将 map 的条目分组到列表中

Question

Let's suppose i have a HashMap with some entries:

Map hm= new HashMap();
hm.put(1,"ss");
hm.put(2,"ss");
hm.put(3,"bb");
hm.put(4,"cc");
hm.put(5,"ss");

i want output like:

[{1,ss},{2,ss},{5,ss}]

Is it possible?

Answer 1

Of course it is:

List<Map.Entry<Integer,String>> list =
    hm.entrySet().stream().collect(Collectors.toList());

You should change the definition of your Map to:

Map<Integer,String> hm = new HashMap<>();

PS You didn't specify whether you want all the entries in the output List , or just some of them. In the sample output you only included entries having "ss" value. This can be achieved by adding a filter:

List<Map.Entry<Integer,String>> list =
    hm.entrySet().stream().filter(e -> e.getValue().equals("ss")).collect(Collectors.toList());
System.out.println (list);

Output:

[1=ss, 2=ss, 5=ss]

EDIT: You can print that List in the desired format as follows:

System.out.println (list.stream ().map(e -> "{" + e.getKey() + "," + e.getValue() + "}").collect (Collectors.joining (",", "[", "]")));

Output:

[{1,ss},{2,ss},{5,ss}]

Answer 2

Firstly, you declare your HashMap like this:

HashMap<Integer, String> hm = new HashMap<>();

Then after putting the key and the values you can print the whole HashMap it like this:

System.out.println("Mappings of HashMap hm1 are : " + hm);

If you want to print the value where the key is equal to 1 then:

if (hm.containsKey(1)) { 
            String s = hm.get(1); 
            System.out.println("value for key 1 is: " + s); 
        }