Use of generics

 

package Generic;

import org.junit.Test;

import java.util.*;

public class GenericTest {
    @Test
    public void test1(){
// Cases where generics are not used ：
        ArrayList list = new ArrayList();
//         demand ： Store student scores 
        list.add(89);
        list.add(87);
        list.add(86);
        list.add(90);
//         problem 1： The type is not safe 
//        list.add("tom");

        for (Object obj : list){
//             problem 2： Type conversion exceptions may occur during strong conversion 
            int stuScore = (int)obj;
            System.out.println(stuScore);
        }

    }
//     Using generics in collections 
//     Generic is a class , Cannot be a basic data type 
    @Test
    public void test2(){
        ArrayList<Integer> list = new ArrayList<Integer>();

        list.add(78);
        list.add(79);
        list.add(80);
        list.add(77);
//         Type checking occurs at compile time , Ensure data security 
//        list.add("Tom");
        int stuScore;
        for(Integer score: list){
//             The forced rotation operation is avoided 
            stuScore = score;
            System.out.println(stuScore);
        }

        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()){
            stuScore = iterator.next();
            System.out.println(stuScore);

        }
    }

//     Using generics in collections ：HashMap
    @Test
    public void test3(){
        Map<String ,Integer> map = new HashMap<String, Integer>();

        map.put("Laowang",40);
        map.put("Xiaodu",10);
        map.put("Xiaoli",3);
        map.put("Laohei",95);

//         The nesting of generics appears 
        Set<Map.Entry<String, Integer>> entrySet = map.entrySet();
        Iterator<Map.Entry<String, Integer>> iterator = entrySet.iterator();
        while (iterator.hasNext()){
            Map.Entry<String, Integer> entry = iterator.next();
            String key = entry.getKey();
            Integer value = entry.getValue();
            System.out.println(key+"---"+value);
        }
    }
}