检查地图中存在的值列表的最佳方法

Question

I have a Collection containing some values.

Collection<String> myList = new ArrayList<String>();
myList.add("a");
myList.add("b");
myList.add("c");

I created a Map which has some values:

Map<String, String> myMap = new HashMap<String, String>();
myMap.put("r", "r");
myMap.put("s","m");
myMap.put("t", "n");
myMap.put("a", "o");

I want to check whether the values in the list are present as a key in the map? The way which I know using Java is to iterate through the list and check if the map contains that particular value using myMap.containsKey(). I want to know if there is anything out there using streams or for-each which saves me either lines of code or an efficient way of doing that! Thanks for thinking through it.

EDIT: I want to store all the elements present in the myList which are not there in myMap . So my output can be a list eg in this scenario [b,c] .

Answer 1

Set<String> mySet = new HashSet<String>(myList);
myMap.keySet().containsAll(set);

Does this answer your question?

After your EDIT: Given the above, you can get the keys difference between two sets using the answer to one of these questions:- What is the best way get the symmetric difference between two sets in java? or Getting the difference between two sets

Answer 2

...whether the values in the list are present as a key in the map?

As I understand this case as you need to verify whether all the elements in the myList are present as the myMap 's keys. Use the Stream::allMatch method:

myMap.keySet().stream()
              .allMatch(myList::contains);

... which is the same as:

myList.keySet().containsAll(myMap);

EDIT: I want to store all the elements present in the myList which are not there in myMap . So my output can be a list eg in this scenario [b,c].

You don't want to store all the elements in the myList since they are already there filled. You want to retain those elements using List::retainAll .

myList.retainAll(myMap.keySet());

Printing out the result of myList would produce only the keys , that have been found in the original myList and also as a key of myMap .

Answer 3

For: I want to store all the elements present in the myList which are not there in myMap. So my output can be a list eg in this scenario [b,c]

Create a duplicate myList and remove all myMap's keys

        Collection<String> newList = new ArrayList<>(myList);
        newList.removeAll(myMap.keySet());
        System.out.println(newList);

Answer 4

Using Streams you can do something like:

List<String> result = myList.stream()                
                            .filter(myMap::containsKey)
                            .collect(Collectors.toList());

The resulting list will contain the values that are present as key in the map.

In case you want to print them out you can do:

result.forEach(System.out::println);