How to get few random keys from HashMap

I have a map with titles. I want to print 10 random keys from my hashmap.

For example my map (String, Object) contains 100 pairs: "A, new Object(...)", "B, ...", "C, ..." etc.

I want to get 10 random keys from this map and append it to one string.

So my string should looks like: "A\nD\nB".

A quick way to get random 10 keys without repetition is putting the keys in a list and using Collections.shuffle to shuffle the list.

Map<String, Object> map = ...yourmap
ArrayList<String> keys = new ArrayList<>(map.keySet());
Collections.shuffle(keys);
List<String> randomTenKeys = keys.subList(0, 10);

Creating a list of all keys and shuffling it is not the most efficient thing you can do. You can do it in a single pass with a reservoir sampling algorithm. I haven't looked into it but you can probably find an implementation in some Apache or Guava library.

Joni's answer is quite good and short. But, here is a fully working example if you'd like. I split your problem into two methods - one to return a list of randomly selected keys and another to print keys in whichever way you like. You could combine the two methods into one. But, its better to keep them separate.

import java.util.*;
import java.util.stream.IntStream;
public class Test {
public static void main(String [] args){
Map<String, Object> map = new HashMap<>();
//You can use for loop instead to make a map of String, Integer. 
IntStream.rangeClosed(0, 9).forEach(i -> map.put(i +"", i));//Map of 10 numbers.
List<String> keys = getRandomKeys(map, 3);
String allKeys = combineKeys(keys, "\n");
System.out.println(allKeys);
}
public static List<String> getRandomKeys(Map<String, Object> map, int keyCount) {
List<String> keys = new ArrayList<>(map.keySet());
for(int i = 0; i < map.size()-keyCount; i++){
int idx = (int) ( Math.random() *  keys.size() );
keys.remove(idx);
}
return keys;
}
public static String combineKeys(List<String> keys, String separator){
String all = "";
for(int i = 0; i < keys.size() - 1; i++){
all = all + keys.get(i) + separator;
}
all += keys.get(keys.size()-1);//last element does not need separator.
return all;
}
}

This is a complementary answer to Joni's answer. Use String:join to join the randomTenKeys.

Given below is Joni's answer:

Map<String, Object> map = ...yourmap
ArrayList<String> keys = new ArrayList<>(map.keySet());
Collections.shuffle(keys);
List<String> randomTenKeys = keys.subList(0, 10);

and the complementary answer is:

String joinedKeys = String.join("\n", randomTenKeys);

HashMap Stores the values already in unsorted order it is random.
you can directly use

for(Map.Entry entry : map.entrySet())
str.append(entry.getKey()+" "+entry.getValue());

however if you want new order every time you can shuffle your data.
For Shuffle you need to get all keys in a array or list
Then you can shuffle that list and iterate over that list to get values from hashmap

Set<String> keys = myMap.keySet();
String combined = "";
for (int i=0; i<10; i++)
{
int random = (int)(Math.random() * keys.size());
String key = keys.get(random);
combined += key + "\n";
keys.remove(random);
}