单例实现思路

1、构造方法私有化；2、实例化的变量引用私有化；3、获取实例的方法共有。

Hello.java
public enum Hello {
    HELLO;
}

通过 javac Hello.java
javap: 是一个字节码反汇编工具，提供类的结构、方法签名和字节码的详细信息，但不生成源代码。
javap -c Hello >1.txt
Compiled from "Hello.java"
public final class test.Hello extends java.lang.Enum<test.Hello> {
  public static final test.Hello HELLO;

  public static test.Hello[] values();
    Code:
       0: getstatic     #1                  // Field $VALUES:[Ltest/Hello;
       3: invokevirtual #2                  // Method "[Ltest/Hello;".clone:()Ljava/lang/Object;
       6: checkcast     #3                  // class "[Ltest/Hello;"
       9: areturn

  public static test.Hello valueOf(java.lang.String);
    Code:
       0: ldc           #4                  // class test/Hello
       2: aload_0
       3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
       6: checkcast     #4                  // class test/Hello
       9: areturn

  static {};
    Code:
       0: new           #4                  // class test/Hello
       3: dup
       4: ldc           #7                  // String HELLO
       6: iconst_0
       7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
      10: putstatic     #9                  // Field HELLO:Ltest/Hello;
      13: iconst_1
      14: anewarray     #4                  // class test/Hello
      17: dup
      18: iconst_0
      19: getstatic     #9                  // Field HELLO:Ltest/Hello;
      22: aastore
      23: putstatic     #1                  // Field $VALUES:[Ltest/Hello;
      26: return
}

jad 反编译器，可以将 .class 文件转换为接近源代码的 .java 文件
jad -p Hello.class >result.java 获取的编译后的数据
public final class Hello extends Enum {
    public static Hello[] values() {
        return (Hello[]) $VALUES.clone();
    }
    public static Hello valueOf(String s) {
        return (Hello) Enum.valueOf(Hello, s);
    }
    private Hello(String s, int i) {
        super(s, i);
    }
    public static final Hello HELLO;
    private static final Hello $VALUES[];
    static {
        HELLO = new Hello("HELLO", 0);
        $VALUES = (new Hello[]{
                HELLO
        });
    }
}

优点：避免反射、序列化问题