Exploring StringBuffer ensureCapacity() Method in Java: Usage and Examples

If you want to preallocate room for a certain number of characters after a StringBuffer has been constructed, you can use ensureCapacity( ) to set the size of the buffer. This is useful if you know in advance that you will be appending a large number of small strings to a StringBuffer.

Syntax

public void ensureCapacity(int minimumCapacity)

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.

If the minimumCapacity argument is nonpositive, this method takes no action and simply returns.

Parameters

minimumCapacity - the minimum desired capacity.

Program

public class MethodensureCapacity
{
  public static void main(String args[])
  {
    StringBuffer sb1 = new StringBuffer("ok");
    System.out.println("sb1 default capacity: " + sb1.capacity());      // 18
    sb1.ensureCapacity(10);
    System.out.println("sb1.ensureCapacity(10): " + sb1.capacity());    // 18

    StringBuffer sb2 = new StringBuffer("abcdef");   // with some length, say 6
    System.out.println("sb2 capacity with six characters size: " + sb2.capacity());  // 22
    sb2.ensureCapacity(30);
    System.out.println("sb2.ensureCapacity(30): " + sb2.capacity());    // 46
                         // with length 0
    StringBuffer sb3 = new StringBuffer();
    System.out.println("sb3 default capacity: " + sb3.capacity());      // 16
    sb3.ensureCapacity(50);
    System.out.println("sb3.ensureCapacity(50): " + sb3.capacity());    // 50

			             // negative value given
    StringBuffer sb4 = new StringBuffer();
    System.out.println("sb4 default capacity: " + sb4.capacity());      // 16
    sb4.ensureCapacity(-50);
    System.out.println("sb3.ensureCapacity(-50): " + sb4.capacity());   // 16
  }
}

Output

sb1 default capacity: 18
sb1.ensureCapacity(10): 18
sb2 capacity with six characters size: 22
sb2.ensureCapacity(30): 46
sb3 default capacity: 16
sb3.ensureCapacity(50): 50
sb4 default capacity: 16
sb3.ensureCapacity(-50): 16
Press any key to continue . . .

The ensure capacity cannot be less than the old capacity.

StringBuffer sb1 = new StringBuffer();
    System.out.println("sb1 default capacity: " + sb1.capacity());  // 16
    sb1.ensureCapacity(10);
    System.out.println("sb1.ensureCapacity(10): " + sb1.capacity());  // 16

The ensure capacity is, now in this case, twice the old capacity plus 2 (but not 30).

StringBuffer sb2 = new StringBuffer("abcdef");        // length is 6
    System.out.println("sb2 capacity with six characters size: " + sb2.capacity());  // 22
    sb2.ensureCapacity(30);     
    System.out.println("sb2.ensureCapacity(30): " + sb2.capacity());  // 46

When ensure capacity is more than the default 16, the ensure capacity will be 50

StringBuffer sb3 = new StringBuffer();
    System.out.println("sb3 default capacity: " + sb3.capacity());  // 16
    sb3.ensureCapacity(50);
    System.out.println("sb3.ensureCapacity(50): " + sb3.capacity());  // 50

The negative capacity value does not have any effect, just return the old existing value.

StringBuffer sb4 = new StringBuffer();
    System.out.println("sb4 default capacity: " + sb4.capacity());  // 16
    sb4.ensureCapacity(-50);
    System.out.println("sb3.ensureCapacity(-50): " + sb4.capacity());  // 16

Another Program

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

	StringBuffer sb=new StringBuffer();
	System.out.println(sb.capacity());//default 16
	sb.append("Hello");
	System.out.println(sb.capacity());//now 16
	sb.append("java is my favourite language");
	System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2
	sb.ensureCapacity(10);//now no change
	System.out.println(sb.capacity());//now 34
	sb.ensureCapacity(50);//now (34*2)+2
	System.out.println(sb.capacity());//now 70

	}
}

Output

16
16
34
34
70
Press any key to continue . . .