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());
    }
}

java.util.Comparator

• used to sort:
  • a List – java.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()); } }

Searching

• In order to search in a Collection or an array of objects, they must be sorted:

  Arrays.sort(anArray);
  Arrays.binarySearch(anArray, "string_to_search");
  

  or

  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)