Cannot compare results of a method to a "T" key java

September 7, 2022

import java.util.*;
public class Searcher<T> {
    // Returns the index of the key in the sorted array, or -1 if the 
    // key is not found.
    public static <T> int binarySearch(T[] array, int arraySize, T key, 
            Comparator<T> comparer) {
               
        int lowBound = 0; 
        int highBound = arraySize - 1;
        int mid = 0;
        //string this.key = key;
        while (highBound > lowBound) {
           mid = (highBound - lowBound) / 2;
           String results = comparer.compare(array[mid], key.toString());
         
           if(results > key){
              lowBound = mid + 1; 
           }
           else if(results > key) {
              highBound = mid - 1;
           }
           else { return mid;
           }
        }
        
        return -1;
    }
}

when i run the program I get this error:

Searcher.java:17: error: bad operand types for binary operator '<'
           if(results < key){
                      ^
  first type:  int
  second type: T
  where T is a type-variable:

For both lines 17 and 20. i cannot for the life of me figure out how to compare the results from line 15 to the key i am searching for in my binary search algorithm.][2]

>Solution :

The result of Comparator::compare is an int, as such you need to change result to be of type int and not String, then you can correct the if else as shown below. a result of 1 indicates that your value is large, 0 is the same, and -1 is less (this depends on how you have implemented the comparator.

import java.util.*;
public class Searcher<T> {
// Returns the index of the key in the sorted array, or -1 if the 
// key is not found.
public static <T> int binarySearch(T[] array, int arraySize, T key, 
        Comparator<T> comparer) {
           
    int lowBound = 0; 
    int highBound = arraySize - 1;
    int mid = 0;
    //string this.key = key;
    while (highBound > lowBound) {
       mid = (highBound - lowBound) / 2;
       int results = comparer.compare(array[mid], key);
     
       if(results > 0){
          lowBound = mid + 1; 
       }
       else if(results < 0) {
          highBound = mid - 1;
       }
       else { return mid;
       }
    }
    
    return -1;
}
}

As mentioned in my comment you can have type T implement comparable and then use the Comparable::compareTo method; this would remove the requirement for the Comparator in the method signature.

Example:

public class Searcher<T extends Comparable<T>> {
// Returns the index of the key in the sorted array, or -1 if the
// key is not found.
public int binarySearch(T[] array, int arraySize, T key) {

    int lowBound = 0;
    int highBound = arraySize - 1;
    int mid = 0;
    //string this.key = key;
    while (highBound > lowBound) {
       mid = (highBound - lowBound) / 2;
       int results = key.compareTo(array[mid]);

       if(results > 0){
          lowBound = mid + 1;
       }
       else if(results < 0) {
          highBound = mid - 1;
       }
       else { return mid;
       }
    }

    return -1;
  }
}