How to provide a lambda mapping function as method parameter?

user_1194415 0
Viewed 94

It is possible to pass a lambda expression as a function parameter? Example as follows: I want to pass the "strategy" that extracts properties from list items. For not having to repeat the stream mapping over and over again.

List<MyObject> list;

list.stream().map(item -> item.field1)).collector(Collectors.joining(","));
list.stream().map(item -> item.field2)).collector(Collectors.joining(","));
list.stream().map(item -> {
         //some complex parser function
    })).collector(Collectors.joining(","));
...

//pseudocode
private String map(List<MyObject> list, Function func) {
    return list.stream.map(item -> func.call()).collector(Collectors.joining(","));
}

Update:

what if the object type changes? I still want to have my general map() method that defines the collecting of the extracted field. But as the object type changes, I just want to supply a function that knows how to extract the desired property.

class Person {
    String name;
} 

class Car {
    String manufacture;
}

map(persons, (person) -> person.name));
map(cars, (car) -> car.manufacture));
map(cars, (MyObject) -> {
   //more complex parsing
}));
java-stream java
2 Answers

use a Function<MyObject, String>:

private String map(List<MyObject> list, Function<MyObject, String> func) {
        return list.stream().map(func).collect(Collectors.joining(","));
}

Update:

here is a generic implementation:

private <T> String map(List<? extends T> list,  
           Function<? super T, ? extends CharSequence> func) {
       return list.stream().map(func).collect(Collectors.joining(","));
}
user_58620710

what if the object type changes? I still want to have my general map() method that defines the collecting of the extracted field.

You're mostly looking for a generic approach here:

private <T> String map(List<T> list, Function<T, String> func) {
    return list.stream().map(func).collect(Collectors.joining(","));
}
user_17461180
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