创建地图<Key,Object>基于列表<Object> - 将循环转换为 Java 流

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));

Answer 1

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));

Answer 2

The flavor of Collectors.toMap() , that expects two arguments, should be provided with Function s: 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