Java StringBuilder(StringBuffer)'s ensureCapacity(): Why is it doubled and incremented by 2?

user_8181644 0
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I have searched about this, but I couldn't find why StringBuilder's ensureCapacity() method won't lengthen the old capacity by just doubling but also adding two.

So, when default capacity of 16 is full, next lengthened value will be 34 unless whole string length doesn't exceed 34. Why shouldn't it be 32?

My best guess is considering for a null character, '\u0000', but I'm not sure. Can anyone tell me why?

capacity stringbuffer stringbuilder java
5 Answers
 public void ensureCapacity(int minimumCapacity) {
     if (minimumCapacity > value.length) {
         expandCapacity(minimumCapacity);
     }
 }



 void expandCapacity(int minimumCapacity) {
     int newCapacity = (value.length + 1) * 2;
     if (newCapacity < 0) {
         newCapacity = Integer.MAX_VALUE;
     } else if (minimumCapacity > newCapacity) {
         newCapacity = minimumCapacity;
     }
    value = Arrays.copyOf(value, newCapacity);
}

NOTE: value.length is the capacity of the StringBuffer, not the length.

It has nothing to do with a null string because minimum capacity is 16.

What I think is, the memory allocations cost so much time, and if we are calling ensureCapacity() frequently with increasing minimumCapacity , (capacity +1)*2 will allocate a bit more memory and may reduce further allocations and save some time.

lets consider initial capacity as 16,

only with doubling 16 , 32 , 64 , 128 , 256 , 512 , 1024 , 2048 , so on...

with double +2 16 , 34 , 70 , 142 , 286 , 574 , 1150 , 2302 , so on...

Thus the memory will gradually keeping increasing every time and may decrease the no of allocations of memory.

user_97240380
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