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Thread: Using Comparable in Binary Search

  1. #1
    Junior Member
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    Aug 2011
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    Default Using Comparable in Binary Search

    So I have written some code which tells whether a certain Integer is within a list of integers. I tried to do this with a comparable type instead of 'integer' type, but I realised that this wouldn't work as we could have say a String and Integer in the same list. I want to be able to have comparables of all the same type, but not limited to just Integers... Does anyone know how I could do this? I'm not very confident with using interfaces as input parameters. This is my code:

    package Recursion;
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Scanner;
    public class BinarySearch {
    	// Takes in a sorted list (ascending order) of comparable objects 
    	// and finds whether a certain object is in that list using binary 
    	// search method.
    	public static void main(String[] args){
    		ArrayList<Integer> l = new ArrayList<Integer>();
    		Scanner s = new Scanner(System.in);
    			int n = s.nextInt();
    			System.out.print(BinarySearch.search(l, n));
    	//Check whether something is in the list
    	public static boolean search(List<Integer> al, Integer c){
    		//Current one we are looking at
    		Integer pivot = al.get(al.size()/2);
    		if (c.equals(pivot)){
    			return true;
    		} else if (c.compareTo(pivot) == -1){
    			if (al.size()/2 == 0){ //if our pivot is the first value in the sorted list
    				return false; // no more to see		
    			} else {
    				return search(al.subList(0,al.size()/2),c);
    		} else {
    			if(al.size()/2 == 0){
    				return false;
    			} else {
    			return search(al.subList(al.size()/2,al.size()),c); 

  2. #2

    Default Re: Using Comparable in Binary Search

    The comparable interface will assist you in comparing objects.

    Comparable (Java 2 Platform SE v1.4.2)

    You will need to implement the methods in your class BinarySearch when you add the interface. You will create the rules in method compareTo() for when a certain BinarySearch is different from another. Be sure to read the rules in the link.
    Kenneth Walter
    Software Developer

  3. #3
    Junior Member
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    Aug 2011
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    Default Re: Using Comparable in Binary Search

    I understand that I have to implement the interface to use a data type as a 'comparable'.. however this is not my issue. The issue is that I want my list to contain any type which implements comparable, but I want it to be so only one type can be in the list at a time. For example, I could use this code to make and search a list of Integers as they are comparable, or a list of Strings, but I dont want to be able to have integers and strings in the same list.

    By making the input list type List<Comparable> this doesnt force the elements to be of one comparable type, but various types.

  4. #4
    Super Moderator helloworld922's Avatar
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    Jun 2009
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    Default Re: Using Comparable in Binary Search

    I would suggest using a Comparator instead.

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