Error while creating LinkedList of LInkedLists

Question

I am trying to create a LinkedList of LinkedLists in Java.

The following code segment is giving an error. I am using java 11 and util.List

No idea why I am getting this error..

N = in.read();
List<List<Integer>> L;
L = new LinkedList<>();
for( i = 0;i<N;i++) L.add(new LinkedList<>());

It gives the following errors:

A.java:25: error: cannot infer type arguments for LinkedList
            L = new LinkedList<>();
                              ^
  reason: cannot use '<>' with non-generic class LinkedList
A.java:26: error: cannot infer type arguments for LinkedList
            for( i = 0;i<N;i++) L.add(new LinkedList<>());
                                                    ^
  reason: cannot use '<>' with non-generic class LinkedList

How should I go on resolving this?

Okay, so just to test I created a dummy class just to create LinkedList of LinkedLists. Here is the full program:

import java.util.*;
class Dummy
{
    public static void main(String[] args) 
    {
        List<List<Integer>> L;
        L = new LinkedList<>();
        for(int i = 0;i<10;i++) L.add(new LinkedList<>());    
    }
}

Again, these errors:

A.java:7: error: cannot infer type arguments for LinkedList
        L = new LinkedList<>();
                          ^
  reason: cannot use '<>' with non-generic class LinkedList
A.java:8: error: cannot infer type arguments for LinkedList
        for(int i = 0;i<10;i++) L.add(new LinkedList<>());    
                                                    ^
  reason: cannot use '<>' with non-generic class LinkedList

Edit: Okay, works fine when I use import java.util.List and import java.util.linkedList instead of import java.util.*

As pointed out in the comments there is probably some issue with my build path

Have you named one of your classes LinkedList, by any chance? — kaya3
– kaya3, Commented Nov 1, 2019 at 16:37
Show your actual imports and the type signature of the class you're running this from. That it says LinkedList isn't generic is a big warning flag. — Dave Newton
– Dave Newton, Commented Nov 1, 2019 at 16:40
Try importing java.util.LinkedList (and java.util.List) explicitly, rather than using a wildcard import statement. Since java.util.LinkedList is a generic class, the compiler is definitely picking up some non-generic class LinkedList from somewhere. — Ted Hopp
– Ted Hopp, Commented Nov 1, 2019 at 16:50
You probably have a (non-generic) class LinkedList somewhere that the compiler is looking. It may only be a left-over .class file. Try cleaning out your build artifacts. — Ted Hopp
– Ted Hopp, Commented Nov 1, 2019 at 16:53

Thomas Bitonti · Accepted Answer · 2019-11-01 19:27:48Z

I've tried your example using java7 and using java8, and it gives me the same error as you are seeing for java7, but works for me for java8.

Why it doesn't work for java7 will have to do with limitations of type inference in that version of the compiler.

I would expect java11 to work at least as well as java8 (which is to say, the code should compile using java11). Can you double check your compiler settings? You may be using a java11 compiler, but may have set it to generate code using the java7 rules.

Here is the version of the code that I tested:

import java.util.List;
import java.util.LinkedList;

public class TypeTest {
    private static final int STORAGE_SIZE = 10;

    private static final List<List<Integer>> storage = new LinkedList<>();

    static {
        for ( int elementNo = 0; elementNo < STORAGE_SIZE; elementNo++ ) {
            storage.add( new LinkedList<>() );
        }
    }
}

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