java的String类下substring方法源码解读

/**
     * Returns a string that is a substring of this string. The
     * substring begins with the character at the specified index and
     * extends to the end of this string. <p>
     * Examples:
     * <blockquote><pre>
     * "unhappy".substring(2) returns "happy"
     * "Harbison".substring(3) returns "bison"
     * "emptiness".substring(9) returns "" (an empty string)
     * </pre></blockquote>
     *
     * @param      beginIndex   the beginning index, inclusive.
     * @return     the specified substring.
     * @exception  IndexOutOfBoundsException  if
     *             {@code beginIndex} is negative or larger than the
     *             length of this {@code String} object.
     */
    public String substring(int beginIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        // 默认长度为字符串的长度减去起始下标，也就是从起始下标一直到末尾
        int subLen = value.length - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);
    }
    /**
     * Returns a string that is a substring of this string. The
     * substring begins at the specified {@code beginIndex} and
     * extends to the character at index {@code endIndex - 1}.
     * Thus the length of the substring is {@code endIndex-beginIndex}.
     * <p>
     * Examples:
     * <blockquote><pre>
     * "hamburger".substring(4, 8) returns "urge"
     * "smiles".substring(1, 5) returns "mile"
     * </pre></blockquote>
     *
     * @param      beginIndex   the beginning index, inclusive.
     * @param      endIndex     the ending index, exclusive.
     * @return     the specified substring.
     * @exception  IndexOutOfBoundsException  if the
     *             {@code beginIndex} is negative, or
     *             {@code endIndex} is larger than the length of
     *             this {@code String} object, or
     *             {@code beginIndex} is larger than
     *             {@code endIndex}.
     */
    public String substring(int beginIndex, int endIndex) {
      // 判断边界条件，如果第一个参数（也就是起始下标小于0,那就下标访问越界抛异常）
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        // 判断边界条件，如果第二个参数（也就是起始截止下标大于字符串的长度,那就下标访问越界抛异常）
        if (endIndex > value.length) {
            throw new StringIndexOutOfBoundsException(endIndex);
        }
        // 截取的长度是后面参数减去前面参数
        int subLen = endIndex - beginIndex;
        // 如果长度小于0，也就是第二个参数大于第一个参数，那就抛异常
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        // 返回从beginIndex开始，截取长度为subLen的字符串
        return ((beginIndex == 0) && (endIndex == value.length)) ? this : new String(value, beginIndex, subLen);
    }