Map集合

Map集合

Map接口的特点：

用于存储任意键值对(Key- Value)

键：无序、无下标、不允许重复(唯一)

值：无序、无下标、允许重复

Map(Intertace)

1.HashMap(Class)

2.SortedMap(Intertace)   TreeMap(Class)

Map父接口

特点：存储一对数据(Key-Value)， 无序、无下标，键不可重复，值可重复

方法：

V put(K key,V value)  //将对象存入到集合中，关联键值。key重复则覆盖原值

Object get (Object key)  //根据键获取对应的值

Set<K>  //返回所有key

Collection<V> values()  //返回包含所有值的Collection集合

Set<Map. Entry<K, V>>  //键值匹配的Set集合

eg : 

public class Demo01 {
    public static void main(String[] args) {
        //创建Map集合
        Map<String,String> map = new HashMap<>();
        //1.添加元素
        map.put("cn","中国");
        map.put("uk","英国");
        map.put("usa","美国");
        map.put("cn","zhongguo");
        System.out.println("元素个数："+map.size());
        System.out.println(map.toString());
        //2.删除
        //map.remove("usa") ;
        //System.out.println(map.toString() );
        //3.遍历
        //3.1使用keySet();
        Set<String> keySet = map.keySet();
        for (String key : map.keySet()) {
            System.out.println(key+"---"+map.get(key));
        }
        //3.2使用entrySet();方法  其效率要高一些
        //Set<Map.Entry<String,String>> entries = map.entrySet();
        for (Map.Entry<String,String> entry : map.entrySet()) {
            System.out.println(entry.getKey()+"--------"+entry.getValue());
        }
        //4.判断
        System.out.println(map.containsKey("cn"));
        System.out.println(map.containsKey("法国"));
        System.out.println(map.isEmpty());//判断是否为空
    }
}

Map集合的实现类

HashMap [ 重点]：

JDK1.2版本，线程不安全，运行效率快；允许用null作为key或是value

Hashtable：

DK1.0版本，线程安全，运行效率慢；不允许null作为key或是value

Properties

Hashtable的子类，要求key和value都是String。通常用于配置文件的读取

eg : 

public class Student implements Comparable<Student>{
    private String name;
    private int stoNo;
    public Student() {
    }
    public Student(String name, int stoNo) {
        this.name = name;
        this.stoNo = stoNo;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getStoNo() {
        return stoNo;
    }
    public void setStoNo(int stoNo) {
        this.stoNo = stoNo;
    }
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", stoNo=" + stoNo +
                '}';
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return stoNo == student.stoNo &&
                Objects.equals(name, student.name);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, stoNo);
    }
    @Override
    public int compareTo(Student o) {
        int n2 = this.stoNo-o.getStoNo();
        return n2;
    }
}

public class Demo02 {
    public static void main(String[] args) {
        //创建集合
        HashMap<Student,String> students = new HashMap<Student,String>();
        //1.添加元素
        Student s1 = new Student("孙悟空",100);
        Student s2 = new Student("猪八戒",80);
        Student s3 = new Student("沙和尚",70);
        students.put(s1,"北京");
        students.put(s2,"上海");
        students.put(s3,"杭州");
        //students.put(s3,"南京");
        students.put(new Student("沙和尚",70),"杭州");//内存地址不一样
        System.out.println("元素个数："+students.size());
        System.out.println(students.toString());
        //2.删除
        //students.remove(s1);
        //System.out.println(students.toString());
        //3.遍历
        //3.1使用keySet();
        for (Student key : students.keySet()) {
            System.out.println(key.toString()+"---"+students.get(key));
        }
        //3.2使用entrySet();方法
        for (HashMap.Entry<Student,String> entry : students.entrySet()) {
            System.out.println(entry.getKey()+"--"+entry.getValue());
        }
        //4.判断
        System.out.println(students.containsKey(new Student("猪八戒",80)));
        System.out.println(students.containsKey("上海"));
        System.out.println(students.isEmpty());//判断是否为空
    }
}

public class Demo03 {
    public static void main(String[] args) {
        //新建集合(定制比较)
        TreeMap<Student,String> treeMap = new TreeMap<Student,String>();
        //1.添加元素
        Student s1 = new Student("孙悟空",100);
        Student s2 = new Student("猪八戒",80);
        Student s3 = new Student("沙和尚",70);
        treeMap.put(s1,"北京");
        treeMap.put(s2,"上海");
        treeMap.put(s3,"杭州");
        System.out.println("元素个数："+treeMap.size());
        System.out.println(treeMap.toString());
        //2.删除
        //treeMap.remove(s1);
        //System.out.println(treeMap.toString());
        //3.遍历
        //3.1使用keySet();
        for (Student key : treeMap.keySet()) {
            System.out.println(key+"---"+treeMap.get(key));
        }
        //3.2使用entrySet();方法
        for (Map.Entry<Student,String> entry : treeMap.entrySet()) {
            System.out.println(entry.getKey()+"-----"+entry.getValue());
        }
        //4.判断
        System.out.println(treeMap.containsKey(new Student("沙和尚",70)));
        System.out.println(treeMap.isEmpty());//判断是否为空
    }
}