Java Collections Framework:ArrayList-6.2

 

Java ArrayList trimToSize() Method example

trimToSize() method is used for memory optimization. It trims the capacity of ArrayList to the current list size. For e.g. An arraylist is having capacity of 15 but there are only 5 elements in it, calling trimToSize() method on this ArrayList would change the capacity from 15 to 5.

public void trimToSize()

Example

Here I have defined the ArrayList of capacity 50. After adding 10 elements I called trimToSize() method which would have reduced the capacity from 50 to 10 (current size of arraylist).

package beginnersbook.com;
import java.util.ArrayList;
public class TrimExample {
  public static void main(String args[]) {
    ArrayList<Integer> arraylist = new ArrayList<Integer>(50);
    arraylist.add(1);
    arraylist.add(2);
    arraylist.add(3);
    arraylist.add(4);
    arraylist.add(5);
    arraylist.add(6);
    arraylist.add(7);
    arraylist.add(1);
    arraylist.add(1);
    arraylist.add(1);
    arraylist.trimToSize();
    System.out.println(arraylist);
  }
}

Output:

[1, 2, 3, 4, 5, 6, 7, 1, 1, 1]

Java ArrayList set() Method example

If there is a need to update the list element based on the index then set method of ArrayList class can be used. The method set(int index, Element E) updates the element of specified index with the given element E.

public E set(int index, Element E)

Example:

In this example I have an ArrayList of Integer Type where I have added few elements and then I’m updating few of elements using set method of java.util.ArrayList class.

package beginnersbook.com;
import java.util.ArrayList;
public class SetExample {
  public static void main(String args[]) {
    ArrayList<Integer> arraylist = new ArrayList<Integer>();
    arraylist.add(1);
    arraylist.add(2);
    arraylist.add(3);
    arraylist.add(4);
    arraylist.add(5);
    arraylist.add(6);
    arraylist.add(7);
    System.out.println("ArrayList before update: "+arraylist);
    //Updating 1st element
    arraylist.set(0, 11);
    //Updating 2nd element
    arraylist.set(1, 22);
    //Updating 3rd element
    arraylist.set(2, 33);
    //Updating 4th element
    arraylist.set(3, 44);
    //Updating 5th element
    arraylist.set(4, 55);
    System.out.println("ArrayList after Update: "+arraylist);
  }
}

Output:

ArrayList before update: [1, 2, 3, 4, 5, 6, 7]
ArrayList after Update: [11, 22, 33, 44, 55, 6, 7]

Java ArrayList ensureCapacity() Method example

ArrayList internally implements growable dynamic array which means it can increase and decrease its size automatically. If we try to add an element to a already full ArrayList then it automatically re-sized internally to accommodate the new element however sometimes its not a good approach.

Consider a scenario when there is a need to add huge number of elements to an already full ArrayList, in such case ArrayList has to be resized several number of times which would result in a poor performance. For such scenarios ensureCapacity() method of Java.util.ArrayList class is very useful as it increases the size of the ArrayList by a specified capacity.

public void ensureCapacity(int minCapacity)

Example

package beginnersbook.com;
import java.util.ArrayList;
public class EnsureCapacityExample {
  public static void main(String args[]) {
      // ArrayList with Capacity 4
      ArrayList<String> al = new ArrayList<String>(4);
      //Added 4 elements
      al.add("Hi");
      al.add("Hello");
      al.add("Bye");
      al.add("GM");

      //Increase capacity to 5
      al.ensureCapacity(5);

      al.add("GE");
      // let us print all the elements available in list
      for (String temp: al) {
            System.out.println(temp);
      }
   }
}

Output:

Hi
Hello
Bye
GM
GE