Create a Map<Key,Object> based on a List<Object> - Converting Loops into Java streams

Question

Facing error while converting a List<Object> to a HashMap<Key,Object>.

I can do with a traditional code, but I'm getting an error while doing using Java 8 streams and ignore the duplicate keys.

Equivalent code:

Map<String, Field> uniqueFields = new HashMap<>();
for (Field field : allFields) {
    uniqueFields.put(field.getName(), field);
}

Tried below stream, but there's a syntactic mistake:

Map<String, Field> uniqueFields1 = allFields.stream()
    .collect(Collectors.toMap(
        Field::getName, 
        (oldValue, newValue) -> oldValue));

You missed the key value like (Field::getName, Field::getXXX, (oldValue,newValue)->oldValue)) — newuserua_ext
– newuserua_ext, Commented Jun 10, 2022 at 14:22

marstran · Accepted Answer · 2022-06-10 14:23:54Z

4

You just need one more argument to the toMap function to tell the collector what should be the values of the Map. You can use Function.identity(), which means it will just pass the Field right through.

 Map<String,Field> uniqueFields = allFields.stream()
    .collect(Collectors.toMap(Field::getName, Function.identity(), 
        (oldValue,newValue) -> oldValue));

answered Jun 10, 2022 at 14:23

marstran

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2 Comments

Shalini Gupta Over a year ago

Getting syntax error in Function " Function cannot be resolved" , @marstran

Shalini Gupta Over a year ago

It working , issue was with import Fixed that Thanks

Alexander Ivanchenko · Accepted Answer · 2022-06-10 14:39:56Z

1

The flavor of Collectors.toMap(), that expects two arguments, should be provided with Functions: keyMapper and valueMapper (that are responsible for expracting a key and a value respectivelly from a stream element).

But in your code snippet, the second argument isn't of type Function, because you've provided two arguments oldValue, newValue, but Function takes only one.

You need to use a version of toMap that in addition to keyMapper and valueMapper functions expects the third argument BinaryOperator mergeFunction which is meant to resolve values mapped to the same key:

Map<String, Field> uniqueFields = allFields.stream()
    .collect(Collectors.toMap(
        Field::getName,      // generating a kay
        Function.identity(), // generating a value
        (oldValue, newValue) -> oldValue))); // resolving duplicates

edited Jun 10, 2022 at 14:39

answered Jun 10, 2022 at 14:18

Alexander Ivanchenko

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3 Comments

Shalini Gupta Over a year ago

Error "Function cannot be resolved"

Alexander Ivanchenko Over a year ago

@ShaliniGupta which expression causes the error?

Shalini Gupta Over a year ago

Its working Thanks , issue was with import Function

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