java8 Map集合对象的遍历

Map集合的遍历
a) 键找值
i. 获取所有键
ii. 遍历键的集合，得到每一个键的值
b) 键值对对象找值和键
i. 遍历所有的键值对对象的集合
ii. 根据键值对对象获取键和值

实例：

public class HashMapDemo {
	public static void main(String[] args) {
		HashMap<String, Object> hm=new HashMap<String,String>();
		hm.put("1", "Hello");
		hm.put("2", "World");
		hm.put("3", "see");
		hm.put("4", "Java");
		
		//键找值遍历
		Set<String> set1=hm.keySet();
		for(String key:set1) {
			String value=hm.get(key);
			System.out.println(key+"--"+value);
		}
		System.out.println("----------------------------");
		
		//键值对象找键和值遍历
		Set<Map.Entry<String, String>> set2=hm.entrySet();
		for(Map.Entry<String, String> me:set2) {
			System.out.println(me.getKey()+"--"+me.getValue());
		}
        
		// 遍历获取相关对象集合
		set1= headMap.entrySet().stream().map(o -> {
			return o.getKey().equals("value匹配值") ? o.getValue().toString() : null;
		}).collect(Collectors.toList());

        // 获取对象
        List<Student> students = new ArrayList<>();
        students.add(new Student(1,"张三",90));
        students.add(new Student(2,"李四",60));
        students.add(new Student(3,"王五",30));
        students.add(new Student(4,"赵六",85));
        int studentId = 3;
        Student student = students.stream().filter(o -> o.getId() == studentId).findAny().orElse(null);
	}
}

public class Student {
    private int id;
    private String name;
    private int score;
    ...
}