Java StringBuffer ensureCapacity() method ensures that the capacity is at least equal to the specified minimum. If the current capacity is less than the argument, then a new internal array is allocated with greater capacity. 

The new capacity is the larger of:

• The minimumCapacity argument.
• Twice the old capacity, plus 2.

Java StringBuffer ensureCapacity() Example

Example: Example to ensure the capacity of string buffer using ensureCapacity() method.

public class EnsureCapacityExample {
 public static void main(String[] args) {
  StringBuffer builder = new StringBuffer();
  builder.ensureCapacity(11);
     System.out.println(builder.capacity());
     builder.ensureCapacity(17);
     System.out.println(builder.capacity());
 }
}

Output:

16
34

Reference

https://www.javaguides.net/2018/08/stringbuffer-class-methods-in-java.html