Baozi series -- Java basics Chapter 10_ Enumerating classes and annotations

baozi series java basics chapter


Chapter ten Enumerating classes and annotations

Use of enumeration class

Enumeration class description

* 1. Enumeration class understanding : Class has only a limited number of objects , affirmatory . We call this class an enumeration class
* 2. When you need to define a set of constants , It is highly recommended to use enumeration classes
* 3. If there is only one object in the enumeration class , It can be used as the implementation of singleton mode .

Define an enumeration class

// Custom enumeration class
class Season{
//1. Statement Season Object properties :private final modification
private final String seasonName;
private final String seasonDesc;
//2. The constructor of the private class , And assign values to the object properties
private Season(String seasonName,String seasonDesc){
this.seasonName = seasonName;
this.seasonDesc = seasonDesc;
}
//3. Provides multiple objects of the current enumeration class :public static final Of
public static final Season SPRING = new Season(" spring "," Spring flowers ");
public static final Season SUMMER = new Season(" summer "," It's hot in summer ");
public static final Season AUTUMN = new Season(" autumn "," autumn ");
public static final Season WINTER = new Season(" winter "," a world of ice and snow ");
//4. Other demands 1: Get the properties of the enumeration class object
public String getSeasonName() {
return seasonName;
}
public String getSeasonDesc() {
return seasonDesc;
}
//4. Other demands 1: Provide toString()
@Override
public String toString() {
return "Season{" +
"seasonName='" + seasonName + '\'' +
", seasonDesc='" + seasonDesc + '\'' +
'}';
}
}

jdk 5.0 New use enum Define an enumeration class

// Use enum Keyword enumeration class
enum Season1 {
//1. Provides the object of the current enumeration class , Use... Between multiple objects "," separate , End object ";" end
SPRING(" spring "," Spring flowers "),
SUMMER(" summer "," It's hot in summer "),
AUTUMN(" autumn "," autumn "),
WINTER(" winter "," a world of ice and snow ");
//2. Statement Season Object properties :private final modification
private final String seasonName;
private final String seasonDesc;
//2. The constructor of the private class , And assign values to the object properties
private Season1(String seasonName,String seasonDesc){
this.seasonName = seasonName;
this.seasonDesc = seasonDesc;
}
//4. Other demands 1: Get the properties of the enumeration class object
public String getSeasonName() {
return seasonName;
}
public String getSeasonDesc() {
return seasonDesc;
}
}

Common methods

Season1 summer = Season1.SUMMER;
//toString(): Returns the name of the enumerated class object
System.out.println(summer.toString());
// System.out.println(Season1.class.getSuperclass());
System.out.println("****************");
//values(): Returns an array of enumerated class objects
Season1[] values = Season1.values();
for(int i = 0;i < values.length;i++){
System.out.println(values[i]);
}
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 name of the object in the enumeration class objName The object of .
Season1 winter = Season1.valueOf("WINTER");
// If not objName Enumeration class objects of , Throw exception :IllegalArgumentException
// Season1 winter = Season1.valueOf("WINTER1");
System.out.println(winter);

Use of annotations

The understanding of annotation

① jdk 5.0 New features
*
② Annotation It's a special tag in the code , These tags can be compiled , Class loading , Read at run time , And carry out the corresponding treatment . By using Annotation,
* Programmers can... Without changing the original logic , Embed some supplementary information in the source file .
*
③ stay JavaSE in , The purpose of annotation is simple , For example, marking outdated features , Ignore warnings, etc . stay JavaEE/Android
* Annotations play an even more important role in Chinese , For example, to configure any aspect of the application , Instead of JavaEE The redundancy left over from the old edition
* Code and XML Configuration etc. .

frame = annotation + The reflex mechanism + Design patterns

Examples of using annotations

* Example 1 : Generate documentation related annotations
* Example 2 : Format check at compile time (JDK Built in basic annotations )
@Override: Qualify override parent method , This annotation can only be used for methods
@Deprecated: Used to represent the decorated element ( class , Such method ) Deprecated . Usually because the modified structure is dangerous or there is a better alternative
@SuppressWarnings: Suppress compiler warnings
* Example : Trace code dependencies , Implement alternative profile functions

Custom annotation

 * ① The annotation states that :@interface
* ② Internally defined members , Usually use value Express
* ③ You can specify default values for members , Use default Definition
* ④ If the custom annotation has no members , It's a sign .
@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";
}
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