JAVA基础之枚举

1、枚举的作用

如果程序运行时需要的数据只能是一定范围内的值，jdk5之前我们可以采用自定义类来解决，jdk5后就可以直接采用枚举来解决了。

例如要限定操作类型只能为增删改查四种，我们可以用自定义类的方式解决如下：

public class Operation 
{  
    private String operType;
    
    private Operation(String operType)
    { 
	this.operType = operType;
    }
    
    public static final Operation ADD = new Operation("add");
    public static final Operation DELETE = new Operation("delete");
    public static final Operation MODIFY= new Operation("modify");
    public static final Operation SELECT = new Operation("select");

    public String getOperType() 
    {
        return operType;
    }

}

jdk5后我们就可以通过枚举来更简洁的解决问题：

 

public enum OperationByEnum
{
    ADD("add"), DELETE("delete"), MODIFY("modify"), SELECT("select");
    public String getOperType()
    {
	return operType;
    }

    private String operType;

    private OperationByEnum(String operType)
    {
	this.operType = operType;
    }

}

 查看上述两个类的class文件可以看出两种方式可以认为是等价的，

 

D:\java\workspace\EnumDemo\bin>javap OperationByEnum.class
Compiled from "OperationByEnum.java"
public final class OperationByEnum extends java.lang.Enum<OperationByEnum> {
  public static final OperationByEnum ADD;
  public static final OperationByEnum DELETE;
  public static final OperationByEnum MODIFY;
  public static final OperationByEnum SELECT;
  static {};
  public java.lang.String getOperType();
  public static OperationByEnum[] values();
  public static OperationByEnum valueOf(java.lang.String);
}

D:\java\workspace\EnumDemo\bin>javap Operation.class
Compiled from "Operation.java"
public class Operation {
  public static final Operation ADD;
  public static final Operation DELETE;
  public static final Operation MODIFY;
  public static final Operation SELECT;
  static {};
  public java.lang.String getOperType();
}

也就是说现在我们定义一个enum类型后，编译器自动帮我们实现一个构造函数为private自定义类，再也不用我们自己去实现一个类似于图1中的自定义类了。

2、带抽象方法的枚举

 

public class EnumTest {

    public static void main(String[] args) 
    {
	print(OperationByEnum.ADD);
    }
    
    public static void print(OperationByEnum arg)
    {
	System.out.println(arg.getOperType());
    }
    
}

输出 add

每个枚举值的获取操作类型方法此时获取的是英文名称，如果我想添加一个方法来获取每个操作类型的中文名称，应该怎么处理呢？此时带抽象方法的枚举就要闪亮登场了

public enum OperationByEnum
{
    ADD("add")
    {

	@Override
	public String getChineseName()
	{
	    return "增";
	}
	
    }, 
    DELETE("delete")
    {

	@Override
	public String getChineseName()
	{
	    return "删";
	}
	
    }, 
    MODIFY("modify")
    {

	@Override
	public String getChineseName()
	{
	    return "改";
	}
	
    },
   SELECT("select")
   {

    @Override
    public String getChineseName()
    {
	return "查";
    }
	
   };
    

    private String operType;
    
    public String getOperType()
    {
	return operType;
    }

    private OperationByEnum(String operType)
    {
	this.operType = operType;
    }
    
    public abstract String getChineseName();

}

此时就可以通过getChineseName来获取每个枚举值的中文名称了。

3、枚举类的常用API

所有枚举类均继承自java.lang.Enum类，其中比较常用的方法为：

下面的例子可以帮助我们理解valueOf方法的作用

 

String str = "ADD";
	OperationByEnum ob = OperationByEnum.valueOf(str);	
	System.out.println(ob == OperationByEnum.ADD);//TRUE
	
	String str2="add";
	OperationByEnum ob1 = OperationByEnum.valueOf(str2);
//Exception in thread "main" java.lang.IllegalArgumentException: No enum constant OperationByEnum.add

另外所有枚举类都会有一个values()方法，用于返回枚举的所有值

OperationByEnum[] array = OperationByEnum.values();
	for(OperationByEnum ob:array)
	{
	    System.out.println(ob.getChineseName());
	}

注：

Where does the Enum.valueOf(String) method come from?

/**
* Returns an array containing the constants of this enum 
* type, in the order they're declared.  This method may be
* used to iterate over the constants as follows:
*
*    for(E c : E.values())
*        System.out.println(c);
*
* @return an array containing the constants of this enum 
* type, in the order they're declared
*/
public static E[] values();

/**
* Returns the enum constant of this type with the specified
* name.
* The string must match exactly an identifier used to declare
* an enum constant in this type.  (Extraneous whitespace 
* characters are not permitted.)
* 
* @return the enum constant with the specified name
* @throws IllegalArgumentException if this enum type has no
* constant with the specified name
*/
public static E valueOf(String name);

4、枚举的特性总结

枚举类是一种特殊的JAVA类，枚举类中每声明一个枚举值就代表枚举类的一个实例对象。

与JAVA普通类一样，声明枚举类时也可以声明类的属性、方法、构造函数，但构造函数必须为私有。

枚举类也可以实现接口，继承抽象类。可以作为switch语句的参数。

若枚举类只有一个枚举值，则可以当做单例设计模式使用。

 

来源：oschina

链接：https://my.oschina.net/u/999023/blog/665398