Strinig.indexOf(String str, Int fromIndex)

Question

Hello.
In the API reference manual J2SE 5.0 (and earlyer also),
"public indexOf(String str, int fromIndex)"
is discribed such as,
| Returns the index within this string of the first occurrence of the specified substring,
| starting at the specified index. The integer returned is the smallest value k for which:
| k >= Math.min(fromIndex, str.length()) && this.startsWith(str, k)
I can't understand why str.length() is needed here.
For example,
String string = "my beautiful dream";
String str = "ea";
int fromIndex = 10;
in the case, str.ng.indexOf(str, fromIndex) returns 15, the value is what I what to get.
But,I think the smallest value k for which
| k >= Math.min(fromIndex, str.length()) && this.startsWith(str, k)
is 4, isn't it?
import junit.framework.TestCase;
* @author phinloda
* test String.indexOf
public class IndexOfTest extends TestCase {
     public final void testIndexOf() {
          String targetString = "my beautiful dream";
          String str = "ea";
          int fromIndex = 10;
          int k1 = myIndexOf(targetString, str, fromIndex);
          assertTrue(k1 >= Math.min(fromIndex, str.length()) && targetString.startsWith(str, k1));
          int k2 = targetString.indexOf(str, fromIndex);
          assertTrue(k2 >= Math.min(fromIndex, str.length()) && targetString.startsWith(str, k2));
          assertEquals(k1, k2);
     private int myIndexOf(String targetStr, String str, int fromIndex) {
          for (int n = 0; n < targetStr.length(); n++) {
               if (n >= Math.min(fromIndex, str.length()) && targetStr.startsWith(str, n)) {
                    return n;
          return -1;
     public static void main(String[] args) {
          junit.textui.TestRunner.run(IndexOfTest.class);
}lastIndexOf(String str, int fromIndex) is more difficult for me.
("this.length()" is right?, but it's nonsense.)

Answer 1

("this.length()" is right?, but it's nonsense.)Yes, it should be this.length() and not str.length().
I don't think it's nonsense because now the empty
string always gives a non-negative index, e.g.
"string".indexOf("", 1000) returns 6...perhaps this
property is useful for some string algorithmsI think it only does this for an empty String, so it would be clearer if the docs said something about returning this.length() if str.length()==0.