Differences in Object modifications

Question

i was just wondering if anybody could help me out with this :

    StringBuilder s=new StringBuilder(\"0123456789\");
    s.substring(1, 2);
    System.out.printl         


        

Answer 1

If you see the substring method definition in AbstractStringBuilder abstract class which later extended by StringBuilder class, you will find below code:

public String substring(int start, int end) {
    if (start < 0)
        throw new StringIndexOutOfBoundsException(start);
    if (end > count)
        throw new StringIndexOutOfBoundsException(end);
    if (start > end)
        throw new StringIndexOutOfBoundsException(end - start);
    return new String(value, start, end - start);
}

From the method definition you can see that it is returning a new String object, the method is not working on actual StringBuilder content. So their will no change in the content of StringBuilder object but rather a new String object will be returned.

Now if you see delete method definition inside StringBuilder class it is:

@Override
public StringBuilder delete(int start, int end) {
    super.delete(start, end);
    return this;
}

And the definition of delete in AbstractStringBuilder (StringBuilder super class) is :

public AbstractStringBuilder delete(int start, int end) {
    if (start < 0)
        throw new StringIndexOutOfBoundsException(start);
    if (end > count)
        end = count;
    if (start > end)
        throw new StringIndexOutOfBoundsException();
    int len = end - start;
    if (len > 0) {
        System.arraycopy(value, start+len, value, start, count-end);
        count -= len;
    }
    return this;
}

From the method definition it could be clearly understood that it is working on same StringBuilder object content and it is not returning a new object but rather the same object reference which is passed to it.