如果具有特定于常量的方法，为什么要为每种枚举类型创建不同的类文件? [英] Why different class files are created for each enum type if they have constant-specific method?

问题描述

我有一个枚举

enum OperationsType {
  ADD("+"), SUB("-"), DIV("/"), MUL("*");

  private String opcodes;

  private OperationsType(String opcodes) {
    this.opcodes = opcodes;
  }

  public String toString() {
    return this.opcodes;
  }
}

这里所有的枚举类型都没有任何constant Specific method，因此javac仅为enum创建一个类文件，作为

Here all enum type doesn't have any constant Specific method so javac is creating only one class file for enum as

OperationsType.class

但是如果我为所有枚举类型在同一代码中添加constant Specific method，则javac正在创建5个类文件.

but if i add constant Specific method in same code for all enum type then javac is creating 5 class files.

Operations.class
Operations$1.class
Operations$2.class
Operations$3.class
Operations$4.class

获取以下代码

enum Operations {
ADD("+") {
    public double apply(double a, double b) {
        return a + b;
    }
},
SUB("-") {
    public double apply(double a, double b) {
        return a - b;
    }
},
DIV("/") {
    public double apply(double a, double b) {
        return a / b;
    }
},
MUL("*") {
    public double apply(double a, double b) {
        return a * b;
    }
};

private String opcodes;

private Operations(String opcodes) {
    this.opcodes = opcodes;
}

public abstract double apply(double a, double b);
}

所以我对此感到怀疑，为什么compiler如果每个enum type具有constant Specific method，则为每个enum type创建4个不同的类，而如果它们没有constant Specific method，却不创建不同的类呢?

So i have doubt that why compiler has created 4 different classes for each enum type if they are having constant Specific method but not creating different classes if they don't have constant Specific method?

推荐答案

具有特定于常量的方法的枚举是使用匿名内部类实现的.如 Java语言规范中所述:

Enums with constant-specific methods are implemented using anonymous inner classes. As mentioned in The Java Language Specification:

枚举常量的可选类主体隐式定义一个 匿名类声明(§15.9. 5 ) 封闭的枚举类型.班级机构受以下一般规则的约束: 匿名类；特别是它不能包含任何构造函数.

The optional class body of an enum constant implicitly defines an anonymous class declaration (§15.9.5) that extends the immediately enclosing enum type. The class body is governed by the usual rules of anonymous classes; in particular it cannot contain any constructors.

匿名内部类是通过创建名称如OuterClass$1，OuterClass$2等的类文件来实现的，在枚举的情况下正是这种情况.

Anonymous inner classes are implemented by creating class files with names like OuterClass$1, OuterClass$2 etc., and this is exactly what happens in the case of the enum.

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