单例模式（确保对象的唯一性）

饿汉式，先初始化对象

class Student{  
	private int age;  
	private Student(){}  
	private final static Student s = new Student();   //加了final staitic 的属性不可以被改变
	public static Student getInstance(){  
		return s;  
	}  
	public void setAge(int age){  
		this.age = age;  
	}  
	public int getAge(){  
		return this.age;  
	}  
}  
public class Demo{  
	public static void main(String[] args){  
		Student s = Student.getInstance();  
		Student s1 = Student.getInstance();  
		s.setAge(20);  
		System.out.println(s1.getAge());  
	}  
}  

懒汉式，第二种是对象是被调用时，才初始化，也叫做对象的延时加载

class Student{  
	private int age; 
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	private Student(){}  
	private static Student s = null;  
	public static Student getInstance(){   //synchronized   
		if(s==null)                                 //判断是否已经存在
			synchronized(Student.class){  
				if(s==null)  
					s = new Student();  
			}  
		return s;                                //返回对象
	}  
}  
public class Demo{  
	public static void main(String[] args){  
		Student s = Student.getInstance();  
		Student s1 = Student.getInstance();  
		s.setAge(20);  
		System.out.println(s1.getAge());  
	}  
}