The Map of traversal methods

 public static void main(String[] args) {
    
        Map<String, String> map = Maps.newHashMap();
        map.put("a", "1");
        map.put("b", "2");
        map.put("c", "3");

        // 方法一 Iterator
        System.out.println("迭代器");
        Iterator it = map.entrySet().iterator();
        while (it.hasNext()) {
    
            Map.Entry entry = (Map.Entry) it.next();
            System.out.println("key=" + entry.getKey() + " value=" + entry.getValue());
        }

        // 方法二 entrySet
        System.out.println("entrySet");
        for (Map.Entry<String, String> entry : map.entrySet()) {
    
            System.out.println("key=" + entry.getKey() + " value=" + entry.getValue());
        }

        // 方法三 keySet
        System.out.println("entrySet");
        for (String s : map.keySet()) {
    
            System.out.println("key=" + s + " value=" + map.get(s));
        }

        // 方法四 values
        System.out.println("values");
        for (String s : map.values()) {
    
            System.out.println("value =" + s);
        }

        //方法五 forEach
        System.out.println("forEach");
        map.forEach((key,value) ->{
    
            System.out.println(key+":"+value);
        });
    }

Map接口概述

Map是一个将键映射到值的对象.映射不能包含重复的键：每个键最多可以映射到一个值.It models mathematical function abstractions.该MapInterfaces are included for basic operations（例如put、get、remove、 containsKey、containsValue、size和empty）、批量操作（例如putAll和clear）and collection view（例如keySet、entrySet和values）的方法.

Java The platform contains three genericsMap实现： HashMap、 TreeMap和 LinkedHashMap.Their behavior and performance are the sameSetas described in the interface HashSet、TreeSet和LinkedHashSet完全类似.

Map集合Lambda操作：

// Group employees by department
Map<Department, List<Employee>> byDept = employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment));

// Calculate the sum of wages by department
Map<Department, Integer> totalByDept = employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment,
Collectors.summingInt(Employee::getSalary)));

// Divide students into qualified and unqualified
Map<Boolean, List<Student>> passFailing = students.stream()
.collect(Collectors.partitioningBy(s -> s.getGrade()>= PASS_THRESHOLD));

// 按城市对 Person 对象进行分类
Map<String, List<Person>> peopleByCity
         = personStream.collect(Collectors.groupingBy(Person::getCity));

// Cascading collectors
Map<String, Map<String, List<Person>>> peopleByStateAndCity
  = personStream.collect(Collectors.groupingBy(Person::getState,
  Collectors.groupingBy(Person::getCity)))

亦余心之所善兮,虽九死其犹未悔.