Java String n-th index of a substring

//package com.demo2s;

public class Main {
    public static void main(String[] argv) throws Exception {
        String str = "demo2s.com demo";
        String searchStr = "mo";
        int ordinal = 2;
        System.out.println(ordinalIndexOf(str, searchStr, ordinal));
    }//from w w  w.  j  av a  2 s. c  o m

    /**
     * Represents a failed index search.
     * @since 2.1
     */
    public static final int INDEX_NOT_FOUND = -1;

    /**
     * <p>Finds the n-th index within a String, handling <code>null</code>.
     * This method uses {@link String#indexOf(String)}.</p>
     *
     * <p>A <code>null</code> String will return <code>-1</code>.</p>
     *
     * <pre>
     * ordinalIndexOf(null, *, *)          = -1
     * ordinalIndexOf(*, null, *)          = -1
     * ordinalIndexOf("", "", *)           = 0
     * ordinalIndexOf("aabaabaa", "a", 1)  = 0
     * ordinalIndexOf("aabaabaa", "a", 2)  = 1
     * ordinalIndexOf("aabaabaa", "b", 1)  = 2
     * ordinalIndexOf("aabaabaa", "b", 2)  = 5
     * ordinalIndexOf("aabaabaa", "ab", 1) = 1
     * ordinalIndexOf("aabaabaa", "ab", 2) = 4
     * ordinalIndexOf("aabaabaa", "", 1)   = 0
     * ordinalIndexOf("aabaabaa", "", 2)   = 0
     * </pre>
     *
     * @param str  the String to check, may be null
     * @param searchStr  the String to find, may be null
     * @param ordinal  the n-th <code>searchStr</code> to find
     * @return the n-th index of the search String,
     *  <code>-1</code> (<code>INDEX_NOT_FOUND</code>) if no match or <code>null</code> string input
     * @since 2.1
     */
    public static int ordinalIndexOf(String str, String searchStr, int ordinal) {
        if (str == null || searchStr == null || ordinal <= 0) {
            return INDEX_NOT_FOUND;
        }
        if (searchStr.length() == 0) {
            return 0;
        }
        int found = 0;
        int index = INDEX_NOT_FOUND;
        do {
            index = str.indexOf(searchStr, index + 1);
            if (index < 0) {
                return index;
            }
            found++;
        } while (found < ordinal);
        return index;
    }

  
}


/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */