[Java知识库]java基础笔记--多敲代码

Map集合的特点

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

public class MapDemo {
    public static void main(String[] args) {
        //创建集合对象
        Map<String,String> map = new HashMap<String,String >();
        //添加对象
        map.put("xuehao001","xiaoming1");
        map.put("xuehao002","xiaoming2");
        map.put("xuehao003","xiaoming3");
        map.put("xuehao003","xiaoming4"); //当键重复时，值被替换掉，即键是唯一的。
        //输出集合对象
        System.out.println(map);//{xuehao003=xiaoming4, xuehao002=xiaoming2, xuehao001=xiaoming1}
    }
}

Map集合的基本功能

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

public class MapDemo {
    public static void main(String[] args) {
        //创建集合对象
        Map<String,String> map = new HashMap<String,String >();
        //添加对象
        map.put("xuehao001","xiaoming1");
        map.put("xuehao002","xiaoming2");
        map.put("xuehao003","xiaoming3");
//        //删除键值对
//        map.remove("xuehao001"); //{xuehao003=xiaoming3, xuehao002=xiaoming2}
//        map.remove("xhehao004"); //{xuehao003=xiaoming3, xuehao002=xiaoming2}
//        //移除所有的键值对
//        map.clear(); //{}
        //判断是否包含指定的键
        System.out.println(map.containsKey("xuehao001")); //true
        System.out.println(map.containsKey("xuehao005")); //false
        //判断是否包含指定的值
        System.out.println(map.containsValue("xiaoming1")); //true
        //判断集合是否普为空
        System.out.println(map.isEmpty()); //false
        //集合的长度
        System.out.println(map.size()); //3

        //输出集合对象
        System.out.println(map);
    }
}

Map集合的获取功能

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapDemo {
    public static void main(String[] args) {
        //创建集合对象
        Map<String,String> map = new HashMap<String,String >();
        //添加对象
        map.put("xuehao001","xiaoming1");
        map.put("xuehao002","xiaoming2");
        map.put("xuehao003","xiaoming3");
        //根据键获取值
        System.out.println(map.get("xuehao001"));  //xiaoming1
        System.out.println(map.get("xuehao"));  //null
        //获取所有键的集合
        Set<String> keySet = map.keySet(); //map.keySet() ctrl+alt+v快捷键，
        for(String key:keySet){
            System.out.println(key);
        }
        //获取所有值得方法
        Collection<String> values = map.values();
        for (String val:values){
            System.out.println(val);
        }


        //输出集合对象
        System.out.println(map);
    }
}

Map集合遍历一

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapDemo {
    public static void main(String[] args) {
        //创建集合对象
        Map<String, String> map = new HashMap<String, String>();
        //添加对象
        map.put("xuehao001", "xiaoming1");
        map.put("xuehao002", "xiaoming2");
        map.put("xuehao003", "xiaoming3");

        //获取所有键的集合
        Set<String> keySet = map.keySet(); //map.keySet() ctrl+alt+v快捷键，
        //遍历集合获取每一个键
        for (String key : keySet) {
            //根据键去找值
            String val = map.get(key);
            System.out.println(key + "," + val);
        }
    }
}

Map集合遍历二

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapDemo {
    public static void main(String[] args) {
        //创建集合对象
        Map<String, String> map = new HashMap<String, String>();
        //添加对象
        map.put("xuehao001", "xiaoming1");
        map.put("xuehao002", "xiaoming2");
        map.put("xuehao003", "xiaoming3");

        //获取所有键值对对象的集合
        Set<Map.Entry<String, String>> entrySet = map.entrySet();
        //遍历键值对对象的集合，得到每个键值对对象
        for (Map.Entry<String, String >me : entrySet) {
            //根据键值对对象获取每个键和值
            String key = me.getKey();
            String val = me.getValue();
            System.out.println(key + "," + val);

        }
    }
}

ArrayLIst嵌套HashMap

import java.util.*;

public class MapDemo {
    public static void main(String[] args) {
        //创建arraylist集合
        ArrayList<HashMap<String,String>> array = new ArrayList<HashMap<String,String>>();
        //创建HashMap集合，并添加元素
        HashMap<String,String> hm = new HashMap<>();
        hm.put("student1","lily1");
        hm.put("student2","lily2");
        //把HashMap作为元素添加到ArrayList集合
        array.add(hm);

        HashMap<String,String> hm2 = new HashMap<>();
        hm2.put("student21","lily21");
        hm2.put("student22","lily22");
        //把HashMap作为元素添加到ArrayList集合
        array.add(hm2);

        HashMap<String,String> hm3 = new HashMap<>();
        hm3.put("student31","lily31");
        hm3.put("student32","lily32");
        //把HashMap作为元素添加到ArrayList集合
        array.add(hm3);

        //遍历ArrayList集合
        for (HashMap<String,String> hsm: array){
            Set<String> keySet = hsm.keySet();
            System.out.println(keySet);
            for (String key:keySet){  //按照HashMap遍历
                String val = hsm.get(key);
                System.out.println(key+","+val);
            }

        }

    }
}

HashMap嵌套ArrayList并遍历

import java.util.*;

public class MapDemo {
    public static void main(String[] args) {
        //创建HashMap集合
        HashMap<String, ArrayList<String>> hm = new HashMap<>();
        //创建ArrayList集合，并添加元素
        ArrayList<String> array1 = new ArrayList<String>();
        array1.add("array1a");
        array1.add("array1b");
        //把arraylist作为元素添加到HashMap
        hm.put("三国", array1);

        ArrayList<String> array2 = new ArrayList<String>();
        array2.add("array2a");
        array2.add("array2b");
        //把arraylist作为元素添加到HashMap
        hm.put("西游", array2);

        ArrayList<String> array3 = new ArrayList<String>();
        array3.add("array3a");
        array3.add("array3b");
        //把arraylist作为元素添加到HashMap
        hm.put("水浒", array3);

        //遍历HashMap集合
        Set<String> keySet = hm.keySet();
        for (String Key : keySet) {
            System.out.println(Key);
            ArrayList<String> val = hm.get(Key);
            for (String s : val) {
                System.out.println("\t" + s);
            }
        }
    }
}

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