How do the StringBuffer insert() and deleteCharAt() methods work

Question

In Java there is methods in either StringBuffer() or StringBuilder() called insert() and deleteCharAt() I am very interested in understanding a little more how these two specific methods work. Or how one would program one using the standard java String methods. I reckon they are pretty simple methods to write yourself?

Answer 1

In the Sun's implementation both methods are delegated to StringBuffer's parent class AbstractStringBuilder:

public synchronized StringBuffer insert(int index, char str[], int offset,
                                        int len) 
{
    super.insert(index, str, offset, len);
    return this;
}

public synchronized StringBuffer deleteCharAt(int index) {
    super.deleteCharAt(index);
    return this;
}

AbstractStringBuffer has the following implementations:

public AbstractStringBuilder insert(int index, char str[], int offset,
                                    int len)
{
    if ((index < 0) || (index > length()))
        throw new StringIndexOutOfBoundsException(index);
    if ((offset < 0) || (len < 0) || (offset > str.length - len))
        throw new StringIndexOutOfBoundsException(
            "offset " + offset + ", len " + len + ", str.length " 
            + str.length);
    int newCount = count + len;
    if (newCount > value.length)
        expandCapacity(newCount);
    System.arraycopy(value, index, value, index + len, count - index);
    System.arraycopy(str, offset, value, index, len);
    count = newCount;
    return this;
}

public AbstractStringBuilder deleteCharAt(int index) {
    if ((index < 0) || (index >= count))
        throw new StringIndexOutOfBoundsException(index);
    System.arraycopy(value, index+1, value, index, count-index-1);
    count--;
    return this;
}

So, nothing special.