Collecting with casts and lambdas

Question

I am running into an issue in java 8 where it is not allowing me to collect objects when using a cast.

definitions.stream()
           .map(Definition.class::cast)
           .map((Definition definition) -> { 
               WonderfulDefinition wd = new WonderfulDefinition();
               wd.name(definition.getName());
               //etc
               return wd;
           }).collect(Collectors.toList())

And I am getting the compiler error:

Error:(71, 23) java: incompatible types: inference variable T has incompatible bounds equality constraints: java.util.List lower bounds: java.lang.Object

Any help would be appreciated.

Updated:

import java.util.*;
import java.util.stream.*;

class Driver {
    public static void main(String[] args) {
        List definitions = new ArrayList<>();
        definitions.add(new Definition());
        definitions.add(new Definition());

        List<WonderfulDefinition> list =
                definitions.stream()
                        .map(Definition.class::cast)
                        .map((Definition definition) -> {
                            WonderfulDefinition wd = new WonderfulDefinition();
                            wd.name(definition.getName());
                            //etc
                            return wd;
                        }).collect(Collectors.toList());

        System.out.println(list);
    }
}
class Definition {
    private String name;

    public String getName() {
        return name;
    }
    public void name(String name) {
        this.name = name;
    }
}
class WonderfulDefinition extends Definition {
}

Object unfortunately. It's coming from a 3rd party lib that returns definitions as List with no type. i.e. dictionary.getDefinitions() returns just List — user1870035
What type the List variable has? It probably marking that part, since there's no other mentions of lists. — M. Prokhorov
Please update your question with an MCVE which reproduces the issue you're having. Here's an example where your code as-is appears to work: ideone.com/IPoexa — Radiodef
Hi Radiodef, I updated it. Due to list being just "List" (lib code can't change it) without a type specifier (object or otherwise) it is failing. — user1870035

Grzegorz Piwowarek Grzegorz Piwowarek · Accepted Answer · 2017-08-24T19:41:00

The problem is that your List is untyped. In this case, simply hint the compiler by performing a cast:

((List<Object>) definitions).stream()
      .map(Definition.class::cast)
      .map(definition -> {
          WonderfulDefinition wd = new WonderfulDefinition();
          wd.name(definition.getName());
          //etc
          return wd;
      }).collect(Collectors.toList());

keep in mind that something like this is perfectly valid:

List definitions = Arrays.asList(new Definition());
List<Object> d = definitions;

Collecting with casts and lambdas

1 Answers