单例模式有哪些实现方式？

publicclassSingleton {
privateSingleton() {}
privatestaticvolatileSingletoninstance=null;
publicstaticSingletongetInstance() {
if (instance==null) {
synchronized (Singleton.class) {
if (instance==null) { 
instance=newSingleton();
                }
            }
        }
returninstance;
    }
}

publicenumSingletonEnum {
INSTANCE;
publicSingletonEnumgetInstance(){
returnINSTANCE;
    }
}

publicclassSingleton {
privateSingleton() {
    }
privatestaticclassSingletonHolder {
privatestaticfinalSingletonINSTANCE=newSingleton();
    }
publicstaticfinalSingletongetInstance() {
returnSingletonHolder.INSTANCE;
    }
}

publicclassSingleton {
privateSingleton() {}
privatestaticSingletoninstance=newSingleton();
publicstaticSingletongetInstance() {
returninstance;
    }
}