java - java中 "java.lang.Enum"类型对象的身份和状态是什么？

Question

在下面的代码中，

class Example{}
Example ex1 = new Example();
Example ex2 = new Example();
bool compareAddress = (ex1 == ex2);

此处，compareAddress给出false，因为identity是ex1指向的对象的内存地址，并且ex2。 class Example{} 类型的对象内的字段值(如果有)定义该对象的状态。

但是在下面的代码中，

enum Company{
    EBAY(30), PAYPAL(10), GOOGLE(15), YAHOO(20), ATT(25);
    private int value;

    private Company(int value){
        this.value = value;
    }

}
Company x1 = Company.EBAY;
Company x1 = Company.EBAY;
bool compareValues = (x1 == x2);

此处，compareValues 给出 true。

那么，x1和x2指向的对象的身份和状态是什么？

Answer 1

An enum is a special type of class.

例如

enum Company {
    EBAY
}

将编译为

final class Company extends Enum<Company> {
    public final static Company EBAY = new Company(); // almost, this constructor invocation will have arguments
}

在 Java 进程的剩余生命周期中，EBAY 字段将保持相同的引用值，即对 Company 实例的引用。

来自JLS ,

Because there is only one instance of each enum constant, it is permitted to use the == operator in place of the equals method when comparing two object references if it is known that at least one of them refers to an enum constant.