CODEWITHSUNDEEP
javagenericscompiler-errors
NateDrake07
NateDrake07

Reputation: 11

Error . operator < cannot be applied

In Line 11 there is an error .

operator < cannot be applied to java.lang.Object,java.lang.Object

I don't quite understand this but then again I'm new to generics in Java . bear with me please.

public void sortbykey()
 {
     LinkedList temp = new LinkedList();
     LinkedList third = new LinkedList();
     temp=head;

     if(temp!=null)
     {
       while(temp.next!=null)
       {
               if (temp.key < temp.next.key)
               {
                 third.item = temp.key;
                 temp.key = temp.next.key;
                 temp.next.key = third.item;
               }
               else
                   temp = temp.next;
       }
     }
 }

Upvotes: 1

Views: 7583

Answers (3)

Andy Senn
Andy Senn

Reputation: 649

To compare non-numeric types, you would need to implement Comparable on that type and use temp.key.compareTo(temp.next.key) < 0 instead.

If you were using a LinkedList<? extends Number>, it would not be applicable due to type erasure. However, if you specify a concrete implementation that wraps a primitive to which the < operator is applicable (see autoboxing) such as LinkedList<Integer>, then it would be possible since Integer is specified as the bound, Integer wraps int, and the < operator is applicable to int.

In your case, since you are using a raw type, the bound is java.lang.Object, to which the < operator is not applicable. Just like those of us reading your question, we can't tell if the LinkedList is of a type to which the < can be applied.

Raw types should be generally avoided anyway as they effectively strip generics of their value.

Upvotes: 5

stewartbracken
stewartbracken

Reputation: 134

I can't tell for sure because you haven't posted the data structure of your Node and LinkedList, but it looks like whatever you're storing in 'key' is not a java primitive so you can't use the '<' operator. make sure key is something like an int or float to use <. Otherwise you will need to use a class method on the type of 'key'. For example if 'key' is a String, you could use ("apple").compareTo("banana");

Upvotes: 0

Konstantin Yovkov
Konstantin Yovkov

Reputation: 62864

Simply, this operator can be applied on numeric types, such as int, byte, double, float, etc and their boxing classes (such as Integer, Byte, etc.).

It doesn't work combined with Generics, because the compiler doesn't know the exact types of the operands. When working with Generics, these types are set at Runtime, after the type erasure has occurred.

A possible solution for you is to make the class of the key property implement the Comparable<T> interface and then you will be able to compare it with:

temp.key.compareTo(temp.next.key) > 0

Upvotes: 2

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