String.indexOf()的使用方法

String.indexOf()的用途：

返回此字符串中第一个出现的指定的子字符串，如果没有找到则返回-1

源码如下:

/**
 * Returns the index within this string of the first occurrence of the
 * specified substring.
 *
 * <p>The returned index is the smallest value <i>k</i> for which:
 * <blockquote><pre>
 * this.startsWith(str, <i>k</i>)
 * </pre></blockquote>
 * If no such value of <i>k</i> exists, then {@code -1} is returned.
 *
 * @param   str   the substring to search for.
 * @return  the index of the first occurrence of the specified substring,
 *          or {@code -1} if there is no such occurrence.
 */

public int indexOf(String str) {
        return indexOf(str, 0);
    }

举例1：包含指定子字符串的情况

String str1="abcdef";
String str2="cd";
System.out.println(str1.indexOf(str2));

输出结果：2

举例2：未包含指定子字符串的情况

String str1="abcdef";
String str2="gh";
System.out.println( str1.indexOf(str2) );

输出结果：-1

它还有一个重载的方法：从指定的索引开始，返回此字符串中第一个出现的指定的子字符串，如果没有找到则返回-1

源码如下：

/**
     * Returns the index within this string of the first occurrence of the
     * specified substring, starting at the specified index.
     *
     * <p>The returned index is the smallest value <i>k</i> for which:
     * <blockquote><pre>
     * <i>k</i> &gt;= fromIndex {@code &&} this.startsWith(str, <i>k</i>)
     * </pre></blockquote>
     * If no such value of <i>k</i> exists, then {@code -1} is returned.
     *
     * @param   str         the substring to search for.
     * @param   fromIndex   the index from which to start the search.
     * @return  the index of the first occurrence of the specified substring,
     *          starting at the specified index,
     *          or {@code -1} if there is no such occurrence.
     */
    public int indexOf(String str, int fromIndex) {
        return indexOf(value, 0, value.length,
                str.value, 0, str.value.length, fromIndex);
    }

举例3：从指定的索引开始，包含指定子字符串的情况

 String str3="abcdef0abcdef";
 String str4="cd";
 System.out.println( str3.indexOf(str4,5) );

输出结果：9

举例3：从指定的索引开始，未包含指定子字符串的情况

String str3="abcdef0abcdef";
String str4="gh";
System.out.println( str3.indexOf(str4,5) );

输出结果：-1