StringBuilder ensureCapacity() in Java with Examples

Last Updated : 10 Jul, 2020
The ensureCapacity(int minimumCapacity) method of StringBuilder class helps us to ensures the capacity is at least equal to the specified minimumCapacity passed as the parameter to the method.
  • If the current capacity of StringBuilder is less than the argument minimumCapacity, then a new internal array is allocated with greater capacity.
  • If the minimumCapacity argument is greater than twice the old capacity, plus 2 then new capacity is equal to minimumCapacity else new capacity is equal to twice the old capacity, plus 2.
  • If the minimumCapacity argument passed as parameter is not-positive, this method takes no action.
Syntax: 
public void ensureCapacity(int minimumCapacity)
Parameters: This method accepts one parameter minimumCapacity represents the minimum desired capacity you want. Return Value: This method do not return anything. Below programs demonstrate the ensureCapacity() method of StringBuilder Class: Example 1: In this program minimumCapacity argument which is equal to 18 is less than twice the old capacity, plus 2 which is equal to 34 then new capacity is equal to 34. Java 
// Java program to demonstrate
// the ensureCapacity() Method.

class GFG {
    public static void main(String[] args)
    {
        // create a StringBuilder object
        StringBuilder str = new StringBuilder();

        // print string capacity
        System.out.println("Before ensureCapacity "
                           + "method capacity = "
                           + str.capacity());

        // apply ensureCapacity()
        str.ensureCapacity(18);

        // print string capacity
        System.out.println("After ensureCapacity"
                           + " method capacity = "
                           + str.capacity());
    }
}
Output: 
Before ensureCapacity method capacity = 16
After ensureCapacity method capacity = 34
Example 2: In this program minimumCapacity argument which is equal to 44 is greater than twice the old capacity, plus 2 which is equal to 34 then new capacity is equal to 34. Java 
// Java program to demonstrate
// the ensureCapacity() Method.

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

        // create a StringBuilder object
        StringBuilder str = new StringBuilder();

        // print string capacity
        System.out.println("Before ensureCapacity"
                           + " method capacity = "
                           + str.capacity());

        // apply ensureCapacity()
        str.ensureCapacity(44);

        // print string capacity
        System.out.println("After ensureCapacity"
                           + " method capacity = "
                           + str.capacity());
    }
}
Output: 
Before ensureCapacity method capacity = 16
After ensureCapacity method capacity = 44
Reference: https://docs.oracle.com/javase/10/docs/api/java/lang/StringBuilder.html#ensureCapacity(int)
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