subString源码分析

Posted 2020-11-07 ming_szu

1. Public String subString(int beginIndex)

    /**返回目标String中截取的指定String
     * 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.　　  开始位(包括在结果中)
     * @return     the specified substring.　　　　　　　　　　　　　　 指定的Strng
     * @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) {　　　　　　　　　　　　　　　　　　　　　　//当开始位beginIndex小于0抛出异常StringIndexOutOfBoundsException()
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        int subLen = value.length - beginIndex;
        if (subLen < 0) {　　　　　　　　　　　　　　　　　　　　　　　　//要截取的String的长度小于0时抛出异常StringIndexOutOfBoundsException()
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);　　//当开始位beginIndex等于0时相当于返回字符串本身否则返回新的String
    }

 

2.public String substring(int beginIndex, int endIndex)

/**
     * 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>
　　　* 返回一个从本字符串截取的新的字符串
     * Examples:
     * <blockquote><pre>
     * "hamburger".substring(4, 8) returns "urge"
     * "smiles".substring(1, 5) returns "mile"
     * </pre></blockquote>
     *
     * @param      beginIndex   the beginning index, inclusive.　　开始截取位置（包括在内）
     * @param      endIndex     the ending index, exclusive.　　　　结束位置（不包括在内）
     * @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) {
        if (beginIndex < 0) {　　　　　　　　　　　　　　　　　　　　　　　　//开始位置小于0时抛出异常
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        if (endIndex > value.length) {　　　　　　　　　　　　　　　　　　　//结束位置大于本字符串长度时抛出异常（String通过char[] value储存）
            throw new StringIndexOutOfBoundsException(endIndex);
        }
        int subLen = endIndex - beginIndex;
        if (subLen < 0) {　　　　　　　　　　　　　　　　　　　　　　　　　　//截取长度小于0时抛出异常
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return ((beginIndex == 0) && (endIndex == value.length)) ? this　　//当开始位置等与0且结束位置等于字符串长度时返回本身，否则新建一个String返回
                : new String(value, beginIndex, subLen);
    }

 

3.public String(char value[],int offset,int count)

/**
     * Allocates a new {@code String} that contains characters from a subarray
     * of the character array argument. The {@code offset} argument is the
     * index of the first character of the subarray and the {@code count}
     * argument specifies the length of the subarray. The contents of the
     * subarray are copied; subsequent modification of the character array does
     * not affect the newly created string.
　　　* 分配一个新的String，从一个char[]数组截取（String类带有char[] value成员，它是String的主要存储结构），
　　　* 参数offset是开始复制的位置，count是复制的长度大小
     *
     * @param  value　　　　
     *         Array that is the source of characters　　源数组
     *
     * @param  offset
     *         The initial offset　初始位
     *
     * @param  count
     *         The length　　　　复制的长度
     *
     * @throws  IndexOutOfBoundsException
     *          If the {@code offset} and {@code count} arguments index
     *          characters outside the bounds of the {@code value} array
     */
    public String(char value[], int offset, int count) {
        if (offset < 0) {　　　　　　　　　　　　　　　　　　　　　　　　//开始位小于0抛出异常
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count <= 0) {　　　　　　　　　　　　　　　　　　　　　　　　//复制长度小于等于0时
            if (count < 0) {　　　　　　　　　　　　　　　　　　　　　　//复制长度小于0抛出异常
                throw new StringIndexOutOfBoundsException(count);
            }
            if (offset <= value.length) {　　　　　　　　　　　　　　 //复制长度等于0且开始位置小于等于value长度时返回空的String
                this.value = "".value;
                return;
            }
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > value.length - count) {　　　　　　　　　　　　 //开始位+长度大于源char数组的长度时抛出错误
            throw new StringIndexOutOfBoundsException(offset + count);
        }
        this.value = Arrays.copyOfRange(value, offset, offset+count);　　//调用Arrays.copyRange进行复制
    }

 

4.public static char[] copyOfRange(char[] original, int from, int to)

    /**
     * Copies the specified range of the specified array into a new array.
     * The initial index of the range (<tt>from</tt>) must lie between zero
     * and <tt>original.length</tt>, inclusive.  The value at
     * <tt>original[from]</tt> is placed into the initial element of the copy
     * (unless <tt>from == original.length</tt> or <tt>from == to</tt>).
     * Values from subsequent elements in the original array are placed into
     * subsequent elements in the copy.  The final index of the range
     * (<tt>to</tt>), which must be greater than or equal to <tt>from</tt>,
     * may be greater than <tt>original.length</tt>, in which case
     * <tt>‘\\u000‘</tt> is placed in all elements of the copy whose index is
     * greater than or equal to <tt>original.length - from</tt>.  The length
     * of the returned array will be <tt>to - from</tt>.
     * 从源char[]数组复制成新的char[]数组
　　　* 
     * @param original the array from which a range is to be copied　　　　　　源char[]数组
     * @param from the initial index of the range to be copied, inclusive　　 源数组的开始复制位置(包括在内)
     * @param to the final index of the range to be copied, exclusive.　　　　 源数组的复制结束位置（不包括在内）
     *     (This index may lie outside the array.)
     * @return a new array containing the specified range from the original array,
     *     truncated or padded with null characters to obtain the required length
     * @throws ArrayIndexOutOfBoundsException if {@code from < 0}
     *     or {@code from > original.length}
     * @throws IllegalArgumentException if <tt>from &gt; to</tt>
     * @throws NullPointerException if <tt>original</tt> is null
     * @since 1.6
     */
    public static char[] copyOfRange(char[] original, int from, int to) {
        int newLength = to - from;
        if (newLength < 0)　　　　　　　　　　　　　　　　　　　　　　　　　　　　//新数组长度小于0抛出异常
            throw new IllegalArgumentException(from + " > " + to);
        char[] copy = new char[newLength];　　　　　　　　　　　　　　　　　　 //新建一个数组
        System.arraycopy(original, from, copy, 0,　　　　　　　　　　　　　　 //调用arraycopy来复制修改新数组的值　　
                         Math.min(original.length - from, newLength));
        return copy;
    }