String源码解析一

---

title: String源码解析一
date: 2018-11-07 15:00:00
tags: JAVA基础

一.类定义、属性与构造函数

类定义

public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {

final说明类是常量，String类实现了Serializable, Comparable, CharSequence接口，Comparable接口有compareTo方法，而CharSequence接口有length、charAt、subSequence、toString方法，jdk1.8新增了chars和codePoints方法。

属性

 /** The value is used for character storage. */
private final char value[];

/** Cache the hash code for the string */
private int hash; // Default to 0

/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;

/**
 * Class String is special cased within the Serialization Stream Protocol.
 *
 * A String instance is written into an ObjectOutputStream according to
 * <a href="{@docRoot}/../platform/serialization/spec/output.html">
 * Object Serialization Specification, Section 6.2, "Stream Elements"</a>
 */
private static final ObjectStreamField[] serialPersistentFields =
    new ObjectStreamField[0];

value：不可变字符数组，用来存储String值

hash：用来缓存计算后的hash 值

常用构造函数

public String() {
    this.value = "".value;
}

public String(String original) {
    this.value = original.value;
    this.hash = original.hash;
}

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);
}

二.常用方法

应用的最多的方法，需要重点关注下！

length、isEmpty、charAt

//字符串的长度
public int length() {
    return value.length;
}

//是否为空
public boolean isEmpty() {
    return value.length == 0;
}

//返回某个索引下的字符
public char charAt(int index) {
    if ((index < 0) || (index >= value.length)) {
        throw new StringIndexOutOfBoundsException(index);
    }
    return value[index];
}

equals：判断两个String对象是否相等
两个String对象的规则如下：

1.首先判断两个对象的内存地址是否相同，若是，返回true

2.判断比较的对象是否是String类型

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3.比较两个对象的长度是否相等

4.比较两个对象的每个字符是否相等

public boolean equals(Object anObject) {

	//1.首先判断是否是对象本身，若是，返回true
    if (this == anObject) {
        return true;
    }
	//2.判断比较的对象是否是String类型
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = value.length;
		//3.比较两个对象的长度是否相等
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
			//4.比较两个对象的每个字符是否相等
            while (n-- != 0) {
                if (v1[i] != v2[i])
                    return false;
                i++;
            }
            return true;
        }
    }
    return false;
}

compareTo：比较两个String对象的大小

public int compareTo(String anotherString) {
    int len1 = value.length;
    int len2 = anotherString.value.length;
	//1.获取两个字符串对象的长度最小值
    int lim = Math.min(len1, len2);
	//2.将两个字符串转换成字符数组
    char v1[] = value;
    char v2[] = anotherString.value;

    int k = 0;
	//3.从两个字符数组的第一个字符开始比较，直到最小值lim为止
    while (k < lim) {
        char c1 = v1[k];
        char c2 = v2[k];
        if (c1 != c2) {
            return c1 - c2;
        }
        k++;
    }
	//4.若之前的字符都相同，则比较两个字符串的长度，长度长的大
    return len1 - len2;
}

startsWith：是否以某个指定的字符串开始

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.
	//1.判定toffset是否符合规范,如果起始地址小于0或者（起始地址+所比较对象长度）大于自身对象长度，返回false
    if ((toffset < 0) || (toffset > value.length - pc)) {
        return false;
    }
	//2.对象从指定位置与比较对象依次比较是否相同
    while (--pc >= 0) {
        if (ta[to++] != pa[po++]) {
            return false;
        }
    }
    return true;
}

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

indexOf：返回第一次出现的指定字符串的索引位置

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

public int indexOf(String str, int fromIndex) {
    return indexOf(value, 0, value.length,
            str.value, 0, str.value.length, fromIndex);
}

static int indexOf(char[] source, int sourceOffset, int sourceCount,
        char[] target, int targetOffset, int targetCount,
        int fromIndex) {

	//1.判断参数fromIndex的合法性
    if (fromIndex >= sourceCount) {
        return (targetCount == 0 ? sourceCount : -1);
    }
    if (fromIndex < 0) {
        fromIndex = 0;
    }
    if (targetCount == 0) {
        return fromIndex;
    }
	
	//2.获取目标字符串第一个字符和源字符串和目标字符串的长度差
    char first = target[targetOffset];
    int max = sourceOffset + (sourceCount - targetCount);
	
    for (int i = sourceOffset + fromIndex; i <= max; i++) {
        /* Look for first character. */
		//3.寻找源字符串中匹配的第一个字符
        if (source[i] != first) {
            while (++i <= max && source[i] != first);
        }

        /* Found first character, now look at the rest of v2 */
		//4.找到第一个字符后，依次匹配剩下的字符，并返回源字符串的索引位置
        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;
            }
        }
    }
	//5.若不匹配，返回-1
    return -1;
}

substring：获取指定索引开始的子字符串

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);
}

concat：将指定的字符串参数连接到字符串上

public String concat(String str) {
    int otherLen = str.length();
	//1.若拼接字符串为空，返回原字符串
    if (otherLen == 0) {
        return this;
    }
    int len = value.length;
	//2.将指定字符串拼接在原字符串上，并返回新的字符串对象
    char buf[] = Arrays.copyOf(value, len + otherLen);
    str.getChars(buf, len);
    return new String(buf, true);
}

replace：替换指定的字符

 public String replace(char oldChar, char newChar) {
    if (oldChar != newChar) {
        int len = value.length;
        int i = -1;
        char[] val = value; /* avoid getfield opcode */
		
		//1.获取源字符串中出现指定字符串的索引位置
        while (++i < len) {
            if (val[i] == oldChar) {
                break;
            }
        }
        if (i < len) {
            char buf[] = new char[len];
			//2.将指定源字符串中出现指定字符之前的字符串记录下来
            for (int j = 0; j < i; j++) {
                buf[j] = val[j];
            }
            while (i < len) {
				//3.将后面的字符依次替换为新字符
                char c = val[i];
                buf[i] = (c == oldChar) ? newChar : c;
                i++;
            }
			//4.返回新创建的字符串
            return new String(buf, true);
        }
    }
    return this;
}

//正则表达式匹配
public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
            this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}

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

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

split：根据所传递的regex字符串分割当前对象

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

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 ".$|()[{^?*+\\", or
     (2)two-char String and the first char is the backslash and
        the second is not the ascii digit or ascii letter.
     */
	//1.字符串匹配的原则一，如果是一个字符的话不能是正则表达式的元字符
	//2.字符串匹配的原则二，如果是两个字符的话第一个不能是反斜杠，第二个不能是数字或字母
	//3.我们这里设定limit=0,也就是split(String regex)方法的使用
    char ch = 0;
    if (((regex.value.length == 1 &&
         ".$|()[{^?*+\\".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) {
			//4.字符串中依次匹配regex字符
            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;
            }
        }
		//5.若一个都没匹配上，返回原字符串数组
        // If no match was found, return this
        if (off == 0)
            return new String[]{this};

        // Add remaining segment
		//6.匹配剩下的字符串拼接到数组后
        if (!limited || list.size() < limit)
            list.add(substring(off, value.length));

        // Construct result
        int resultSize = list.size();
        if (limit == 0) {
		//7.确定匹配后实际的字符串数组长度
            while (resultSize > 0 && list.get(resultSize - 1).length() == 0) {
                resultSize--;
            }
        }
		//8.返回最终匹配的字符串数组
        String[] result = new String[resultSize];
        return list.subList(0, resultSize).toArray(result);
    }
	//9.若不是上面的匹配情况，进行正则匹配
    return Pattern.compile(regex).split(this, limit);
}

toLowerCase：将所有字符串转换成小写

使用给定的Locale的规则将此String中的所有字符转换为小写。

大小写映射基于Character类指定的Unicode标准版本，由于大小写映射并不总是1：1字符映射，因此生成的字符串可能与原始字符串的长度不同

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

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. */
	//1.扫描字符串是否存在大写的字符
    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. */
	//2.拷贝前面的小写字符集
    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;
	//3.由于大写与小写的字符比例不一定是1:1，所以转换成小写时需要统计转换后的字符串长度再转换
    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);
}

toLowerCase：将所有字符串转换成大写，源码类似于toLowerCase

trim：过滤掉字符串首尾空格

public String trim() {
    int len = value.length;
    int st = 0;
    char[] val = value;    /* avoid getfield opcode */
	//1.去掉起始空格
    while ((st < len) && (val[st] <= ' ')) {
        st++;
    }
	//2.去掉末尾空格
    while ((st < len) && (val[len - 1] <= ' ')) {
        len--;
    }
    return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}

valueOf：将其他类型的值转换为String类型

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

public static String valueOf(char data[]) {
    return new String(data);
}

public static String valueOf(boolean b) {
    return b ? "true" : "false";
}

public static String valueOf(char c) {
    char data[] = {c};
    return new String(data, true);
}

public static String valueOf(int i) {
    return Integer.toString(i);
}

public static String valueOf(long l) {
    return Long.toString(l);
}

public static String valueOf(float f) {
    return Float.toString(f);
}

public static String valueOf(double d) {
    return Double.toString(d);
}