How to use Collectors.toMap() in Java8

How to use Collectors.toMap() in Java8

You can convert from a List to a Map by using Collectors.toMap().

List<Dto> dataList = Arrays.asList(
  new Dto(1, "sample1"),
  new Dto(2, "sample2"),
  new Dto(3, "sample3"),
  new Dto(4, "sample4")
);

Map<Integer, String> map = dataList.stream().collect(
Collectors.toMap(
  d -> d.getAge(),
  d -> d.getName(),
  (oldVal,newVal) -> newVal,
  LinkedHashMap::new));

The meanings of the arguments are as follows

| Argument | Meaning |
| :– | :– | first argument
| First Argument | Map’s key |
| Second Argument | Map’s value |
| Third Argument | First or second win in case of key overlap |
| Fourth argument | Class that implements the Map interface |

The third argument specifies which of the keys takes precedence in the case of duplicate keys. In the above case, the latter wins.

List<Dto> dataList = Arrays.asList(
 new Dto(1, "sample1"),
 new Dto(2, "sample2"),
 new Dto(3, "sample3"),
 new Dto(4, "sample4")
);

If dataList is changed to the above, the contents of the map will look like this

{1=sample1, 2=sample2, 3=sample3, 4=sample4}

Next is the case of duplication.

List<Dto> dataList = Arrays.asList(
  new Dto(1, "sample1"),
  new Dto(2, "sample2"),
  new Dto(3, "sample3"),
  new Dto(3, "sample4")
);

If dataList is changed to the above, the contents of the map will look like this

{1=sample1, 2=sample2, 3=sample4}

java.lang.IllegalStateException: Duplicate key

If the third argument is omitted, the default is a duplicate error.

final List<Dto> dataList = Arrays.asList(new Dto(1, "sample1"),
    new Dto(2, "sample2"),
    new Dto(3, "sample3"),
    new Dto(3, "sample4"));
final Map<Integer, String> result = dataList.stream()
    .collect(Collectors.toMap(d -> d.getAge(), d -> d.getName()));

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