Java enumeration_Power node summary

The full name of enum is enumeration, which is a new feature introduced in JDK 5 and is stored in the java.lang package. This article will introduce you to the knowledge related to Java enumeration. Friends who need it can refer to it.

The full name of enum is enumeration. It is a new feature introduced in JDK 5 and is stored in the java.lang package.

Original interface definition constant

public interface IConstants {
  String MON = "Mon";
  String TUE = "Tue";
  String WED = "Wed";
  String THU = "Thu";
  String FRI = "Fri";
  String SAT = "Sat";
  String SUN = "Sun";
}

Syntax (Definition)

To create an enumeration type, use the enum keyword. It is implicit that the types created are subclasses of the java.lang.Enum class (java.lang.Enum is an abstract class). Enumeration types conform to the general pattern Class Enum>, where E represents the name of the enumeration type. Each value of the enumeration type will be mapped to the protected Enum(String name, int ordinal) constructor, where the name of each value is converted to a string, and the ordinal setting represents the setting in which this setting was created order.

package com.bjpowernode.test;
/**
 * 枚举测试类
 */
public enum EnumTest {
  MON, TUE, WED, THU, FRI, SAT, SUN;
}

This code actually calls Enum(String name, int ordinal) 7 times:

new Enum<EnumTest>("MON",0);
new Enum<EnumTest>("TUE",1);
new Enum<EnumTest>("WED",2);
  ... ...

Common operations such as traversal and switch

Sample code for enum traversal and switch operation:

public class Test {
  public static void main(String[] args) {
    for (EnumTest e : EnumTest.values()) {
      System.out.println(e.toString());
    }
    System.out.println("----------------我是分隔线------------------");
    EnumTest test = EnumTest.TUE;
    switch (test) {
    case MON:
      System.out.println("今天是星期一");
      break;
    case TUE:
      System.out.println("今天是星期二");
      break;
    // ... ...
    default:
      System.out.println(test);
      break;
    }
  }
}

Output result:

MON
TUE
WED
THU
FRI
SAT
SUN

----------------I am the divider------------------

Today is Tuesday

Introduction to common methods of enum objects

int compareTo(E o)

Compare this enumeration with the order of the specified object.

Class<E> getDeclaringClass()

                                  Returns the Class object corresponding to the enumeration type of this enumeration constant.

String name()

                                                                                                                           can be returned.

int ordinal()

Returns the ordinal of the enumeration constant (its position in the enumeration declaration, where the initial constant ordinal is zero).

String toString()

Returns the name of the enumeration constant, which is contained in the declaration.

static > T valueOf(Class enumType, String name)

Returns the enumeration constant of the specified enumeration type with the specified name.

public class Test {
  public static void main(String[] args) {
    EnumTest test = EnumTest.TUE;
    //compareTo(E o)
    switch (test.compareTo(EnumTest.MON)) {
    case -1:
      System.out.println("TUE 在 MON 之前");
      break;
    case 1:
      System.out.println("TUE 在 MON 之后");
      break;
    default:
      System.out.println("TUE 与 MON 在同一位置");
      break;
    }
    //getDeclaringClass()
    System.out.println("getDeclaringClass(): " + test.getDeclaringClass().getName());
    //name() 和 toString()
    System.out.println("name(): " + test.name());
    System.out.println("toString(): " + test.toString());
    //ordinal()， 返回值是从 0 开始
    System.out.println("ordinal(): " + test.ordinal());
  }
}

Output result:

TUE after MON

getDeclaringClass(): com.hmw.test.EnumTest
name(): TUE
toString(): TUE
ordinal(): 1

Add custom properties and methods to enum

Add enum object Let’s look at the value attributes and the getValue() method:

package com.bjpowernode.test;
 /**
 * 枚举测试类
 * 
 */
public enum EnumTest {
  MON(1), TUE(2), WED(3), THU(4), FRI(5), SAT(6) {
    @Override
    public boolean isRest() {
      return true;
    }
  },
  SUN(0) {
    @Override
    public boolean isRest() {
      return true;
    }
  };
  private int value;
  private EnumTest(int value) {
    this.value = value;
  }
  public int getValue() {
    return value;
  }
  public boolean isRest() {
    return false;
  }
}
public class Test {
  public static void main(String[] args) {
    System.out.println("EnumTest.FRI 的 value = " + EnumTest.FRI.getValue());
  }
}

Output results:

EnumTest.FRI 的 value = 5
EnumSet，EnumMap 的应用
public class Test {
  public static void main(String[] args) {
    // EnumSet的使用
    EnumSet<EnumTest> weekSet = EnumSet.allOf(EnumTest.class);
    for (EnumTest day : weekSet) {
      System.out.println(day);
    }
    // EnumMap的使用
    EnumMap<EnumTest, String> weekMap = new EnumMap(EnumTest.class);
    weekMap.put(EnumTest.MON, "星期一");
    weekMap.put(EnumTest.TUE, "星期二");
    // ... ...
    for (Iterator<Entry<EnumTest, String>> iter = weekMap.entrySet().iterator(); iter.hasNext();) {
      Entry<EnumTest, String> entry = iter.next();
      System.out.println(entry.getKey().name() + ":" + entry.getValue());
    }
  }
}

Principle analysis

Although the syntax structure of enum is the same as that of class The syntax is different, but what is generated after compilation by the compiler is a class file. After decompiling the class file, you can see that a class is actually generated, which inherits java.lang.Enum. The content obtained after EnumTest is decompiled (javap com.hmw.test.EnumTest command) is as follows:

public class com.hmw.test.EnumTest extends java.lang.Enum{
  public static final com.hmw.test.EnumTest MON;
  public static final com.hmw.test.EnumTest TUE;
  public static final com.hmw.test.EnumTest WED;
  public static final com.hmw.test.EnumTest THU;
  public static final com.hmw.test.EnumTest FRI;
  public static final com.hmw.test.EnumTest SAT;
  public static final com.hmw.test.EnumTest SUN;
  static {};
  public int getValue();
  public boolean isRest();
  public static com.hmw.test.EnumTest[] values();
  public static com.hmw.test.EnumTest valueOf(java.lang.String);
  com.hmw.test.EnumTest(java.lang.String, int, int, com.hmw.test.EnumTest);
}

So, in fact, enum is a class, but the java compiler helps us do the syntax analysis and Just compiled.

Summary

You can think of enum as an ordinary class. They can all define some properties and methods. The difference is: enum cannot use the extends keyword. Inherit other classes, because enum has inherited java.lang.Enum (java is single inheritance).

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