, that is, the annotation can be seen when the document is generated

2.4.4 @Inherited

• The annotation class modified by the modified annotation has inheritance, that is, a class is modified by the annotation modified by @ Inherited, and its subclass automatically obtains the annotation.