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

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