java - StringBuilder capacity()

Question

I noticed that the capacity method returns StringBuilder capacity without a logic way ... sometime its value is equals to the string length other time it's greater...

is there an equation for know which is its logic?

Answer 1

When you append to the StringBuilder, the following logic happens:

if (newCount > value.length) {
    expandCapacity(newCount);
}

where newCount is the number of characters needed, and value.length is the current size of the buffer.

expandCapacity simply increases the size of the backing char[]

The ensureCapacity() method is the public way to call expandCapacity(), and its docs say:

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.