Java源码分析之String

仰慕了已久的String类

成员变量

//内部就是char数组保存，注意是final哦private final char value[];private int hash; // Default to 0

构造函数

public String() {    this.value = "".value;}public String(String original) {    this.value = original.value;    this.hash = original.hash;}//不需要计算hashcode？public String(char value[]) {    this.value = Arrays.copyOf(value, value.length);}

public String(char value[], int offset, int count) {    if (offset < 0) {        throw new StringIndexOutOfBoundsException(offset);    }    if (count <= 0) {        if (count < 0) {            throw new StringIndexOutOfBoundsException(count);        }        if (offset <= value.length) {            this.value = "".value;            return;        }    }    // Note: offset or count might be near -1>>>1.    if (offset > value.length - count) {        throw new StringIndexOutOfBoundsException(offset + count);    }    this.value = Arrays.copyOfRange(value, offset, offset+count);}

//hibyte是什么意思，不太懂，byte转char，必须得& 0xff,否则会出问题public String(byte ascii[], int hibyte, int offset, int count) {    checkBounds(ascii, offset, count);    char value[] = new char[count];    if (hibyte == 0) {        for (int i = count; i-- > 0;) {            value[i] = (char)(ascii[i + offset] & 0xff);        }    } else {        hibyte <<= 8;        for (int i = count; i-- > 0;) {            value[i] = (char)(hibyte | (ascii[i + offset] & 0xff));        }    }    this.value = value;}

public String(byte bytes[], int offset, int length, String charsetName)        throws UnsupportedEncodingException {    if (charsetName == null)        throw new NullPointerException("charsetName");    checkBounds(bytes, offset, length);    this.value = StringCoding.decode(charsetName, bytes, offset, length);}public String(byte bytes[], int offset, int length) {    checkBounds(bytes, offset, length);    this.value = StringCoding.decode(bytes, offset, length);}

//注意是synchronizedpublic String(StringBuffer buffer) {    synchronized(buffer) {        this.value = Arrays.copyOf(buffer.getValue(), buffer.length());    }}public String(StringBuilder builder) {    this.value = Arrays.copyOf(builder.getValue(), builder.length());}

基本方法

length && isEmpty

public int length() {    return value.length;}public boolean isEmpty() {    return value.length == 0;}

charAt

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

getChars

void getChars(char dst[], int dstBegin) {    System.arraycopy(value, 0, dst, dstBegin, value.length);}//将srcBegin-srcEnd拷贝至dstpublic void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {    if (srcBegin < 0) {        throw new StringIndexOutOfBoundsException(srcBegin);    }    if (srcEnd > value.length) {        throw new StringIndexOutOfBoundsException(srcEnd);    }    if (srcBegin > srcEnd) {        throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);    }    System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);}

equals

//如果先比hashcode是不更好一点儿？如果相等，再一位一位比较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;}

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

public boolean contentEquals(CharSequence cs) {    // Argument is a StringBuffer, StringBuilder    if (cs instanceof AbstractStringBuilder) {        if (cs instanceof StringBuffer) {            synchronized(cs) {               return nonSyncContentEquals((AbstractStringBuilder)cs);            }        } else {            return nonSyncContentEquals((AbstractStringBuilder)cs);        }    }    // Argument is a String    if (cs instanceof String) {        return equals(cs);    }    // Argument is a generic CharSequence    char v1[] = value;    int n = v1.length;    if (n != cs.length()) {        return false;    }    for (int i = 0; i < n; i++) {        if (v1[i] != cs.charAt(i)) {            return false;        }    }    return true;}

compareTo

//按字典序比较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;}

startsWith

//找value从toffest开始，与prefix是否相等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;}public boolean startsWith(String prefix) {    return startsWith(prefix, 0);}public boolean endsWith(String suffix) {    return startsWith(suffix, value.length - suffix.value.length);}

hashCode

//string的hashcode的算法，依旧用的31，这里初始是0，hashmap里初始是1public int hashCode() {    int h = hash;    if (h == 0 && value.length > 0) {        char val[] = value;        for (int i = 0; i < value.length; i++) {            h = 31 * h + val[i];        }        hash = h;    }    return h;}

indexOf

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);    }}

//为何参数这么多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);    //暴力找，居然不是kmp什么的    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;}

subString

// 当子串刚好是本串时，就返回this，这也是String的一大思想，不可变，对于一个确定的字符串，可以用同一个对象表示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);}

concat

//讲str添加到value的后面，这里也给出了一种字符串相加的方法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

//将字符串中所有的oldChar替换为newCharpublic String replace(char oldChar, char newChar) {    //如果两个字符相等，直接返回本串    if (oldChar != newChar) {        int len = value.length;        int i = -1;        char[] val = value; /* avoid getfield opcode */        while (++i < len) {            if (val[i] == oldChar) {                break;            }        }        //如果本串中没有oldChar，直接返回本串        if (i < len) {            char buf[] = new char[len];            for (int j = 0; j < i; j++) {                buf[j] = val[j];            }            while (i < len) {                char c = val[i];                buf[i] = (c == oldChar) ? newChar : c;                i++;            }            return new String(buf, true);        }    }    return this;}

toLowerCase

//locale，居然可以选语言！真厉害了！当然，默认是英语的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);}//默认英语public String toLowerCase() {    returntoLowerCase(Locale.getDefault());}//toUpperCase基本同理，就不贴了

trim

//判断条件是 <=' '，所以前面的tab，换行之类的，都可以去掉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;}

toString

//String.toString，感觉蛮搞笑的public String toString() {    return this;}

toCharArray

public char[] toCharArray() {    // Cannotuse Arrays.copyOf because of class initialization order issues    char result[] = new char[value.length];    System.arraycopy(value, 0, result, 0, value.length);    return result;}

valueOf

//对于char，直接构造就好public static String valueOf(char data[]) {    return new String(data);}//咦，这俩不一样的吗public static String copyValueOf(char data[]) {    return new String(data);}public static String valueOf(char data[], int offset, int count) {    return new String(data, offset, count);}//咦，这俩不一样的吗public static String copyValueOf(char data[], int offset, int count) {    return new String(data, offset, count);}

//booleanpublic static String valueOf(boolean b) {    return b ? "true" : "false";}//int，调用的Integer.toStringpublic static String valueOf(int i) {    return Integer.toString(i);}//对于long，float，double都是调用toString

matches

//匹配正则public boolean matches(String regex) {    return Pattern.matches(regex, this);}

split

//有点儿复杂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);}

getBytes

public void getBytes(int srcBegin, int srcEnd, byte dst[], int dstBegin) {    if (srcBegin < 0) {        throw new StringIndexOutOfBoundsException(srcBegin);    }    if (srcEnd > value.length) {        throw new StringIndexOutOfBoundsException(srcEnd);    }    if (srcBegin > srcEnd) {        throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);    }    Objects.requireNonNull(dst);    //为何要重新赋值？    int j = dstBegin;    int n = srcEnd;    int i = srcBegin;    char[] val = value;   /* avoid getfield opcode */    while (i < n) {        dst[j++] = (byte)val[i++];    }}public byte[] getBytes(String charsetName)        throws UnsupportedEncodingException {    if (charsetName == null) throw new NullPointerException();    return StringCoding.encode(charsetName, value, 0, value.length);}public byte[] getBytes(Charset charset) {    if (charset == null) throw new NullPointerException();    return StringCoding.encode(charset, value, 0, value.length);}public byte[] getBytes() {    return StringCoding.encode(value, 0, value.length);}