Create Map in Java

Question

I'd like to create a map that contains entries consisting of (int, Point2D)

How can I do this in Java?

I tried the following unsuccessfully.

HashMap hm = new HashMap();

hm.put(1, new Point2D.Double(50, 50));

score 152 · Accepted Answer · answered Feb 07 '13 at 04:26

152

Map <Integer, Point2D.Double> hm = new HashMap<Integer, Point2D>();
hm.put(1, new Point2D.Double(50, 50));

answered Feb 07 '13 at 04:26

hd1

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12

You also must do `import java.util.Map; import java.util.HashMap;` or `import java.util.*;` – Max Apr 06 '17 at 16:49
you may choose avoid the second pair of class names, i.e. `Map hm = new HashMap<>();` – user0221441 Jul 07 '22 at 13:48

score 35 · Answer 2 · answered Jun 04 '17 at 08:30

35

There is even a better way to create a Map along with initialization:

Map<String, String> rightHereMap = new HashMap<String, String>()
{
    {
        put("key1", "value1");
        put("key2", "value2");
    }
};

For more options take a look here How can I initialise a static Map?

answered Jun 04 '17 at 08:30

webdizz

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This is a really **bad** way to initalize a map. – Kao Mar 04 '19 at 07:51
2

@Kao Why? This just visually contains the operation, otherwise it's identical to the top answer on this question. – Anthony Mansour Jun 14 '19 at 13:30
19

No, it's not. It is a double-brace initialization, which is, generally speaking, an anti-pattern. [Here is why](https://blog.jooq.org/2014/12/08/dont-be-clever-the-double-curly-braces-anti-pattern/). – Kao Jun 17 '19 at 09:31
It's a weird code, I agree. – Igor Kanshyn Nov 11 '21 at 21:43
This is perfect for creating a static map in unit test – BabyishTank Jan 31 '23 at 01:29

score 30 · Answer 3 · answered Apr 28 '18 at 06:59

30

Java 9

public static void main(String[] args) {
    Map<Integer,String> map = Map.ofEntries(entry(1,"A"), entry(2,"B"), entry(3,"C"));
}

answered Apr 28 '18 at 06:59

Durgpal Singh

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dreamcrash · Answer 4 · 2020-12-19T11:52:33.283

With the newer Java versions (i.e., Java 9 and forwards) you can use :

Map.of(1, new Point2D.Double(50, 50), 2, new Point2D.Double(100, 50), ...)

generically:

Map.of(Key1, Value1, Key2, Value2, KeyN, ValueN)

Bear in mind however that Map.of only works for at most 10 entries, if you have more than 10 entries that you can use :

Map.ofEntries(entry(1, new Point2D.Double(50, 50)), entry(2,  new Point2D.Double(100, 50)), ...);

Achrome · Answer 5 · 2013-02-07T04:36:36.060

11

Map<Integer, Point2D> hm = new HashMap<Integer, Point2D>();

edited Feb 07 '13 at 04:36

answered Feb 07 '13 at 04:25

Achrome

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`Point2D.Double` doesn't look as a `Point2D` =\ – Luiggi Mendoza Feb 07 '13 at 04:26
using `Map hm = new HashMap()`, I get this error: Syntax error on token "int", Dimensions expected after this token. – Kevin Meredith Feb 07 '13 at 04:28
Forgot that Java maps don't support primitive types. Fixed that. – Achrome Feb 07 '13 at 04:29
but I need Point2D as the value in the [key, value]. :/ – Kevin Meredith Feb 07 '13 at 04:35
That was what I initially wrote. – Achrome Feb 07 '13 at 04:36

score 5 · Answer 6 · edited Apr 02 '20 at 10:41

I use such kind of a Map population thanks to Java 9. In my honest opinion, this approach provides more readability to the code.

  public static void main(String[] args) {
    Map<Integer, Point2D.Double> map = Map.of(
        1, new Point2D.Double(1, 1),
        2, new Point2D.Double(2, 2),
        3, new Point2D.Double(3, 3),
        4, new Point2D.Double(4, 4));
    map.entrySet().forEach(System.out::println);
  }

Create Map in Java

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