코드 위치: ch1/4.java

class Singleton {
    private static class singleInstanceHolder {
        private static final Singleton INSTANCE = new Singleton();
    }
    public static synchronized Singleton getInstance() {
        return singleInstanceHolder.INSTANCE;
    }
}

public class HelloWorld { 
     public static void main(String[] args) { 
        Singleton a = Singleton.getInstance(); 
        Singleton b = Singleton.getInstance(); 
        System.out.println(a.hashCode());
        System.out.println(b.hashCode());  
        if (a == b) {
         System.out.println(true); 
        } 
     }
}
/*
705927765
705927765
true
*/