java枚举类的定义,使用,本质

我会带着你远行 2024-04-17 05:31 93阅读 0赞

## 关于枚举类 ##

参考[反编译那些事儿（二）—枚举的反编译][Link 1]  
[Java枚举类，你真的了解吗？][Java]

#### 定义枚举类 ####

public enum ResultCode { 
    
        /** 有没有发现写枚举类的这一部分就像是在调用构造方法 */
        SUCCESS(1, "成功"),
        PARAM_IS_INVALID(10001, "参数无效"),
        PARAM_IS_BLANK(10002, "参数为空"),
        PARAM_TYPE_BIND_ERROR(10003, "参数类型错误"),
        PARAM_NOT_COMPLETE(10004, "参数缺失"),
        //上面的逗号可去可不去, 并不会导致编译错误
        ;
    
        private Integer code;
    
        private String message;
    
        //构造函数默认并且必须是private
        ResultCode(Integer code, String message) { 
            this.code = code;
            this.message = message;
        }
    
        public Integer code() { 
            return this.code;
        }
    
        public String message() { 
            return this.message;
        }
    }

#### 枚举类的使用 ####

所有的枚举类在编译之后会自动继承enum类, 并且会自动生成一个values()方法

//只放出部分常见代码
    public abstract class Enum<E extends Enum<E>>
            implements Comparable<E>, Serializable { 
    
    	/** 实例的名字, 类型为字符串 即ResultCode.SUCCESS为SUCCESS, ResultCode.PARAM_IS_INVALID为PARAM_IS_INVALID */
        private final String name;
    
        public final String name() { 
            return name;
        }
    
    	/** 实例的索引, 从0开始 即ResultCode.SUCCESS.ordinal()为0, ResultCode.PARAM_IS_INVALID.ordinal()为1 */
        private final int ordinal;
    
        public final int ordinal() { 
            return ordinal;
        }
        //toString方法与name()一样
        public String toString() { 
            return name;
        }
    	//判断两个枚举实例是否一样, 注意这里使用的是==, 直接比引用, 说明枚举类的实例是单例的
        public final boolean equals(Object other) { 
            return this==other;
        }
        /** 比较两个枚举实例的索引详见, 返回的int 上面我们知道ResultCode.SUCCESS.ordinal()为0, ResultCode.PARAM_IS_INVALID.ordinal()为1 所以ResultCode.SUCCESS.compareTo(ResultCode.PARAM_IS_INVALID)为0-1=-1 */
        public final int compareTo(E o) { 
    		//略去实现代码
        }
    }

验证:

//ResultCode.values()会返回ResultCode这个枚举类的所有实例, 类型为ResultCode[]
    for (ResultCode resultCode : ResultCode.values()) { 
                System.out.println(resultCode.name());
                System.out.println(resultCode.ordinal());
                System.out.println(resultCode.compareTo(ResultCode.SUCCESS));
    }
    //ResultCode.valueOf()会返回name与参数相等的枚举实例
    ResultCode SUCCESS = ResultCode.valueOf("SUCCESS");
    System.out.println(SUCCESS.equals(ResultCode.SUCCESS));   //输出为true

#### 枚举类中定义抽象方法 ####

/** * 枚举类可以有抽象方法，但是必须在它的实例中实现 */
    public enum AbstractWeek { 
    
        MONDAY(0,"星期一") { 
            @Override
            public AbstractWeek getNextDay() { 
                return TUESDAY;
            }
        }, TUESDAY(1,"星期二") { 
            @Override
            public AbstractWeek getNextDay() { 
                return WEDNESDAY;
            }
        }, WEDNESDAY(2,"星期三") { 
            @Override
            public AbstractWeek getNextDay() { 
                return THURSDAY;
            }
        }, THURSDAY(3,"星期四") { 
            @Override
            public AbstractWeek getNextDay() { 
                return FRIDAY;
            }
        }, FRIDAY(4,"星期五") { 
            @Override
            public AbstractWeek getNextDay() { 
                return SATURDAY;
            }
        }, SATURDAY(5,"星期六") { 
            @Override
            public AbstractWeek getNextDay() { 
                return SUNDAY;
            }
        }, SUNDAY(6,"星期日") { 
            @Override
            public AbstractWeek getNextDay() { 
                return MONDAY;
            }
        };
    
        private int num;
        private String desc;
    
        AbstractWeek(int num,String desc) { 
            this.num = num;
            this.desc=desc;
        }
    
        //一个抽象方法
        public abstract AbstractWeek getNextDay();
    }

#### 反编译与语法糖 ####

public enum Weather { 
    	
    	Sunny(1, "晴天") { 
    		@Override
    		public String test() { 
    			return "今天阳光明媚！！！\n";
    		}
    	},
    	Rainy(2, "雨天") { 
    		@Override
    		public String test() { 
    			return "出门别忘带把雨伞哦！！！\n";
    		}
    	},
    	Cloudy(3, "多云") { 
    		@Override
    		public String test() { 
    			return "别出去登高望远了，你看不远的，呵呵呵！！！\n";
    		}
    	};
     
    	private int value;
    	private String label;
     
    	private Weather(int value, String label) { 
    		this.value = value;
    		this.label = label;
    	}
     
    	public int getValue() { 
    		return value;
    	}
     
    	public String getLabel() { 
    		return label;
    	}
     
    	public abstract String test();
    
    }

我们将上述代码进行反编译:

import java.io.PrintStream;
    import java.util.*;
     
    //枚举类继承了Enum, 所以我们在定义的时候不能再继承其他的类了
    //非抽象枚举类默认是final的, 所以不要去继承非抽象的枚举类
    public abstract class Weather extends Enum { 
     
        //这里s表示name, i表示ordinal
        //构造函数为私有的
    	private Weather(String s, int i, int value, String label) { 
    		super(s, i);
    		this.value = value;
    		this.label = label;
    	}
     
    	public int getValue() { 
    		return value;
    	}
     
    	public String getLabel() { 
    		return label;
    	}
     
        //我们自己定义的抽象方法
    	public abstract String test();
    
     	//values()返回所有枚举类的实例
    	public static Weather[] values() { 
    		Weather aweather[];
    		int i;
    		Weather aweather1[];
    		System.arraycopy(aweather = ENUM$VALUES, 0, aweather1 = new Weather[i = aweather.length], 0, i);
    		return aweather1;
    	}
     
    	public static Weather valueOf(String s) { 
    		return (Weather) Enum.valueOf(Weather, s);
    	}
     
    	Weather(String s, int i, int j, String s1, Weather weather) { 
    		this(s, i, j, s1);
    	}
     
        //这是我们所定义的枚举类实例, 这也是为什么我们在定义枚举类的时候就像是在调用构造函数
        //每一个变量都是public static final
    	public static final Weather Sunny;
    	public static final Weather Rainy;
    	public static final Weather Cloudy;
        
    	private int value;
    	private String label;
        
    	private static final Weather ENUM$VALUES[];
     
    	static { 
    		Sunny = new Weather("Sunny", 0, 1, "\u6674\u5929") { 
    			public String test() { 
    				return "\u4ECA\u5929\u9633\u5149\u660E\u5A9A\uFF01\uFF01\uFF01\n";
    			}
    		};
    		Rainy = new Weather("Rainy", 1, 2, "\u96E8\u5929") { 
    			public String test() { 
    				return "\u51FA\u95E8\u522B\u5FD8\u5E26\u628A\u96E8\u4F1E\u54E6\uFF01\uFF01\uFF01\n";
    			}
    		};
    		Cloudy = new Weather("Cloudy", 2, 3, "\u591A\u4E91") { 
    			public String test() { 
    				return "\u522B\u51FA\u53BB\u767B\u9AD8\u671B\u8FDC\u4E86\uFF0C\u4F60\u770B\u4E0D\u8FDC\u7684\uFF0C\u5475\u5475\u5475\uFF01\uFF01\uFF01\n";
    			}
    		};
    		ENUM$VALUES = (new Weather[] {  Sunny, Rainy, Cloudy });
    	}
    }

从上述我们可以看出, 枚举类的实现还是普通的java类, 只不过这个java类的构造函数是私有的, 而且实例的个数是有限个的, 当这个枚举类的实例个数只有一个的时候, 这个类就变成了单例模式了.

[Link 1]: https://blog.csdn.net/gaohuanjie/article/details/18140279
[Java]: https://zhuanlan.zhihu.com/p/63859957