java泛型

//泛型
//Collection,map接口中都可以使用
//作用
//1)数据安全
//2)防止类型转换时出错
//list.add(默认是object)
//如果加了Double泛型则自动变成list.add(double...)会直接返回double类型的数据
//以Double泛型为例,如果不加泛型,则默认操作的是Object,如果加了Double泛型,则默认操作的是double类型
package daystudy;

import java.util.*;

public class Demo04 {
    private String key;

    public static void main(String[] args) {
       // List<String> list=new ArrayList<String>();
        List<String> list = new ArrayList<>() ;//jdk1.7之后提供类型推断
        //list.add("a");
        list.add("1");
        String s=list.get(0);
        System.out.println(s);

        Set<String> set=new HashSet<>();
        set.add("aa");
        //学号,名次
        Map<String,Integer> map=new HashMap<>();
        map.put("S01",3);
        map.put("S02",2);
        map.put("S03",1);

        Set<Map.Entry<String,Integer>> entries=map.entrySet();
        for (Map.Entry<String,Integer> entry:entries){
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        }
        System.out.println("迭代器......");
        //取key
        Set<String> keys=map.keySet();
        //遍历key
        Iterator<String> iterator=keys.iterator();
        while (iterator.hasNext()){
            String key= iterator.next();
            Integer value=map.get(key);
            System.out.println(key+"--"+value);
        }

    }
}

 

package col;

public class Person {
       private String name;
       private int age;
       private String city;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getCity() {
        return city;
    }
    public void setCity(String city) {
        this.city = city;
    }
    
    public Person(String name, int age, String city) {
        this.name = name;
        this.age = age;
        this.city = city;
    }
    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + ", city=" + city + "]";
    }
       
}

 

//根据人名,查询这个人的全部信息
package col;

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Demo5 {

    public static void main(String[] args) {
        Map<String, Person> map = new HashMap<>();
        Person p1 = new Person("张三", 23, "阜阳");
        Person p2 = new Person("李四", 24, "亳州");
        Person p3 = new Person("王二", 25, "宿州");
        Person p4 = new Person("麻子", 26, "安庆");
        Person p5 = new Person("大傻", 27, "泰安");
        // key:名字 value:人
        map.put("张三", p1);
        map.put("李四", p2);
        map.put("王二", p3);
        map.put("麻子", p4);
        map.put("大傻", p5);

        System.out.println("请输入人的名字");
        Scanner in = new Scanner(System.in);
        String name = in.next();
        Person person = map.get(name);
        System.out.println(person);
        // 如果不是null,在打印时会调用toString；如果是null，则不调用。
//     System.out.println(person.toString());

//     Set<String> names = map.keySet();
//     //在names中查找name
//     for(String n :names){
//         if(n.equals( name )){
//             Person person = map.get(n);
//             System.out.println(person);
//         }
//     }

    }
}