是否包含字符串

public boolean contains(CharSequence s) {
    return indexOf(s.toString()) > -1;
}

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

public int lastIndexOf(String str) {
    return lastIndexOf(str, value.length);
}

public boolean startsWith(String prefix) {
    return startsWith(prefix, 0);
}

public boolean endsWith(String suffix) {
    return startsWith(suffix, value.length - suffix.value.length);
}

去除首位空白字符串

public String trim() {
    int len = value.length;
    int st = 0;
    char[] val = value;    /* avoid getfield opcode */

    while ((st < len) && (val[st] <= ' ')) {
        st++;
    }
    while ((st < len) && (val[len - 1] <= ' ')) {
        len--;
    }
    return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}

提取字符串

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);
}

大小写转换

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

替换字符串

public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
    this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}

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

分割与拼接

public static String join(CharSequence delimiter, CharSequence... elements) {
    Objects.requireNonNull(delimiter);
    Objects.requireNonNull(elements);
    // Number of elements not likely worth Arrays.stream overhead.
    StringJoiner joiner = new StringJoiner(delimiter);
    for (CharSequence cs: elements) {
        joiner.add(cs);
    }
    return joiner.toString();
}

public String[] split(String regex) {
    return split(regex, 0);
}

字符串与byte转换

byte[] data = "中文ABC".getBytes(StandardCharsets.UTF_8);
String str  = new String(data,StandardCharsets.UTF_8);