Show multiple values for multiple keys in Java Hashmap

Question

public Map<String, List<String>> getContactMap() {
    Set<String> keys = contactMap.keySet(); //Get Key Set
    for(String key: keys){
        List<String> a=contactMap.get(key); //Get List for each Key
        a.forEach(System.out::println);     //Print the list
    }
    return null;
}

I have a Hashmap, with multiple values for a single key. I am using this code to print those values, but this does not seem to work. What am I doing wrong?

I am only getting the last inserted value as output.

This is my addContact method:

private Map<String, List<String>> contactMap = new HashMap<String, List<String>>();

@Override
public void addContact(String name, List<String> list) {
    // TODO Auto-generated method stub
    contactMap.put(name, list);
}

And this is my main():

List <String> list= new ArrayList<String>();
    list.add("0321564988");
    list.add("7891268429");
    contacts.addContact("Name1",list);
    list.clear();
    list.add("1891219122");
    list.add("9495198929");
    contacts.addContact("Name2",list);
    list.clear();
    list.add("8949219912");
    contacts.addContact("Name3",list);

This is my desired output:

Name1   Phone1
        Phone2
        Phone3
Name2   Phone1
        Phone2
        .....

Answer 1

Your code:

List <String> list= new ArrayList<String>();
list.add("0321564988");
list.add("7891268429");
contacts.addContact("Name1",list);
list.clear();
list.add("1891219122");
list.add("9495198929");
contacts.addContact("Name2",list);
list.clear();
list.add("8949219912");
contacts.addContact("Name3",list);

is wrong.

You are reusing the same List .

Look at a simple example:

List <String> list = new ArrayList<String>();
List <String> list2 = list;

list.add("0321564988");
list.add("7891268429");
list.clear();

list2.add("1891219122");
System.out.println(list2);

Output:

1891219122

Because, in Java, a List is a reference to a memory location containing that List . Java passes references (by value) so that when you pass the List to your Map you only pass a reference to that List . When you clear the List you clear the memory location and the reference in the Map still points to the same location.

I would suggest you change your method to:

public void addNumberForContact(String name, String number) {
    contactMap.computeIfAbsent(name, k -> new ArrayList<>()).add(number);
}

This way you abstract the creation of the List to a class that controls the Map so you don't have this problem.

Further, looping over a Map by key and then getting the value is an anti-pattern. A Map stores key -> value pairs together so you can use the EntrySet :

contactMap.entrySet().forEach(System.out::println); 

Answer 2

If you only want to print the values of the values , you can do:

contactMap.entrySet().stream()
          .map(Entry::getValue)
          .flatMap(List::stream)
          .forEach(System.out::println); 

Also, if this is the only purpose of the method, it doesn't have to return anything, so change the return-type to void and remove the return statement.

Answer 3

The reason why you have only last list included is because of your insert method:

List <String> list= new ArrayList<String>();
    list.add("0321564988");
    list.add("7891268429");
    contacts.addContact("Name1",list);
    list.clear();
    list.add("1891219122");
    list.add("9495198929");
    contacts.addContact("Name2",list);
    list.clear();
    list.add("8949219912");
    contacts.addContact("Name3",list);

Map binds key- value pairs by referencing key (in your case String key) to object's memory address (in your case List <String> ). Your map contains three different keys, but each one points to exactly the same object in memory. As a result, when you modify one Map entry, the change is valid for all keys. To correct your code you should initialize your list every time you want to add it:

List <String> list= new ArrayList<String>();
    list.add("0321564988");
    list.add("7891268429");
    contacts.addContact("Name1",list);
List <String> list1= new ArrayList<String>();
    list1.add("1891219122");
    list1.add("9495198929");
    contacts.addContact("Name2",list1);
List <String> list2= new ArrayList<String>();
    list2.add("8949219912");
    contacts.addContact("Name3",list2);

At the end it is worth pointing out, that "memory reference" is valid for elements derived from Object class. Basic types (ie int, boolean, double ) are referenced by value, so for this sort of data your code would work fine.