Stream and map a list to a map in java

Question

I want to do something like the following with java 8 and lambda expression.

Map<String, String> headers = service.getFieldNamesInOrder(eventType).stream()
.map(f -> serviceClassInfo.getNameforField(f).collect(Collectors.toMap(<streamed field>, <result of map>)));

Like this:

Map<String, String> headers = new HashMap<>();
for (String field : headerFieldNames) {
    String name = service.getNameforField(field);
    headers.put(field, name);
}

I want to stream a list, take one element and get another value for it. And afterwards I want to add the streamed element as key and the result from the method as value. Can anyone help?

Answer 1

Try this.

• the (a,b)->b is a merge function in case of duplicates. It uses the most recent one.

Map<String, String> headers = headerFieldNames.stream()
                .collect(Collectors.toMap(field -> field,
                        field -> service.getNamefoField(field),
                        (a, b) -> b));

Answer 2

You should use something like

  Map<String, String> headers = headerFieldNames.stream()
                .collect(Collectors.toMap(f-> f, f -> service.getNameforField(f)));

Or

Map<String, String> headers = headerFieldNames.stream()
            .collect(Collectors.toMap(Function.identity(), f-> service.getNameforField(f)));

Warning: It is code throws expection in case of you have duplicate.