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 t

Answer 1

From the API:

Every string builder has a capacity. As long as the length of the character sequence contained in the string builder does not exceed the capacity, it is not necessary to allocate a new internal buffer. If the internal buffer overflows, it is automatically made larger.

Whenever you append something, there is a check to make sure that the updated StringBuilder won't exceed its capacity, and if it does, the internal storage of the StringBuilder is resized:

int len = str.length();
int newCount = count + len;
if (newCount > value.length)
  expandCapacity(newCount);

When data is added to it that exceeds its capacity it is re-sized according to the following formula:

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);
}

See the src.zip file that comes with the JDK for more information. (Above snippets taken from the 1.6 JDK)

Answer 2

in Java 1.8

public AbstractStringBuilder append(String str) {
    if (str == null)
        return appendNull();
    int len = str.length();
    ensureCapacityInternal(count + len);
    str.getChars(0, len, value, count);
    count += len;
    return this;
}

private void ensureCapacityInternal(int minimumCapacity) {
    // overflow-conscious code
    if (minimumCapacity - value.length > 0) {
        value = Arrays.copyOf(value,
                newCapacity(minimumCapacity));
    }
}

for example :

StringBuilder str = new StringBuilder();  
System.out.println(str.capacity()); //16

str.append("123456789012345"); 
System.out.println(str.capacity()); //16

str.append("12345678901234567890"); 
System.out.println(str.capacity()); // 15 + 20 = 35