Android设计模式-单例模式

public class Singleton {
private static Singleton instance;

private Singleton() {
}

public static synchronized Singleton getInstance(){
if(instance == null){
instance = new Singleton();
}
return  instance;
}
}

public class Singleton {
private static Singleton mInstance = null;

private Singleton() {
}

public void doSomething() {
System.out.println("do sth.");
}

public static Singleton getInstance() {
if (mInstance == null) {
synchronized (Singleton.class) {
if (mInstance == null) {
mInstance = new Singleton();
}
}
}
return mInstance;
}
}

public class Singleton {
private Singleton() {
}

public static Singleton getInstance() {
return SingletonHolder.sInstance;
}

public static class SingletonHolder {
private static final Singleton sInstance = new Singleton();
}
}