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

You will have to first make sure the "Teachers" key exists in courses map, then you have to do courses.get("Teachers").add("adam"). The only downside to using Map<String, List<String>> is that all keys have to be lists now, so you'll have to update all references to courses variable. — Andrew Cotton
– Andrew Cotton, Commented Feb 14, 2019 at 3:05

Squirrely Wrath · Accepted Answer · 2019-02-14 03:11:13Z

2

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

answered Feb 14, 2019 at 3:11

Squirrely Wrath

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3 Comments

salmaan Over a year ago

how do you add new key

Squirrely Wrath Over a year ago

map.put("Teachers", new ArrayList ());

Squirrely Wrath Over a year ago

map.get("Teachers") gives you an ArrayList, so you can access it's values using an index or .contains("Bob") or .indexOf("Bob")

khachik · Accepted Answer · 2019-02-14 03:08:20Z

0

You can benefit from computeIfAbsent:

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

answered Feb 14, 2019 at 3:08

khachik

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Comments

TongChen · Accepted Answer · 2019-02-14 03:48:02Z

0

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>

answered Feb 14, 2019 at 3:48

TongChen

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