单例

饿汉式
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton (){}
public static Singleton getInstance() {
return instance;
}
}

延迟加载
public class Singleton {
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
private Singleton (){}
public static final Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}