Java中String、StringBuilder、StringBuffer常用源码分析及比较（一）：String源码分析

String：

一、成员变量：

/** 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;

其中字符数组value[]是String用来存贮字符串的容器，换句话说String是使用字符数组实现的，值得注意的是这个字符数组用到了final修饰，意味着其中的字符串一旦在构造方法中初始化将不能被修改，这也是String字符串在做拼接时，要新建很多String对象的原因；

hash这个变量则是存贮一个String对象的hash值；

String类型实现了Serializable序列化标记接口，所以拥有序列化ID即serialVersionUID。

二、构造方法：

/**
     * Initializes a newly created {@code String} object so that it represents
     * an empty character sequence.  Note that use of this constructor is
     * unnecessary since Strings are immutable.
     */
    public String() {
        this.value = new char[0];
    }

    /**
     * Initializes a newly created {@code String} object so that it represents
     * the same sequence of characters as the argument; in other words, the
     * newly created string is a copy of the argument string. Unless an
     * explicit copy of {@code original} is needed, use of this constructor is
     * unnecessary since Strings are immutable.
     *
     * @param  original
     *         A {@code String}
     */
    public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }

    /**
     * Allocates a new {@code String} so that it represents the sequence of
     * characters currently contained in the character array argument. The
     * contents of the character array are copied; subsequent modification of
     * the character array does not affect the newly created string.
     *
     * @param  value
     *         The initial value of the string
     */
    public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }

    /**
     * Allocates a new {@code String} that contains characters from a subarray
     * of the character array argument. The {@code offset} argument is the
     * index of the first character of the subarray and the {@code count}
     * argument specifies the length of the subarray. The contents of the
     * subarray are copied; subsequent modification of the character array does
     * not affect the newly created string.
     *
     * @param  value
     *         Array that is the source of characters
     *
     * @param  offset
     *         The initial offset
     *
     * @param  count
     *         The length
     *
     * @throws  IndexOutOfBoundsException
     *          If the {@code offset} and {@code count} arguments index
     *          characters outside the bounds of the {@code value} array
     */
    public String(char value[], int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        // 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);
    }

其中无参构造方法，可以看到是创建了一个容量为0的char数组，可用性不高；

String(String original)方法，则是将original直接拷贝了一份到新的String中，但别看是拷贝若用“==”去对两个String做判断，结果还是会返回false，因为两者指向的并非同一个地址，都是一个新的String对象；

String(char value[])方法直接调用Arrays.copy方法将参数value拷贝进了String中的value，而Arrays.copy内部的调用如下，是创建了一个新的char数组，而非直接将参数value赋给String中的value，故参数的value与String中的values没有指向同一个地址；

public static char[] copyOf(char[] original, int newLength) {
        char[] copy = new char[newLength];
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

public String(char value[], int offset, int count)则是将value中的一部分（以下标offset开头，长度为count），复制给了String；

构造方法中还有一个以整型数组为参数的构造方法，如下：

/**
     * Allocates a new {@code String} that contains characters from a subarray
     * of the <a href="Character.html#unicode">Unicode code point</a> array
     * argument.  The {@code offset} argument is the index of the first code
     * point of the subarray and the {@code count} argument specifies the
     * length of the subarray.  The contents of the subarray are converted to
     * {@code char}s; subsequent modification of the {@code int} array does not
     * affect the newly created string.
     *
     * @param  codePoints
     *         Array that is the source of Unicode code points
     *
     * @param  offset
     *         The initial offset
     *
     * @param  count
     *         The length
     *
     * @throws  IllegalArgumentException
     *          If any invalid Unicode code point is found in {@code
     *          codePoints}
     *
     * @throws  IndexOutOfBoundsException
     *          If the {@code offset} and {@code count} arguments index
     *          characters outside the bounds of the {@code codePoints} array
     *
     * @since  1.5
     */
    public String(int[] codePoints, int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > codePoints.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }

        final int end = offset + count;

        // Pass 1: Compute precise size of char[]
        int n = count;
        for (int i = offset; i < end; i++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                continue;
            else if (Character.isValidCodePoint(c))
                n++;
            else throw new IllegalArgumentException(Integer.toString(c));
        }

        // Pass 2: Allocate and fill in char[]
        final char[] v = new char[n];

        for (int i = offset, j = 0; i < end; i++, j++) {
            int c = codePoints[i];
            if (Character.isBmpCodePoint(c))
                v[j] = (char)c;
            else
                Character.toSurrogates(c, v, j++);
        }

        this.value = v;
    }

可知：这个方法并非简单的将整型数组中的数转成字符（如1转成‘1’），而是找该整数UniCode码对应的字符，传入value数组中，其中isBmpCodePoint方法则是判定这个整数是否在一个码点之内（这个码点是UniCode码表中字符是一个字节还是两个字节的分解线，Unicode码中有一些字符是两个字节，如汉字），若不在这个范围内，自然容量要+1，否则char数组value将装不下。

还有一些构造方法不常用，在此就不做描述；

三、成员方法（列举常用的几个方法）

1.indexOf方法

/**
     * Returns the index within this string of the first occurrence of
     * the specified character. If a character with value
     * <code>ch</code> occurs in the character sequence represented by
     * this <code>String</code> object, then the index (in Unicode
     * code units) of the first such occurrence is returned. For
     * values of <code>ch</code> in the range from 0 to 0xFFFF
     * (inclusive), this is the smallest value <i>k</i> such that:
     * <blockquote><pre>
     * this.charAt(<i>k</i>) == ch
     * </pre></blockquote>
     * is true. For other values of <code>ch</code>, it is the
     * smallest value <i>k</i> such that:
     * <blockquote><pre>
     * this.codePointAt(<i>k</i>) == ch
     * </pre></blockquote>
     * is true. In either case, if no such character occurs in this
     * string, then <code>-1</code> is returned.
     *
     * @param   ch   a character (Unicode code point).
     * @return  the index of the first occurrence of the character in the
     *          character sequence represented by this object, or
     *          <code>-1</code> if the character does not occur.
     */
    public int indexOf(int ch) {
        return indexOf(ch, 0);
    }

    /**
     * Returns the index within this string of the first occurrence of the
     * specified character, starting the search at the specified index.
     * <p>
     * If a character with value <code>ch</code> occurs in the
     * character sequence represented by this <code>String</code>
     * object at an index no smaller than <code>fromIndex</code>, then
     * the index of the first such occurrence is returned. For values
     * of <code>ch</code> in the range from 0 to 0xFFFF (inclusive),
     * this is the smallest value <i>k</i> such that:
     * <blockquote><pre>
     * (this.charAt(<i>k</i>) == ch) && (<i>k</i> &gt;= fromIndex)
     * </pre></blockquote>
     * is true. For other values of <code>ch</code>, it is the
     * smallest value <i>k</i> such that:
     * <blockquote><pre>
     * (this.codePointAt(<i>k</i>) == ch) && (<i>k</i> &gt;= fromIndex)
     * </pre></blockquote>
     * is true. In either case, if no such character occurs in this
     * string at or after position <code>fromIndex</code>, then
     * <code>-1</code> is returned.
     *
     * <p>
     * There is no restriction on the value of <code>fromIndex</code>. If it
     * is negative, it has the same effect as if it were zero: this entire
     * string may be searched. If it is greater than the length of this
     * string, it has the same effect as if it were equal to the length of
     * this string: <code>-1</code> is returned.
     *
     * <p>All indices are specified in <code>char</code> values
     * (Unicode code units).
     *
     * @param   ch          a character (Unicode code point).
     * @param   fromIndex   the index to start the search from.
     * @return  the index of the first occurrence of the character in the
     *          character sequence represented by this object that is greater
     *          than or equal to <code>fromIndex</code>, or <code>-1</code>
     *          if the character does not occur.
     */
    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);
        }
    }

由于第一个方法调用的是第二个方法，主要看第二个方法：

其实我们从之前看到的代码就可以知道，Java的健壮性非常好，因为每一个具体的方法都有对数组是否越界、参数是否合理做了判断并处理。

这个方法是根据传入整型根据Unicode码来查找对应字符串的，fromIndex则是表示从第几个字符开始查找，该方法同样要考虑到有些字占两个字节的情况，不过看完这个方法，可以看出，Java语言对于查找这件事情也没有太好的方法，它也只能遍历char数组来查找对应字符。

/**
     * Returns the index within this string of the first occurrence of the
     * specified substring.
     *
     * <p>The returned index is the smallest value <i>k</i> for which:
     * <blockquote><pre>
     * this.startsWith(str, <i>k</i>)
     * </pre></blockquote>
     * If no such value of <i>k</i> exists, then {@code -1} is returned.
     *
     * @param   str   the substring to search for.
     * @return  the index of the first occurrence of the specified substring,
     *          or {@code -1} if there is no such occurrence.
     */
    public int indexOf(String str) {
        return indexOf(str, 0);
    }

    /**
     * Returns the index within this string of the first occurrence of the
     * specified substring, starting at the specified index.
     *
     * <p>The returned index is the smallest value <i>k</i> for which:
     * <blockquote><pre>
     * <i>k</i> &gt;= fromIndex && this.startsWith(str, <i>k</i>)
     * </pre></blockquote>
     * If no such value of <i>k</i> exists, then {@code -1} is returned.
     *
     * @param   str         the substring to search for.
     * @param   fromIndex   the index from which to start the search.
     * @return  the index of the first occurrence of the specified substring,
     *          starting at the specified index,
     *          or {@code -1} if there is no such occurrence.
     */
    public int indexOf(String str, int fromIndex) {
        return indexOf(value, 0, value.length,
                str.value, 0, str.value.length, fromIndex);
    }

    /**
     * Code shared by String and StringBuffer to do searches. The
     * source is the character array being searched, and the target
     * is the string being searched for.
     *
     * @param   source       the characters being searched.
     * @param   sourceOffset offset of the source string.
     * @param   sourceCount  count of the source string.
     * @param   target       the characters being searched for.
     * @param   targetOffset offset of the target string.
     * @param   targetCount  count of the target string.
     * @param   fromIndex    the index to begin searching from.
     */
    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;
    }

参数为String类型的同理，不过是变成了查找匹配字符串的第一个下标，同样用的是遍历，不过在此我发现一个我原来不知道的情况，如下：

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

str竟然可以可以直接用过‘.’语法直接得到value，value可是用private修饰的，OH！，于是我用一个Test来验证了一下，如下：

public class StrInnerTestBean {

    private final char[] str;
    
    public StrInnerTestBean() {
        this.str = new char[]{'h','e','l','l','o'};
    }
    
    public static void test(StrInnerTestBean bean){
        System.out.println(bean.str);
    }
    
    public static void main(String[] args) {
        test(new StrInnerTestBean());
    }
}

可以看出若传入参数是该类一个对象，就可直接通过'.'语法获得private修饰的值，若不是该类的对象则不可。

2.subString方法

/**
     * Returns a new string that is a substring of this string. The
     * substring begins with the character at the specified index and
     * extends to the end of this string. <p>
     * Examples:
     * <blockquote><pre>
     * "unhappy".substring(2) returns "happy"
     * "Harbison".substring(3) returns "bison"
     * "emptiness".substring(9) returns "" (an empty string)
     * </pre></blockquote>
     *
     * @param      beginIndex   the beginning index, inclusive.
     * @return     the specified substring.
     * @exception  IndexOutOfBoundsException  if
     *             <code>beginIndex</code> is negative or larger than the
     *             length of this <code>String</code> object.
     */
    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);
    }

    /**
     * Returns a new string that is a substring of this string. The
     * substring begins at the specified <code>beginIndex</code> and
     * extends to the character at index <code>endIndex - 1</code>.
     * Thus the length of the substring is <code>endIndex-beginIndex</code>.
     * <p>
     * Examples:
     * <blockquote><pre>
     * "hamburger".substring(4, 8) returns "urge"
     * "smiles".substring(1, 5) returns "mile"
     * </pre></blockquote>
     *
     * @param      beginIndex   the beginning index, inclusive.
     * @param      endIndex     the ending index, exclusive.
     * @return     the specified substring.
     * @exception  IndexOutOfBoundsException  if the
     *             <code>beginIndex</code> is negative, or
     *             <code>endIndex</code> is larger than the length of
     *             this <code>String</code> object, or
     *             <code>beginIndex</code> is larger than
     *             <code>endIndex</code>.
     */
    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);
    }

这个方法很简单，就是创建一个String将value根据下标进行截取再传入就可，但是需要注意的是若没有对String进行截取，会怎么做，还会创建一个新String，copy一份吗，从代码中可以看到，若没有截取，是直接将当前对象返回并未新建String。