List binary search

In this chapter you will learn:

How to do binary search on a List
How to use the not-in index value returned from binary search to insert value

Sort a list

We can do binary search on an ordered list with the following methods.

static<T> int binarySearch(List<? extends Comparable<? super T>> list, T key)
Searches list for an object using the binary search algorithm.
static<T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c)
Searches list for an object using the binary search algorithm and passed in comparator.

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
/* j a  v  a  2s  . c o m*/
public class Main {

  public static void main(String args[]) {
    List<Character> ll = new LinkedList<Character>();

    for (char n = 'A'; n <= 'Z'; n++)
      ll.add(n);

    Collections.shuffle(ll);

    for (Character x : ll)
      System.out.print(x + " ");
    Collections.sort(ll);

    for (Character x : ll)
      System.out.print(x + " ");

    System.out.println("Searching for F.");
    int i = Collections.binarySearch(ll, 'F');

    if (i >= 0) {
      System.out.println("Found at index " + i);
      System.out.println("Object is " + ll.get(i));
    }
  }
}

The code above generates the following result.

Search value not in the List

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
//from  j  a  v a  2 s. com
public class MainClass {
  public static void main(String args[]) {
    String simpsons[] = { "B", "H", "L", "M", "H", "M", "R" };

    List list = new ArrayList(Arrays.asList(simpsons));

    // Ensure list sorted
    Collections.sort(list);
    System.out.println("Sorted list: [length: " + list.size() + "]");
    System.out.println(list);

    // Search for element in list
    int index = Collections.binarySearch(list, "M");
    System.out.println("Found M @ " + index);

    // Search for element not in list
    index = Collections.binarySearch(list, "J");
    System.out.println("Didn't find J @ " + index);

  }
}

The output:

The -4 is returned from the binary search when searching J since J is not in the list. -4 is negative and negative indicates that the value is missing. The absolute value of -4 tells where should we insert and can still keep the list ordered.