Java Map双列集合使代码更高效

public static void main(String[ ] args) {
        //  ○ Object put(Object key,Object value)：将指定key-value添加到(或修 改)当前map对象中
        //  ○ void putAll(Map m):将m中的所有key-value对存放到当前map中
        //  ○ Object remove(Object key)：移除指定key的key-value对，并返回value
        //  ○ void clear()：清空当前map中的所有数据
        //创建集合对象
        Map<String, Student> studentMap = new HashMap<>();
        //将对应的对象放到map中
        studentMap.put("N001", new Student("叮当", 7));
        studentMap.put("N002", new Student("糖糖", 7));
        studentMap.put("N003", new Student("熙熙", 7));
        //remove
        studentMap.remove("N002");
        System.out.println("remove==" + studentMap);
        //put
        studentMap.put("N004", new Student("狗蛋", 7));
        System.out.println("put==" + studentMap);
        //putAll 如果键重复，会把原map中的节点替换
        Map<String, Student> studentMap2 = new HashMap<>();
        studentMap.put("N001", new Student("一一", 7));
        studentMap.put("N002", new Student("陌陌", 7));
        studentMap.putAll(studentMap2);
        System.out.println("putAll == " + studentMap);
        //clear
        studentMap.clear();
        System.out.println("clear == " + studentMap);
    }
}

public class Demo2Map {
    public static void main(String[ ] args) {
        //创建集合对象
        Map<String, Student> studentMap = new HashMap<>();
        //将对应的对象放到map中
        studentMap.put("N001", new Student("叮当", 7));
        studentMap.put("N002", new Student("糖糖", 7));
        studentMap.put("N003", new Student("熙熙", 7));
        //get
        Student n001 = studentMap.get("N001");
        System.out.println("001==" + n001 );
        //containsKey
        boolean isContainsKey = studentMap.containsKey("N002");
        System.out.println("containsKey结果 == " + isContainsKey);
        //containsValue
        boolean containsValue = studentMap.containsValue(new Student("熙熙", 7));
        System.out.println("containsValue结果 == " + containsValue);
        //size
        int size = studentMap.size();
        System.out.println("size == "+ size);
        //isEmpty
        boolean isEmpty = studentMap.isEmpty();
        System.out.println("isEmpty == " + isEmpty);
        //equals
        boolean equals = studentMap.get("N002").equals(new Student("糖糖", 7));
        System.out.println("equals == " + equals);
    }
}

public class Demo3Map {
        public static void main(String[ ] args) {
                //方法一:keySet遍历key+value：
                //创建集合对象
                Map<String, Student> studentMap = new HashMap<>();
                //将对应的对象放到map中
                studentMap.put("N001", new Student("叮当", 7));
                studentMap.put("N002", new Student("糖糖", 7));
                studentMap.put("N003", new Student("熙熙", 7));
                //获取 map集合中的所有key
                Set<String> strings = studentMap.keySet();
                //遍历 key的集合，通过get()获取没一个value
                for (String key : strings) {
                    Student student = studentMap.get(key);
                    System.out.println("studentmap == key="+key+" value="+ student);
                }
                //方法二:entrySet遍历key+value：
                Set<Map.Entry<String, Student>> entries = studentMap.entrySet();
                for (Map.Entry<String, Student> entry : entries) {
                    String key = entry.getKey();
                    Student value = entry.getValue();
                    System.out.println("studentmap 方式2 == key="+key+" value="+ value);
                }
        }
    }