读面试题有感

本区域专门总结遇到的各种面试题，本来都很熟悉了，谁知道几天过后我啥都忘了，只能记录下来

京北方：这个是外包，外包的考题都很基础，但是代码量少的人肯定是过不了关的。它考了一个String的大部分方法，我居然一个也做不出来。

1.

public String toUpperCase() {
    return toUpperCase(Locale.getDefault());
}

2.

public int length() {
    return value.length;
}

3.截取子串-------

public String substring(int beginIndex) {
    if (beginIndex < 0) {
        throw new StringIndexOutOfBoundsException(beginIndex);
    }
    int subLen = value.length - beginIndex;
    if (subLen < 0) {
        throw new StringIndexOutOfBoundsException(subLen);
    }
    return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);
}

------------------.

public String substring(int beginIndex, int endIndex) {
    if (beginIndex < 0) {
        throw new StringIndexOutOfBoundsException(beginIndex);
    }
    if (endIndex > value.length) {
        throw new StringIndexOutOfBoundsException(endIndex);
    }
    int subLen = endIndex - beginIndex;
    if (subLen < 0) {
        throw new StringIndexOutOfBoundsException(subLen);
    }
    return ((beginIndex == 0) && (endIndex == value.length)) ? this
            : new String(value, beginIndex, subLen);
}

4.替换字符串

public String replaceAll(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceAll(replacement);
}

5.转成小写

public String toLowerCase(Locale locale) {
    if (locale == null) {
        throw new NullPointerException();
    }

    int firstUpper;
    final int len = value.length;

    /* Now check if there are any characters that need to be changed. */
    scan: {
        for (firstUpper = 0 ; firstUpper < len; ) {
            char c = value[firstUpper];
            if ((c >= Character.MIN_HIGH_SURROGATE)
                    && (c <= Character.MAX_HIGH_SURROGATE)) {
                int supplChar = codePointAt(firstUpper);
                if (supplChar != Character.toLowerCase(supplChar)) {
                    break scan;
                }
                firstUpper += Character.charCount(supplChar);
            } else {
                if (c != Character.toLowerCase(c)) {
                    break scan;
                }
                firstUpper++;
            }
        }
        return this;
    }

    char[] result = new char[len];
    int resultOffset = 0;  /* result may grow, so i+resultOffset
                            * is the write location in result */

    /* Just copy the first few lowerCase characters. */
    System.arraycopy(value, 0, result, 0, firstUpper);

    String lang = locale.getLanguage();
    boolean localeDependent =
            (lang == "tr" || lang == "az" || lang == "lt");
    char[] lowerCharArray;
    int lowerChar;
    int srcChar;
    int srcCount;
    for (int i = firstUpper; i < len; i += srcCount) {
        srcChar = (int)value[i];
        if ((char)srcChar >= Character.MIN_HIGH_SURROGATE
                && (char)srcChar <= Character.MAX_HIGH_SURROGATE) {
            srcChar = codePointAt(i);
            srcCount = Character.charCount(srcChar);
        } else {
            srcCount = 1;
        }
        if (localeDependent ||
            srcChar == '\u03A3' || // GREEK CAPITAL LETTER SIGMA
            srcChar == '\u0130') { // LATIN CAPITAL LETTER I WITH DOT ABOVE
            lowerChar = ConditionalSpecialCasing.toLowerCaseEx(this, i, locale);
        } else {
            lowerChar = Character.toLowerCase(srcChar);
        }
        if ((lowerChar == Character.ERROR)
                || (lowerChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {
            if (lowerChar == Character.ERROR) {
                lowerCharArray =
                        ConditionalSpecialCasing.toLowerCaseCharArray(this, i, locale);
            } else if (srcCount == 2) {
                resultOffset += Character.toChars(lowerChar, result, i + resultOffset) - srcCount;
                continue;
            } else {
                lowerCharArray = Character.toChars(lowerChar);
            }

            /* Grow result if needed */
            int mapLen = lowerCharArray.length;
            if (mapLen > srcCount) {
                char[] result2 = new char[result.length + mapLen - srcCount];
                System.arraycopy(result, 0, result2, 0, i + resultOffset);
                result = result2;
            }
            for (int x = 0; x < mapLen; ++x) {
                result[i + resultOffset + x] = lowerCharArray[x];
            }
            resultOffset += (mapLen - srcCount);
        } else {
            result[i + resultOffset] = (char)lowerChar;
        }
    }
    return new String(result, 0, len + resultOffset);
}