Java字符串：StringBuilder使用详解及源码分析

1 使用方法

　　StringBuilder是可变字符串，和String的主要区别是他的字符串是可变的，例如拼接等操作不会重返回新的StringBuilder实例。

public final class StringBuilder        extends AbstractStringBuilder        implements java.io.Serializable, CharSequence{}

　　StringBuilder继承了AbstractStringBuilder并且实现了Serializable和CharSequence。

1.1 方法介绍

　　StringBuilder提供的的API主要如下：

//构造函数StringBuilder()StringBuilder(int capacity)StringBuilder(CharSequence seq)StringBuilder(String str)//尾部添加字符(串)StringBuilder     append(float f)StringBuilder     append(double d)StringBuilder     append(boolean b)StringBuilder     append(int i)StringBuilder     append(long l)StringBuilder     append(char c)StringBuilder     append(char[] chars)StringBuilder     append(char[] str, int offset, int len)StringBuilder     append(String str)StringBuilder     append(Object obj)StringBuilder     append(StringBuffer sb)StringBuilder     append(CharSequence csq)StringBuilder     append(CharSequence csq, int start, int end)StringBuilder     appendCodePoint(int codePoint)int     capacity() //容量char     charAt(int index) //获取index下标的字符int     codePointAt(int index) //获取index下标字符的Unicode编码int     codePointBefore(int index)int     codePointCount(int start, int end)StringBuilder     delete(int start, int end) //删除[start,end)之间的字符StringBuilder     deleteCharAt(int index) //删除index下标的字符void     getChars(int start, int end, char[] dst, int dstStart) //获取将[start,end)间的字符填充到到dst中,dstStart为开始位置int     indexOf(String subString, int start) //子串的第一次出现位置, 从start开始查找int     indexOf(String string) //子串的第一次出现位置//插入字符StringBuilder     insert(int offset, boolean b)StringBuilder     insert(int offset, int i)StringBuilder     insert(int offset, long l)StringBuilder     insert(int offset, float f)StringBuilder     insert(int offset, double d)StringBuilder     insert(int offset, char c)StringBuilder     insert(int offset, char[] ch)StringBuilder     insert(int offset, char[] str, int strOffset, int strLen)StringBuilder     insert(int offset, String str)StringBuilder     insert(int offset, Object obj)StringBuilder     insert(int offset, CharSequence s)StringBuilder     insert(int offset, CharSequence s, int start, int end)int     lastIndexOf(String string) //子串从后往前第一次出现的位置int     lastIndexOf(String subString, int start)int     length() //StringBuilder中字符数量StringBuilder     replace(int start, int end, String string) //将[start,end)替换为stringStringBuilder     reverse() //反转字符串void     setCharAt(int index, char ch) //将index字符设置为chCharSequence     subSequence(int start, int end) //获取子串String     substring(int start)String     substring(int start, int end)String     toString()void     trimToSize()

1.2 使用示例

@Componentpublic class TestStringBuilder {    public void testStringBuilder() {        //构造并初始化        StringBuilder stringBuilder = new StringBuilder("0123456");        //获取容量        System.out.println("stringBuilder的容量为: " + stringBuilder.capacity());        //获取字符数量        System.out.println("stringBuilder的字符数量为: " + stringBuilder.length());        //获取指定index的字符        System.out.println("stringBuilder的第2个字符为: " + stringBuilder.charAt(1));        //子串第一次出现的位置        System.out.println("\"23\"在stringBuilder中第一次出现的位置为: " + stringBuilder.indexOf("23"));        //子串最后一次出现的位置,从后往前        System.out.println("\"34\"在stringBuilder从第5个字符以前的字符串中第一次一次出现的位置为: "                + stringBuilder.lastIndexOf("34", 5));        //替换字符串        System.out.println("将stringBuilder的第2-3个字符替换为abcde: " + stringBuilder.replace(1, 3, "abcde"));        //设置指定位置字符        stringBuilder.setCharAt(1, 'A');        System.out.println("将第2个字符设置为A: " + stringBuilder);        //删除滴定位置的字符串        stringBuilder.delete(2,5);        System.out.println("删除第3到第5个字符: " + stringBuilder);        //追加字符        System.out.println("stringBuilder尾部追加一个7" + stringBuilder.append("7"));        //追加double        System.out.println("stringBuilder尾部追加8.0d" + stringBuilder.append(8.0d));        //插入字符串        System.out.println("stringBuilder第3个字符看是追加test: " + stringBuilder.insert(2, "test"));    }}

　　运行结果如下：

stringBuilder的容量为: 23stringBuilder的字符数量为: 7stringBuilder的第2个字符为: 1"23"在stringBuilder中第一次出现的位置为: 2"34"在stringBuilder从第5个字符以前的字符串中第一次一次出现的位置为: 3将stringBuilder的第2-3个字符替换为abcde: 0abcde3456将第2个字符设置为A: 0Abcde3456删除第3到第5个字符: 0Ae3456stringBuilder尾部追加一个70Ae34567stringBuilder尾部追加8.0d0Ae345678.0stringBuilder第3个字符看是追加test: 0Ateste345678.0

2 源码分析

2.1构造函数

　　StringBuilder的构造函数有多个，基本的区别是初始容量大小和是否用字符串进行初始化，下面列举3个典型的构造方法。

/** * Constructs a string builder with no characters in it and an * initial capacity of 16 characters. */public StringBuilder() {    super(16);}/** * Constructs a string builder with no characters in it and an * initial capacity specified by the {@code capacity} argument. * * @param      capacity  the initial capacity. * @throws     NegativeArraySizeException  if the {@code capacity} *               argument is less than {@code 0}. */public StringBuilder(int capacity) {    super(capacity);}/** * Constructs a string builder initialized to the contents of the * specified string. The initial capacity of the string builder is * {@code 16} plus the length of the string argument. * * @param   str   the initial contents of the buffer. */public StringBuilder(String str) {    super(str.length() + 16);    append(str);}

2.2 insert方法

　　insert方法有插入字符串、整形、布尔型等多个重载方法，实现方法都是调用父类AbstractStringBuilder的insert方法。

2.2.1 插入一个对象

/** * 插入一个对象(int/boolean/double....etc) * @throws StringIndexOutOfBoundsException {@inheritDoc} */@Overridepublic StringBuilder insert(int offset, Object obj) {    super.insert(offset, obj); //调用AbstractStringBuilder类中的insert方法    return this;}/** * 父类AbstractStringBuilder中的方法 * @param offset * @param obj * @return */public AbstractStringBuilder insert(int offset, Object obj) {    return insert(offset, String.valueOf(obj)); //调用insert(int offset, String str)}/** * Inserts the string into this character sequence. * <p> * The characters of the {@code String} argument are inserted, in * order, into this sequence at the indicated offset, moving up any * characters originally above that position and increasing the length * of this sequence by the length of the argument. If * {@code str} is {@code null}, then the four characters * {@code "null"} are inserted into this sequence. * <p> * The character at index <i>k</i> in the new character sequence is * equal to: * <ul> * <li>the character at index <i>k</i> in the old character sequence, if * <i>k</i> is less than {@code offset} * <li>the character at index <i>k</i>{@code -offset} in the * argument {@code str}, if <i>k</i> is not less than * {@code offset} but is less than {@code offset+str.length()} * <li>the character at index <i>k</i>{@code -str.length()} in the * old character sequence, if <i>k</i> is not less than * {@code offset+str.length()} * </ul><p> * The {@code offset} argument must be greater than or equal to * {@code 0}, and less than or equal to the {@linkplain #length() length} * of this sequence. * * @param      offset   the offset. * @param      str      a string. * @return     a reference to this object. * @throws     StringIndexOutOfBoundsException  if the offset is invalid. */public AbstractStringBuilder insert(int offset, String str) {    if ((offset < 0) || (offset > length()))        throw new StringIndexOutOfBoundsException(offset);    if (str == null)        str = "null"; //str为null时    int len = str.length();    ensureCapacityInternal(count + len); //确定容量是否够用,不够则增加(length*2 + 2)    System.arraycopy(value, offset, value, offset + len, count - offset); //offset开始的字符后移len位    str.getChars(value, offset); //str字符填充    count += len;    return this;}

2.2.2 插入一个字符串

/** * 插入字符串 * @throws IndexOutOfBoundsException {@inheritDoc} */@Overridepublic StringBuilder insert(int dstOffset, CharSequence s,                            int start, int end){    super.insert(dstOffset, s, start, end);    return this;}/** * Inserts the string representation of a subarray of the {@code str} * array argument into this sequence. The subarray begins at the * specified {@code offset} and extends {@code len} {@code char}s. * The characters of the subarray are inserted into this sequence at * the position indicated by {@code index}. The length of this * sequence increases by {@code len} {@code char}s. * * @param      index    position at which to insert subarray. * @param      str       A {@code char} array. * @param      offset   the index of the first {@code char} in subarray to *             be inserted. * @param      len      the number of {@code char}s in the subarray to *             be inserted. * @return     This object * @throws     StringIndexOutOfBoundsException  if {@code index} *             is negative or greater than {@code length()}, or *             {@code offset} or {@code len} are negative, or *             {@code (offset+len)} is greater than *             {@code str.length}. */public AbstractStringBuilder insert(int index, char[] str, int offset,                                    int len){    if ((index < 0) || (index > length()))        throw new StringIndexOutOfBoundsException(index);    if ((offset < 0) || (len < 0) || (offset > str.length - len))        throw new StringIndexOutOfBoundsException(                "offset " + offset + ", len " + len + ", str.length "                        + str.length);    ensureCapacityInternal(count + len); //确定容量    System.arraycopy(value, index, value, index + len, count - index); //后移len位    System.arraycopy(str, offset, value, index, len); //将str从offset开始的len个字符复制到value    count += len;    return this;}

2.3 append方法

　　append和insert方法类似，也有很多重载版本，主要是追加对象或者字符串，也是调用父类的方法。

/** * Appends the specified {@code StringBuffer} to this sequence. * <p> * The characters of the {@code StringBuffer} argument are appended, * in order, to this sequence, increasing the * length of this sequence by the length of the argument. * If {@code sb} is {@code null}, then the four characters * {@code "null"} are appended to this sequence. * <p> * Let <i>n</i> be the length of this character sequence just prior to * execution of the {@code append} method. Then the character at index * <i>k</i> in the new character sequence is equal to the character at * index <i>k</i> in the old character sequence, if <i>k</i> is less than * <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i> * in the argument {@code sb}. * * @param   sb   the {@code StringBuffer} to append. * @return  a reference to this object. */public StringBuilder append(StringBuffer sb) {    super.append(sb);    return this;}/** * Appends the specified string to this character sequence. * <p> * The characters of the {@code String} argument are appended, in * order, increasing the length of this sequence by the length of the * argument. If {@code str} is {@code null}, then the four * characters {@code "null"} are appended. * <p> * Let <i>n</i> be the length of this character sequence just prior to * execution of the {@code append} method. Then the character at * index <i>k</i> in the new character sequence is equal to the character * at index <i>k</i> in the old character sequence, if <i>k</i> is less * than <i>n</i>; otherwise, it is equal to the character at index * <i>k-n</i> in the argument {@code str}. * * @param   str   a string. * @return  a reference to this object. */public AbstractStringBuilder append(String str) {    if (str == null)        return appendNull();    int len = str.length();    ensureCapacityInternal(count + len); //确定容量    str.getChars(0, len, value, count); //将str追缴到value最后    count += len;    return this;}

2.4 replace方法

/** * @throws StringIndexOutOfBoundsException {@inheritDoc} */@Overridepublic StringBuilder replace(int start, int end, String str) {    super.replace(start, end, str);    return this;}/** * Replaces the characters in a substring of this sequence * with characters in the specified {@code String}. The substring * begins at the specified {@code start} and extends to the character * at index {@code end - 1} or to the end of the * sequence if no such character exists. First the * characters in the substring are removed and then the specified * {@code String} is inserted at {@code start}. (This * sequence will be lengthened to accommodate the * specified String if necessary.) * * @param      start    The beginning index, inclusive. * @param      end      The ending index, exclusive. * @param      str   String that will replace previous contents. * @return     This object. * @throws     StringIndexOutOfBoundsException  if {@code start} *             is negative, greater than {@code length()}, or *             greater than {@code end}. */public AbstractStringBuilder replace(int start, int end, String str) {    if (start < 0)        throw new StringIndexOutOfBoundsException(start);    if (start > count)        throw new StringIndexOutOfBoundsException("start > length()");    if (start > end)        throw new StringIndexOutOfBoundsException("start > end");    if (end > count)        end = count;    int len = str.length();    int newCount = count + len - (end - start); //新数组长度    ensureCapacityInternal(newCount); //确定容量    System.arraycopy(value, end, value, start + len, count - end); //将value从end开始的字符移动到start+len的位置,共count -end个    str.getChars(value, start); //填充字符    count = newCount;    return this;}

2.5 delete方法

/** * @throws StringIndexOutOfBoundsException {@inheritDoc} */@Overridepublic StringBuilder delete(int start, int end) {    super.delete(start, end);    return this;}/** * Removes the characters in a substring of this sequence. * The substring begins at the specified {@code start} and extends to * the character at index {@code end - 1} or to the end of the * sequence if no such character exists. If * {@code start} is equal to {@code end}, no changes are made. * * @param      start  The beginning index, inclusive. * @param      end    The ending index, exclusive. * @return     This object. * @throws     StringIndexOutOfBoundsException  if {@code start} *             is negative, greater than {@code length()}, or *             greater than {@code end}. */public AbstractStringBuilder delete(int start, int end) {    if (start < 0)        throw new StringIndexOutOfBoundsException(start);    if (end > count)        end = count;    if (start > end)        throw new StringIndexOutOfBoundsException();    int len = end - start;    if (len > 0) {        //将value从start+len开始的count-end个字符复制到start开始的位置,即向前覆盖掉要删除的字符串        System.arraycopy(value, start+len, value, start, count-end);        count -= len;    }    return this;}

参考：

[1] http://blog.csdn.net/jiutianhe/article/details/42171103
[2] http://www.cnblogs.com/skywang12345/p/string02.html