读源码-String

2015-05-04 Leave a comment

String操作

public class Test {
    public static void main(String[] args) {
        String str = "abCb";
        System.out.println(str.length());
        System.out.println(str.charAt(2));
        System.out.println(str.getBytes());
        System.out.println(str.equals("abc"));
        System.out.println(str.equalsIgnoreCase("aBc"));
        System.out.println(str.compareTo("bbb"));
        System.out.println(str.contains("b"));
        System.out.println(str.startsWith("a"));
        System.out.println(str.endsWith("c"));
        System.out.println(str.indexOf(0));
        System.out.println(str.lastIndexOf(0));
        System.out.println(str.substring(1));
        System.out.println(str.concat("d"));
        System.out.println(str.replace("b", "B"));
        System.out.println(str.replaceAll("b", "B"));
        System.out.println(str.toLowerCase());
        System.out.println(str.toUpperCase());
        System.out.println(str.trim());
        System.out.println(str.split("b"));
    }
}

String实现

String是不可变的，其是一个final类

public final class String

其实现了java.io.Serializable, Comparable, CharSequence三个接口

String是围绕char数组来实现和操作

private final char value[];

length()

length()方法即获取字符数组的长度

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

charAt()

charAt()方法，即是获取字符数组对应下标的值

public char charAt(int index) {
    if ((index < 0) || (index >= value.length)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index];
}

getBytes()

getBytes()方法，通过StringCoding实现

public byte[] getBytes() {
    return StringCoding.encode(value, 0, value.length);
}

equals()

equals()方法，比较两个字符串对应的char数组是否相同

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String) anObject;
        int n = value.length;
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                        return false;
                i++;
            }
            return true;
        }
    }
    return false;
}

equalsIgnoreCase()

equalsIgnoreCase()方法，与equals()方法类似，只不过对每个char都调用了toUpperCase()方法

//核心方法在regionMatches中
public boolean equalsIgnoreCase(String anotherString) {
    return (this == anotherString) ? true
            : (anotherString != null)
            && (anotherString.value.length == value.length)
            && regionMatches(true, 0, anotherString, 0, value.length);
}

compareTo()

compareTo()方法是Comparable接口中的方法，String中的实现如下:

//遍历每个char，根据对应的整形进行比较
public int compareTo(String anotherString) {
    int len1 = value.length;
    int len2 = anotherString.value.length;
    int lim = Math.min(len1, len2);
    char v1[] = value;
    char v2[] = anotherString.value;
    int k = 0;
    while (k < lim) {
        char c1 = v1[k];
        char c2 = v2[k];
        if (c1 != c2) {
            return c1 - c2;
        }
        k++;
    }
    return len1 - len2;
}

contains()

contains()方法实际上根据indexOf方法来进行判断:

static int indexOf(char[] source, int sourceOffset, int sourceCount,
        char[] target, int targetOffset, int targetCount,
        int fromIndex) {
    if (fromIndex >= sourceCount) {
        return (targetCount == 0 ? sourceCount : -1);
    }
    if (fromIndex < 0) {
        fromIndex = 0;
    }
    if (targetCount == 0) {
        return fromIndex;
    }
    char first = target[targetOffset];
    int max = sourceOffset + (sourceCount - targetCount);
    for (int i = sourceOffset + fromIndex; i <= max; i++) {
        /* Look for first character. */
        if (source[i] != first) {
            while (++i <= max && source[i] != first);
        }
        /* Found first character, now look at the rest of v2 */
        if (i <= max) {
            int j = i + 1;
            int end = j + targetCount - 1;
            for (int k = targetOffset + 1; j < end && source[j]
                    == target[k]; j++, k++);
            if (j == end) {
                /* Found whole string. */
                return i - sourceOffset;
            }
        }
    }
    return -1;
}

startsWith()

public boolean startsWith(String prefix, int toffset) {
    char ta[] = value;
    int to = toffset;
    char pa[] = prefix.value;
    int po = 0;
    int pc = prefix.value.length;
    // Note: toffset might be near -1>>>1.
    if ((toffset < 0) || (toffset > value.length - pc)) {
        return false;
    }
    while (--pc >= 0) {
        if (ta[to++] != pa[po++]) {
            return false;
        }
    }
    return true;
}

endsWith()

endsWith()是对startsWith()方法的委托调用

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

indexOf()

indexOf()简单的遍历比较

public int indexOf(int ch, int fromIndex) {
    final int max = value.length;
    if (fromIndex < 0) {
        fromIndex = 0;
    } else if (fromIndex >= max) {
        // Note: fromIndex might be near -1>>>1.
        return -1;
    }
    if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
        // handle most cases here (ch is a BMP code point or a
        // negative value (invalid code point))
        final char[] value = this.value;
        for (int i = fromIndex; i < max; i++) {
            if (value[i] == ch) {
                return i;
            }
        }
        return -1;
    } else {
        return indexOfSupplementary(ch, fromIndex);
    }
}

lastIndexOf()

lastIndexOf()与indexOf()类似

public int lastIndexOf(int ch, int fromIndex) {
    if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
        // handle most cases here (ch is a BMP code point or a
        // negative value (invalid code point))
        final char[] value = this.value;
        int i = Math.min(fromIndex, value.length - 1);
        for (; i >= 0; i--) {
            if (value[i] == ch) {
                return i;
            }
        }
        return -1;
    } else {
        return lastIndexOfSupplementary(ch, fromIndex);
    }
}

substring()

substring()通过截取对应长度的char数组，根据截取后的char数组构建一个新的String实现

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);
}

concat()

concat()构建一个长度为字符串总长度的字符数组，通过此数组构建一个新的字符串

public String concat(String str) {
    int otherLen = str.length();
    if (otherLen == 0) {
        return this;
    }
    int len = value.length;
    char buf[] = Arrays.copyOf(value, len + otherLen);
    str.getChars(buf, len);
    return new String(buf, true);
}

replace(),replaceAll()

replace(),replaceAll()通过正则表达式实现

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);
}

toLowerCase(),toUpperCase()

toLowerCase(),toUpperCase()主要通过Character类中的方法实现，代码过长，不列出。

trim()

trim()通过比较空格字符来处理

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;
}

split()

split()正则表达式实现

public String[] split(String regex, int limit) {
    /* fastpath if the regex is a
     (1)one-char String and this character is not one of the
        RegEx's meta characters ".$|()[{^?*+\\", or
     (2)two-char String and the first char is the backslash and
        the second is not the ascii digit or ascii letter.
     */
    char ch = 0;
    if (((regex.value.length == 1 &&
         ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) ||
         (regex.length() == 2 &&
          regex.charAt(0) == '\\' &&
          (((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 &&
          ((ch-'a')|('z'-ch)) < 0 &&
          ((ch-'A')|('Z'-ch)) < 0)) &&
        (ch < Character.MIN_HIGH_SURROGATE ||
         ch > Character.MAX_LOW_SURROGATE)){
        int off = 0;
        int next = 0;
        boolean limited = limit > 0;
        ArrayList<String> list = new ArrayList<>();
        while ((next = indexOf(ch, off)) != -1) {
            if (!limited || list.size() < limit - 1) {
                list.add(substring(off, next));
                off = next + 1;
            } else {    // last one
                //assert (list.size() == limit - 1);
                list.add(substring(off, value.length));
                off = value.length;
                break;
            }
        }
        // If no match was found, return this
        if (off == 0)
            return new String[]{this};
        // Add remaining segment
        if (!limited || list.size() < limit)
            list.add(substring(off, value.length));
        // Construct result
        int resultSize = list.size();
        if (limit == 0)
            while (resultSize > 0 && list.get(resultSize - 1).length() == 0)
                resultSize--;
        String[] result = new String[resultSize];
        return list.subList(0, resultSize).toArray(result);
    }
    return Pattern.compile(regex).split(this, limit);
}