Java ArrayList ensureCapacity() Method

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Description

The Java ArrayList ensureCapacity(int minCapacity) method increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.

Declaration

Following is the declaration for java.util.ArrayList.ensureCapacity() method

public void ensureCapacity(int minCapacity)

Parameters

minCapacity − This is the desired minimum capacity.

Return Value

This method does not return any value.

Exception

NA

Increasing Capacity of an ArrayList of Integers Example

The following example shows the usage of Java ArrayList ensureCapacity(E) method to add Integers. We're adding couple of Integers to the ArrayList object using add() method calls per element and using ensureCapacity(), we're increasing its capacity to hold more items then print each element to show the elements added.

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {
      
      // create an empty array list 
      ArrayList<Integer> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add(20);
      arrayList.add(30);
      arrayList.add(20);
      arrayList.add(30);
      arrayList.add(15);
      arrayList.add(22);
      arrayList.add(11);
	  
      // this will increase the capacity of the ArrayList to 15 elements
      arrayList.ensureCapacity(15);

      // let us print all the elements available in arrayList
      System.out.println("ArrayList = " + arrayList);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

ArrayList = [20, 30, 20, 30, 15, 22, 11]

Increasing Capacity of an ArrayList of Strings Example

The following example shows the usage of Java ArrayList ensureCapacity(E) method to add Strings. We're adding couple of Strings to the ArrayList object using add() method calls per element and using ensureCapacity(), we're increasing its capacity to hold more items then print each element to show the elements added.

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {
      
      // create an empty array list
      ArrayList<String> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add("Welcome");
      arrayList.add("To");
      arrayList.add("Tutorialspoint");
	  
      // this will increase the capacity of the ArrayList to 15 elements
      arrayList.ensureCapacity(15);

      System.out.println("ArrayList = " + arrayList);      
   }
}

Output

Let us compile and run the above program, this will produce the following result −

ArrayList = [Welcome, To, Tutorialspoint]

Increasing Capacity of an ArrayList of Objects Example

The following example shows the usage of Java ArrayList ensureCapacity(E) method to add Student objects. We're adding couple of Student objects to the ArrayList object using add() method calls per element and using ensureCapacity(), we're increasing its capacity to hold more items then print each element to show the elements added.

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {

      // create an empty arrayList
      ArrayList<Student> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add(new Student(1, "Julie"));
      arrayList.add(new Student(2, "Robert"));
      arrayList.add(new Student(3, "Adam"));


      // this will increase the capacity of the ArrayList to 15 elements
      arrayList.ensureCapacity(15);
	  
      System.out.println("ArrayList = " + arrayList);      
   }
}

class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
}

Output

Let us compile and run the above program, this will produce the following result −

ArrayList = [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]