Java-Map集合

基本使用

public class Demomap {
    public static void main(String[] args) {
        Map<String,Integer> maps = new HashMap<>();//无序
//        Map<String,Integer> maps = new LinkedHashMap<>();有序
        maps.put("a",1);
        maps.put("b",2);
        maps.put("c",3);
        maps.put(null,null);
        System.out.println(maps);
    }
}

常用api

public class Demomap {
    public static void main(String[] args) {
        Map<String,Integer> maps = new HashMap<>();
        Map<String,Integer> maps1 = new HashMap<>();
        maps.put("a",1);
        maps.put("b",2);
        maps.put("c",3);
        maps1.put("d",1);
        maps1.put("f",2);
        maps1.put("g",3);
        System.out.println(maps);
//        maps.clear();
        maps.remove("a");// 根据key删除
        System.out.println(maps.get("b")); //根据key取值
        Set<String> keySets = maps.keySet();// 取出所有的key
        System.out.println(keySets);
        Collection<Integer>  values = maps.values(); //取出所有的value
        System.out.println(values);
        System.out.println(maps.containsKey("b"));
        System.out.println(maps.containsValue(1));
        maps.putAll(maps1); //maps1所有元素添加到mpas
        System.out.println(maps);
        System.out.println(maps.size());
        System.out.println(maps.isEmpty());
    }
}

遍历

public class Demomap1 {
    public static void main(String[] args) {
        Map<String,Integer> map = new HashMap<>();
        map.put("a",1);
        map.put("b",2);
        map.put("c",3);
//        遍历方式一,通过key取值
        Set<String> mapkey = map.keySet();
        for (String key : mapkey) {
            System.out.println(map.get(key));
        }

//        遍历方式二
        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        for (Map.Entry<String, Integer> entrie : entries){
            System.out.println("key:" + entrie.getKey());
            System.out.println("value:" + entrie.getValue());
        }

//       遍历方式三
        map.forEach(new BiConsumer<String, Integer>() {
            @Override
            public void accept(String key, Integer value) {
                System.out.println("这是key"+key);
                System.out.println("这是value"+value);
            }
        });
//      遍历方式三，简写
        map.forEach((key,value)-> {
            System.out.println("这是key"+key);
            System.out.println("value"+value);
        });
    }
}

排序

//只能针对key进行排序
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.TreeMap;

public class Treesort {
    public static void main(String[] args) {
       //自定义排序 
        Map<Student,Integer> maps = new TreeMap<>((o1,o2)->o2.getHeight() - o1.getHeight());
        maps.put(new Student(1),1);
        maps.put(new Student(2),1);
        maps.put(new Student(3),1);
        System.out.println(maps);
    }
}


class Student{
    private int height;

    public Student(int height) {
        this.height = height;
    }

    public int getHeight() {
        return height;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return height == student.height;
    }

    @Override
    public int hashCode() {
        return Objects.hash(height);
    }

    @Override
    public String toString() {
        return "Student{" +
                "height=" + height +
                '}';
    }
}