Time Complexity = O(log n)

binarySearch.java

public static int binarySearch(int arr[], int key){
  int start = 0, end = arr.length - 1;
  while(start<=end){
    int mid = start + (end-start)/2;
    if(arr[mid]==key) return mid;
    else if(arr[mid]>key) end = mid - 1;
    else start = mid + 1;
  }
  return -1;
}