Collections

Collections

It is an utility class present in java.util package. It defines several utility methods for collection implemented class object.

Sorting a list

public static void sort(List l), to sort the elements of List, according to natural sorting order. In this case compulsory Objects should be Homogeneous & comparable otherwise we will get ClassCastException. The list should not null, otherwise NullPointerException.

Public static void sort(List l, Comparator c): To sort the elements of list according to customized sorting order described by comparative.

CollectionSort.java

package com.narayanatutorial.collections.list;

import java.util.ArrayList;
import java.util.Collections;

public class CollectionSort {

	public static void main(String[] args) {
		ArrayList<String> al = new ArrayList<String>();
		al.add("Z");
		al.add("B");
		al.add("K");
		al.add("N");
		// al.add (new integer(10));// ClassCastException
		// al.add(null);// NullPointerException
		System.out.println("before sorting" + al);
		Collections.sort(al);// [ZAKN]
		System.out.println("after sorting" + al);// [AKNZ]
	}
}

Output

before sorting[Z, B, K, N]
after sorting[B, K, N, Z]

MyComparator.java

package com.narayanatutorial.collections.list;

import java.util.Comparator;

public class MyComparator implements Comparator<Object>{

	public int compare(Object Obj1, Object Obj2)
    {
     String s1 = (String)Obj1;
     String s2 = (String)Obj2;
      return s1.compareTo(s2);
    }

}

CollectionsSortDemo.java

package com.narayanatutorial.collections.list;

import java.util.ArrayList;
import java.util.Collections;

public class CollectionsSortDemo {
	public static void main (String[] args) 
    {
          ArrayList<String> al= new ArrayList<String>(); 
          al.add("Z"); 
          al.add("B");
          al.add("K");
          al.add("L");
          
        System.out.println("before sorting"+al);// [ZAKLL] 
         Collections.sort(al, new MyComparator ());
      System.out.println("after sorting"+al);// [ZLLKA]
     }
}

Output

before sorting[Z, B, K, L]

after sorting[B, K, L, Z]

Searching the list

Collections class defines the following binary search methods.

Public static int binarySearch(List l, Object O): We have to use this method, if list is sorted according to natural sorting order.

Public static in binarySearch(List l, Object O, Comparator c): We have to use this method, if the list is sorted according to comparator.

• Before calling binarySearch() method, compulsory the list should be sorted, otherwise we will get Unpredictable results.
• Successful search returns index, unsuccessful search returns insertion point. Insertion point is the location, where we can place the element.
• If the list is sorted, according to comparator then at the time of search operation also, we have to provide same comparator. Otherwise we will get Unpredictable result.

CollectionsSearchDemo.java

package com.narayanatutorial.collections.list;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class CollectionsSearchDemo {
	public static void main(String[] args) {
		List<String> al = new ArrayList<String>();
		al.add("Z");
		al.add("A");
		al.add("M");
		al.add("K");
		al.add("a");
		Collections.sort(al);
		System.out.println("al:"+al);
		System.out.println(Collections.binarySearch(al, "Z"));
		System.out.println(Collections.binarySearch(al, "j"));
	}
}

Output

al:[A, K, M, Z, a]

3

-6

MyComparatorInteger.java

package com.narayanatutorial.collections.list;

import java.util.Comparator;

public class MyComparatorInteger implements Comparator<Object> {

	@Override
	public int compare(Object o1, Object o2) {
		// TODO Auto-generated method stub
		Integer i1=(Integer)o1;
		Integer i2=(Integer)o2;
		
		return i1.compareTo(i2);
	}

}

CollectionsSearchDemoComparator.java

package com.narayanatutorial.collections.list;

import java.util.ArrayList;
import java.util.Collections;

public class CollectionsSearchDemoComparator {

	public static void main(String args[]) {
		ArrayList<Integer> al = new ArrayList<Integer>();
		al.add(15);
		al.add(0);
		al.add(20);
		al.add(5);
		Collections.sort(al, new MyComparatorInteger());
		System.out.println("al:"+al);
		System.out.println(Collections.binarySearch(al, 10, new MyComparatorInteger()));
		System.out.println(Collections.binarySearch(al, 13, new MyComparatorInteger()));
		System.out.println(Collections.binarySearch(al, 17));// unPredicatable

	}
}

Output

al:[0, 5, 15, 20]
-3
-3
-4

Note

For the list, with ‘n’ elements, the possible values for successful binary search methods on 0 to n-1.

⇒ Unsuccessful search -1 to -(n+1).

⇒ total range -(n+1) to n-1.

Reversing the elements of a List

Collections class contain reverse() method public static void reverse(List l).

ExonReverse.java

package com.narayanatutorial.collections.list;

import java.util.ArrayList;
import java.util.Collections;

public class ExonReverse {

	public static void main(String args[]) {

		ArrayList<Integer> al = new ArrayList<Integer>();
		al.add(15);
		al.add(0);
		al.add(20);
		al.add(10);
		al.add(5);
		Collections.reverse(al);
		System.out.println("Rerverse Order:"+al);
	}
}

Output

Rerverse Order:[5, 10, 20, 0, 15]