Collections – sorting and searching

In order to sort a Collection, one should use java.util.Collections class as follows (assuming ArrayList will be used):

List another object

  • the drawback: there’s only one way of sorting
Example of the class implementing Comparable interface:
class Book implements Comparable {
    private String title;
    public String getTitle() {
        return title;

    public void setTitle(String title_p) {
        title = title_p;

    public int compareTo(Book o) {
        return this.title.compareTo(o.getTitle());


  • used to sort:
    • a Listjava.util.Collections.sort(A_COLLECTION, Comparator)
    • an array of objects – java.util.Arrays.sort(AN_ARRAY, Comparator)
  • a special class that implements java.util.Comparator interface needs to be created; this class defines public int compare(MyClass o1, MyClass o2) method (only one), which returns:
    • negative value – this object < another object
    • 0 – this object == another object
    • positive value – this object > another object
  • allows many one ways of sorting
  • allows to sort even those class that can’t be modified
Example of the class implementing Comparator interface:
 class NameSort implements Comparator { public int compare(Book book1, Book book2) { return book1.getTitle().compareTo(book2.getTitle()); } }


  • In order to search in a Collection or an array of objects, they must be sorted:
    Arrays.binarySearch(anArray, "string_to_search");


    Arrays.sort(anArray, aComparator);
    Arrays.binarySearch(anArray, "string_to_search", aComparator);
  • when search succeeds, the index of the serched element is returned
  • when search fails, insertionPoint is returned: -(insertion point) - 1 (if the element should be placed as a firston, so at 0 index, the returned value is -1)
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