Add multiple values to map using same key?

Question

This is what I have so far

Map<String, String> courses = new HashMap();
courses.put("Teachers","adam");

so how do I add more teachers using the same key

Answer 1

You can use Map<String, ArrayList<String>> . Then add additional information by doing map.get("Teachers").add("Bob")

Answer 2

You can benefit from computeIfAbsent :

Map<String, List<String>> courses = new HashMap<>();
courses.computeIfAbsent("Teachers", k -> new ArrayList<>()).add("adam");

Answer 3

if you can use google guava, Interface Multimap<K,V> is a good way to do this,and all opereation is simple。

To add a dependency on Guava using Maven, use the following:

<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>27.0.1-jre</version>
  <!-- or, for Android: -->
  <version>27.0.1-android</version>
</dependency>