总结：Java后端开发知识点

课程：宋红康 JAVA

如何定义枚举类

jdk5.0之前，自定义枚举类

//自定义枚举类
class Season{
    //1.声明Season对象的属性:private final修饰
    private final String seasonName;
    private final String seasonDesc;
    //2.私有化类的构造器,并给对象属性赋值
    private Season(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }
    //3.提供当前枚举类的多个对象：public static final的
    public static final Season SPRING = new Season("春天","春暖花开");
    public static final Season SUMMER = new Season("夏天","夏日炎炎");
    public static final Season AUTUMN = new Season("秋天","秋高气爽");
    public static final Season WINTER = new Season("冬天","冰天雪地");
    //4.其他诉求1：获取枚举类对象的属性
    public String getSeasonName() {
        return seasonName;
    }
    public String getSeasonDesc() {
        return seasonDesc;
    }
    //4.其他诉求1：提供toString()
    @Override
    public String toString() {
        return "Season{" +
                "seasonName='" + seasonName + '\'' +
                ", seasonDesc='" + seasonDesc + '\'' +
                '}';
    }
}

//测试
public class SeasonTest {
    public static void main(String[] args) {
        Season spring = Season.SPRING;
        System.out.println(spring);
    }
}

jdk5.0，可以使用enum关键字定义枚举类

//使用enum关键字枚举类
public enum Season2{
    //1.提供当前枚举类的对象，多个对象之间用","隔开，末尾对象";"结束
    SPRING("春天","春暖花开"),
    SUMMER("夏天","夏日炎炎"),
    AUTUMN("秋天","秋高气爽"),
    WINTER("冬天","冰天雪地");

    //2.声明Season对象的属性:private final修饰
    private final String seasonName;
    private final String seasonDesc;

    //2.私有化类的构造器,并给对象属性赋值
    private Season2(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    //4.其他诉求1：获取枚举类对象的属性
    public String getSeasonName() {
        return seasonName;
    }
    public String getSeasonDesc() {
        return seasonDesc;
    }
    //4.其他诉求1：提供toString()  
    public String toString() {
        return "Season1{" +
                "seasonName='" + seasonName + '\'' +
                ", seasonDesc='" + seasonDesc + '\'' +
                '}';
    }
}

//测试
public class Test {
    public static void main(String[] args) {
        Season2 spring = Season2.SPRING;
        System.out.println(spring);
    }
}

Enum类中的常用方法：

toString()

toString()：返回当前枚举类对象常量的名称

当我们把之前写的toString方法注释掉，打印的就是枚举类对象常量的名称

因为定义的枚举类默认继承于java.lang.Enum类

@Test
public void test43(){
    Season2 spring = Season2.SPRING;
    System.out.println(spring.getSeasonName());//春天
    System.out.println(spring.getSeasonDesc());//春暖花开
    System.out.println(spring);//SPRING
}

values()

values()方法：返回枚举类型的对象数组。该方法可以很方便地遍历所有的枚举值

@Test
public void test44(){
    Season2[] values = Season2.values();
    for (int i = 0; i < values.length; i++) {
        System.out.println(values[i].getSeasonName());
    }
}

valueOf(String str)

valueOf(String str)：可以把一个字符串转为对应的枚举类对象。要求字符串必须是枚举类对象的“名字”。如不是，会有运行时异常：IllegalArgumentException。

@Test
public void test45(){
    Season2 summer = Season2.valueOf("SUMMER");
    System.out.println(summer.getSeasonName());//夏天
    Season2 autumn = Season2.valueOf("AUTUMN1");
    System.out.println(autumn.getSeasonName());//IllegalArgumentException
}

使用enum关键字定义的枚举类实现接口的情况

实现接口，在enum类中实现抽象方法

SPRING，SUMMER，AUTUMN和WINTER四个枚举对象都共同执行同一个show方法

interface Info{
    void show();
}

public enum Season2 implements Info{
    //1.提供当前枚举类的对象，多个对象之间用","隔开，末尾对象";"结束
    SPRING("春天","春暖花开"),
    SUMMER("夏天","夏日炎炎"),
    AUTUMN("秋天","秋高气爽"),
    WINTER("冬天","冰天雪地");

    //2.声明Season对象的属性:private final修饰
    private final String seasonName;
    private final String seasonDesc;

    //2.私有化类的构造器,并给对象属性赋值
    private Season2(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }

    @Override
    public void show() {
        System.out.println("这是什么季节");
    }
}

@Test
public void test43(){
    Season2 spring = Season2.SPRING;
    spring.show();// 这是什么季节
}

让枚举类的对象分别实现接口中的抽象方法

public enum Season2 implements Info{
    //1.提供当前枚举类的对象，多个对象之间用","隔开，末尾对象";"结束
    SPRING("春天","春暖花开"){
        @Override
        public void show() {
            System.out.println("这是春天");
        }
    },
    SUMMER("夏天","夏日炎炎"){
        @Override
        public void show() {
            System.out.println("这是夏天");
        }
    },
    AUTUMN("秋天","秋高气爽"){
        @Override
        public void show() {
            System.out.println("这是秋天");
        }
    },
    WINTER("冬天","冰天雪地"){
        @Override
        public void show() {
            System.out.println("这是冬天");
        }
    };

    //2.声明Season对象的属性:private final修饰
    private final String seasonName;
    private final String seasonDesc;

    //2.私有化类的构造器,并给对象属性赋值
    private Season2(String seasonName,String seasonDesc){
        this.seasonName = seasonName;
        this.seasonDesc = seasonDesc;
    }
}

@Test
public void test43(){
    Season2 spring = Season2.SPRING;
    spring.show();//这是春天
}

不设值的枚举：

enum Color
{
    RED, GREEN, BLUE;
}
 
public class Test
{
    public static void main(String[] args)
    {
        // 调用 values() 返回枚举类中所有的值。
        Color[] arr = Color.values();
 
        // 迭代枚举
        for (Color col : arr)
        {
            // 查看索引 ordinal()方法可以找到每个枚举常量的索引，就像数组索引一样。
            System.out.println(col + " at index " + col.ordinal());
            //RED at index 0
            //GREEN at index 1
            //BLUE at index 2
        }
 
        // 使用 valueOf() 返回枚举常量，不存在的会报错 IllegalArgumentException
        //valueOf()方法返回指定字符串值的枚举常量。
        Color red = Color.valueOf("RED");
        System.out.println(red);
        // System.out.println(Color.valueOf("WHITE"));//IllegalArgumentException
    }
}

其实枚举未必都要设置其他属性，也可以不设置属性。每个枚举都会有一个索引，通过ordinal()方法获得

索引从0开始，从自己定义的顺序开始。

@Test
public void test45(){
    Season2[] values = Season2.values();
    for (int i = 0; i < values.length; i++) {
        System.out.println(values[i].getSeasonName()+":"+values[i].ordinal());
        /*春天:0
          夏天:1
          秋天:2
          冬天:3*/
    }
}

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