java 学习范例，实现空对象，和重写了，Comparator 的类实现了对于容器的比较

java 学习实例，实现空对象，和重写了，Comparator 的类实现了对于容器的比较！

少说废话，上代码：这是我学习的代码，如果有用，希望能帮助到大家！争取每个例子都是涉及的到新的技术，新的知识点！

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少说废话，上代码：

package com.xuyaowen.student;

import java.util.*;

//利用空对象的方式实现RTTI 的形式；
 
interface Null{
	
}

class Student{
	public static int allNum=0;
	private  int  id;
	private String name;
	private int age;
	Student(){
		System.out.println("这是普通的对象的形式");
	}
	Student(int id, String name, int age){
		allNum++;
		this.id = id;
		this.name = name;
		this.age = age;
	}
	
	public String toString(){
		return "ID " + this.id + " Name "+ this.name + " age " + this.age;
	}
	
	public void modifyAge(int addNum){
		System.out.println("你正在修改年龄！");
		this.age = this.age + addNum;
	}
	
	public int getAge(){
		return this.age;
	}
	
	public int getId(){
		return this.id;
	}
	
	public String getName(){
		return this.name;
	}
	public static class NullStudent extends Student implements Null {
		private NullStudent(){
			super(0, "none", 0);
		}
		
		public String toString(){
			return "NullPerson";
		}
	}
	public static final Student NULL = new NullStudent();
	//由此可见可以直接调用  Student.NULL ; 即可以实现其中的情况；
}
//定义一个比较的接口；
class StudentIdComparator implements Comparator<Student>{
	public int compare(Student std1, Student std2){
		return (std1.getId() - std2.getId());
	}
}

class StudentAgeComparator implements Comparator<Student>{
	public int compare(Student std1, Student std2){
		return (std1.getAge() - std2.getAge());
	}
}

class Admin{
	private LinkedList<Student> students = new LinkedList<Student>();
	Admin(){
		System.out.println("启动学生管理员模式！");
	}
	//添加一个学生；
	public void addStudent(Student std){
		students.add(std);
		System.out.println("成功添加学生："+std.getName());
	}
	//增加所有人的年龄；
	public void addAge(int num){
		if(num<0){
			System.out.println("年龄增长应为正数！");
		}else{
			for(Student a: students){
				a.modifyAge(num);
			}
			System.out.println("年龄增长了  "+ num);
		}
		
	}
	//得到人群之中年龄大于20 的人数；
	public int getNum20(){
		int count = 0;
		for(Student a: students){
			if(a.getAge()>20){
				count++;
			}
		}
		System.out.println("当前学生中年龄大于20的有  "+ count + " 人");
		return count;
	}
	//通过id来给学生进行排序的操作
	public void sortStudents(String tag){
		if("id" == tag){
			System.out.println("你是以id作为排序依据");
			Collections.sort(students, new StudentIdComparator());//利用id进行排序；
		}else if("age" == tag){
			Collections.sort(students, new StudentAgeComparator());//利用年龄进行排序
			System.out.println("你是以age作为排序依据");
		}else{
			 System.out.println("警告!这个类不能通过 以  "+ tag + " 为依据进行排序");
		}
	}
	
	public String toString(){
		StringBuilder result = new StringBuilder();
		for(Student a: students){
			result.append(a.toString()+" ");
			result.append('\n');//这之中换行符还真得可以被解析出来啊！
		}
		return result.toString();
	}
}

public class School {
	public static Admin admin = new Admin();
	private static Scanner scan =  new Scanner(System.in);
	public static void main(String[] args){
		System.out.println("本程序只是涉及接口展示，不涉及程序的交互！");
		scan.nextLine();
		System.out.println("这是学生类的测试");
			admin.addStudent(new Student(123, "jack0", 12));
			admin.addStudent(new Student(125, "jack2", 13));
			admin.addStudent(new Student(121, "jack3", 16));
			admin.addStudent(new Student(120, "jack4", 20));
			admin.addStudent(new Student(124, "jack5", 22));
			admin.addStudent(new Student(126, "jack6", 21));
			System.out.println(admin);
			scan.nextLine();
		System.out.println("按照学生id进行排序：");
			admin.sortStudents("id");
			System.out.println(admin);
			scan.nextLine();
		System.out.println("按照学生age进行排序：");
			admin.sortStudents("age");
			System.out.println(admin);
			scan.nextLine();
		System.out.println("所有学生年龄都增加1");
			admin.addAge(1);
			System.out.println(admin);
			scan.nextLine();
		System.out.println("统计所有年龄大于20岁的人数");
			admin.getNum20();
			scan.nextLine();
		System.out.println("程序接口展示结束！");
	}
}