static {
    语句
}

饿汉式(开发建议使用)
class Student{
    private int age;
    private Student(){};
    private static Student s = new Student();
    public static Student getInstance(){
        return s;
    }
    public void setAge(int age){
        this.age = age;
    }
    public int getAge(){
        return age;
    }
}

懒汉式(对象延迟加载,调用实例化方法时才实例化对象)
class Single {
    private static Single s = null;
    private Single(){};
    public static Single getInstance(){
        if (s === null){
            synchronized(Single.class){
                if (s === null) {
                    s = new Single();
                }
            }
        }
        return s;
    }
}

abstract class Student {
    abstract void study();
}

abstract class GetTime{
    public final void getTime(){
        long start = System.currentTimeMills();
        runcode();
        long end = System.currentTimeMills();
        System.out.println(end-start);
    }
    public abstract void runcode();
}

interface Inter{
    public static final int x = 3;
    public abstract void show();
}

Animal a = new Cat();
Cat c = Cat(a);

synchronized(obj){ //obj 同步锁
     
}

class Single{
    private static Single s = null;
    private Single(){};
    public static Single getInstence(){
        if (s === null){
           syncnorized(Single.class){
               if (s === null) {
                   s = new Single();
               }
           }
        }
       return s;
    }
}