AbstractStringBuilder源码分析

简介

• 这个抽象类是StringBuilder和StringBuffer的直接父类，而且定义了很多方法，因此在学习这两个类之间建议先学习 AbstractStringBuilder抽象类
• 该类在源码中注释是以JDK1.5开始作为前两个类的父类存在的，可是直到JDK1.8的API中，关于StringBuilder和StringBuffer的父类还是Object

类声明：

abstract class AbstractStringBuilder implements Appendable, CharSequence 

实现了两个接口，其中CharSequence这个字符序列的接口已经很熟悉了：

• 该接口规定了需要实现该字符序列的长度:length()；
• 可以取得下标为index的的字符：charAt(int index)；
• 可以得到该字符序列的一个子字符序列： subSequence(int start, int end)；
• 规定了该字符序列的String版本（重写了父类Object的toString()）：toString();

Appendable接口顾名思义，定义添加的’规则’：

• append(CharSequence csq) throws IOException:如何添加一个字符序列
• append(CharSequence csq, int start, int end) throws IOException：如何添加一个字符序列的一部分
• append(char c) throws IOException:如何添加一个字符

变量：

    /**     * The value is used for character storage.     */    char value[];    /**      * The count is the number of characters used.     */    int count;

value为该字符序列的具体存储，count为实际存储的数量
注意：（和String中的value和count不同，String中的这两者都是不可变的(final修饰)，并且不能对value[]直接操作；而AbstractStringBuilder的两者都是可变的,并且也定义了getValues方法让我们可以直接拿到value[],value实际上是个动态数组，和ArrayList的实现有很多相似的地方）

构造器：

    /**      * This no-arg constructor is necessary for serialization of subclasses.     */     AbstractStringBuilder() {    }    /**      * Creates an AbstractStringBuilder of the specified capacity.     */    AbstractStringBuilder(int capacity) {        value = new char[capacity];    }

方法(序号是按照方法在源码中出现的顺序来的，所以不是连着的)：

1.length():返回已经存储的实际长度（就是count值）

public int length() {    return count;    }

2.capacity():这个单词是’容量’的意思,得到目前该value数组的实际大小

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

3.ensureCapacity(int minimumCapacity):确保value数组的容量是否够用，如果不够用，调用expandCapacity(minimumCapacity)方法扩容，参数为需要的容量

 public void ensureCapacity(int minimumCapacity) {    if (minimumCapacity > value.length) {        expandCapacity(minimumCapacity);    }    }

4.expandCapacity(int minimumCapacity):对数组进行扩容，参数为需要的容量

 void expandCapacity(int minimumCapacity) {    int newCapacity = (value.length + 1) * 2;        if (newCapacity < 0) {            newCapacity = Integer.MAX_VALUE;        } else if (minimumCapacity > newCapacity) {        newCapacity = minimumCapacity;    }        value = Arrays.copyOf(value, newCapacity);    }

扩容的算法：
如果调用了该函数，说明容量不够用了，先将当前容量+1的二倍(newCapacity)与需要的容量(minimumCapacity)比较。
如果比需要的容量大，那就将容量扩大到容量+1的二倍；如果比需要的容量小，那就直接扩大到需要的容量。
使用Arrays.copyOf()这个非常熟悉的方法来使数组容量动态扩大

5.trimToSize():如果value数组的容量有多余的，那么就把多余的全部都释放掉

public void trimToSize() {        if (count < value.length) {            value = Arrays.copyOf(value, count);        }    }

6.setLength(int newLength):强制增大实际长度count的大小，容量如果不够就用 expandCapacity()扩大；将扩大的部分全部用’\0’（ASCII码中的null）来初始化

 public void setLength(int newLength) {    if (newLength < 0)        throw new StringIndexOutOfBoundsException(newLength);    if (newLength > value.length)        expandCapacity(newLength);    if (count < newLength) {        for (; count < newLength; count++)        value[count] = '\0';    } else {            count = newLength;        }    }

7.charAt(int index):得到下标为index的字符

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

8.codePointAt(int index):得到代码点

public int codePointAt(int index) {        if ((index < 0) || (index >= count)) {            throw new StringIndexOutOfBoundsException(index);        }        return Character.codePointAt(value, index);    }

9.getChars(int srcBegin, int srcEnd, char dst[],int dstBegin):将value[]的[srcBegin, srcEnd)拷贝到dst[]数组的desBegin开始处

public void getChars(int srcBegin, int srcEnd, char dst[],int dstBegin)    {    if (srcBegin < 0)        throw new StringIndexOutOfBoundsException(srcBegin);    if ((srcEnd < 0) || (srcEnd > count))        throw new StringIndexOutOfBoundsException(srcEnd);        if (srcBegin > srcEnd)            throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");    System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);    }

24.substring(int start)/substring(int start, int end):得到子字符串

 public String substring(int start) {        return substring(start, count);    }public String substring(int start, int end) {    if (start < 0)        throw new StringIndexOutOfBoundsException(start);    if (end > count)        throw new StringIndexOutOfBoundsException(end);    if (start > end)        throw new StringIndexOutOfBoundsException(end - start);        return new String(value, start, end - start);    }

25.subSequence(int start, int end):得到一个子字符序列

public CharSequence subSequence(int start, int end) {        return substring(start, end);    }

36.reverse():将value给倒序存放（注意改变的就是本value，而不是创建了一个新的AbstractStringBuilder然后value为倒序）

public AbstractStringBuilder reverse() {    boolean hasSurrogate = false;    int n = count - 1;    for (int j = (n-1) >> 1; j >= 0; --j) {        char temp = value[j];        char temp2 = value[n - j];        if (!hasSurrogate) {        hasSurrogate = (temp >= Character.MIN_SURROGATE && temp <= Character.MAX_SURROGATE)            || (temp2 >= Character.MIN_SURROGATE && temp2 <= Character.MAX_SURROGATE);        }        value[j] = temp2;        value[n - j] = temp;    }    if (hasSurrogate) {        // Reverse back all valid surrogate pairs        for (int i = 0; i < count - 1; i++) {        char c2 = value[i];        if (Character.isLowSurrogate(c2)) {            char c1 = value[i + 1];            if (Character.isHighSurrogate(c1)) {            value[i++] = c1;            value[i] = c2;            }        }        }    }    return this;    }

toString():唯一的一个抽象方法：toString()

    public abstract String toString();

唯一的一个final方法：getValue()，得到value数组。可以对其直接操作

final char[] getValue() {        return value;    }

CRUD操作：

改：

10.setCharAt(int index, char ch)：直接设置下标为index的字符为ch

 public void setCharAt(int index, char ch) {    if ((index < 0) || (index >= count))        throw new StringIndexOutOfBoundsException(index);    value[index] = ch;    }

23.replace(int start, int end, String str)：用字符串str替换掉value[]数组的[start,end)部分

 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);    if (newCount > value.length)        expandCapacity(newCount);        System.arraycopy(value, end, value, start + len, count - end);        str.getChars(value, start);        count = newCount;        return this;    }

增(AbstractStringBuilder类及其子类中最重要的操作；Appendable接口的具体实现)

其中append都表示’追加’，insert都表示’插入’：

11.append(Object obj)：利用Object（或任何对象）的toString方法转成字符串然后添加到该value[]中

 public AbstractStringBuilder append(Object obj) {    return append(String.valueOf(obj));    }

12.append()的核心代码：append(String str)/append(StringBuffer sb)/append(CharSequence s)。直接修改value[]，并且’添加’的意思为链接到原value[]的实际count的后面

 public AbstractStringBuilder append(String str) {    if (str == null) str = "null";        int len = str.length();    if (len == 0) return this;    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    str.getChars(0, len, value, count);    count = newCount;    return this;    }    // Documentation in subclasses because of synchro difference    public AbstractStringBuilder append(StringBuffer sb) {    if (sb == null)            return append("null");    int len = sb.length();    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    sb.getChars(0, len, value, count);    count = newCount;    return this;    }    // Documentation in subclasses because of synchro difference    public AbstractStringBuilder append(CharSequence s) {        if (s == null)            s = "null";        if (s instanceof String)            return this.append((String)s);        if (s instanceof StringBuffer)            return this.append((StringBuffer)s);        return this.append(s, 0, s.length());    }

同时注意返回的都是AbstractStringBuilder，意味着append方法可以连续无限调用，即AbstractStringBuilder对象.append(参数1).append(参数2).append(参数三)…………;

13.append(CharSequence s, int start, int end):添加字符序列s的部分序列，范围是[start,end)

 public AbstractStringBuilder append(CharSequence s, int start, int end) {        if (s == null)            s = "null";    if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))        throw new IndexOutOfBoundsException(                "start " + start + ", end " + end + ", s.length() "                 + s.length());    int len = end - start;    if (len == 0)            return this;    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);        for (int i=start; i<end; i++)            value[count++] = s.charAt(i);        count = newCount;    return this;    }

14.append(char str[]):添加一个字符数组

public AbstractStringBuilder append(char str[]) {     int newCount = count + str.length;    if (newCount > value.length)        expandCapacity(newCount);        System.arraycopy(str, 0, value, count, str.length);        count = newCount;        return this;    }

15.append(char str[], int offset, int len):添加一个字符数组的一部分，该部分的范围是[offset,offset+len);

public AbstractStringBuilder append(char str[], int offset, int len) {        int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    System.arraycopy(str, offset, value, count, len);    count = newCount;    return this;    }

16.append(boolean b):添加布尔值。

public AbstractStringBuilder append(boolean b) {        if (b) {            int newCount = count + 4;            if (newCount > value.length)                expandCapacity(newCount);            value[count++] = 't';            value[count++] = 'r';            value[count++] = 'u';            value[count++] = 'e';        } else {            int newCount = count + 5;            if (newCount > value.length)                expandCapacity(newCount);            value[count++] = 'f';            value[count++] = 'a';            value[count++] = 'l';            value[count++] = 's';            value[count++] = 'e';        }    return this;    }

17.append(char c):添加一个字符

public AbstractStringBuilder append(char c) {        int newCount = count + 1;    if (newCount > value.length)        expandCapacity(newCount);    value[count++] = c;    return this;    }

18.append(int i):添加一个整数

public AbstractStringBuilder append(int i) {        if (i == Integer.MIN_VALUE) {            append("-2147483648");            return this;        }        int appendedLength = (i < 0) ? stringSizeOfInt(-i) + 1                                      : stringSizeOfInt(i);          //stringSizeOfInt方法在下面，得到参数整数的位数。        int spaceNeeded = count + appendedLength;        if (spaceNeeded > value.length)            expandCapacity(spaceNeeded);    Integer.getChars(i, spaceNeeded, value);        count = spaceNeeded;        return this;    } final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,                                     99999999, 999999999, Integer.MAX_VALUE };    static int stringSizeOfInt(int x) {        for (int i=0; ; i++)            if (x <= sizeTable[i])                return i+1;    }

19.append(long l):添加一个长整型的数据，原理同上一个append

public AbstractStringBuilder append(long l) {        if (l == Long.MIN_VALUE) {            append("-9223372036854775808");            return this;        }        int appendedLength = (l < 0) ? stringSizeOfLong(-l) + 1                                      : stringSizeOfLong(l);        int spaceNeeded = count + appendedLength;        if (spaceNeeded > value.length)            expandCapacity(spaceNeeded);    Long.getChars(l, spaceNeeded, value);        count = spaceNeeded;        return this;    }    // Requires positive x    static int stringSizeOfLong(long x) {        long p = 10;        for (int i=1; i<19; i++) {            if (x < p)                return i;            p = 10*p;        }        return 19;    }

20.append(float f)/append(double d):添加一个浮点数。

 public AbstractStringBuilder append(float f) {    new FloatingDecimal(f).appendTo(this);    return this;    } public AbstractStringBuilder append(double d) {    new FloatingDecimal(d).appendTo(this);    return this;    }

以上是append

---

以下是insert

26.insert(int index, char str[], int offset,int len):(insert的核心代码)在value[]的下标为index位置插入数组str的一部分，该部分的范围为：[offset,offset+len);

 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);    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    System.arraycopy(value, index, value, index + len, count - index);    System.arraycopy(str, offset, value, index, len);    count = newCount;    return this;    }

27.insert(int offset, Object obj)：在value[]的offset位置插入Object（或者说所有对象）的String版。

public AbstractStringBuilder insert(int offset, Object obj) {    return insert(offset, String.valueOf(obj));    }

28.insert(int offset, String str):在value[]的offset位置插入字符串

public AbstractStringBuilder insert(int offset, String str) {    if ((offset < 0) || (offset > length()))        throw new StringIndexOutOfBoundsException(offset);    if (str == null)        str = "null";    int len = str.length();    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    System.arraycopy(value, offset, value, offset + len, count - offset);    str.getChars(value, offset);    count = newCount;    return this;    }

29.insert(int offset, char str[]):在value[]的offset位置插入字符数组

public AbstractStringBuilder insert(int offset, char str[]) {    if ((offset < 0) || (offset > length()))        throw new StringIndexOutOfBoundsException(offset);    int len = str.length;    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    System.arraycopy(value, offset, value, offset + len, count - offset);    System.arraycopy(str, 0, value, offset, len);    count = newCount;    return this;    }

30.insert(int dstOffset, CharSequence s)/insert(int dstOffset, CharSequence s,int start, int end):插入字符序列

public AbstractStringBuilder insert(int dstOffset, CharSequence s) {        if (s == null)            s = "null";        if (s instanceof String)            return this.insert(dstOffset, (String)s);        return this.insert(dstOffset, s, 0, s.length());    }public AbstractStringBuilder insert(int dstOffset, CharSequence s,                                           int start, int end) {        if (s == null)            s = "null";    if ((dstOffset < 0) || (dstOffset > this.length()))        throw new IndexOutOfBoundsException("dstOffset "+dstOffset);    if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))            throw new IndexOutOfBoundsException(                "start " + start + ", end " + end + ", s.length() "                 + s.length());    int len = end - start;        if (len == 0)            return this;    int newCount = count + len;    if (newCount > value.length)        expandCapacity(newCount);    System.arraycopy(value, dstOffset, value, dstOffset + len,                         count - dstOffset);    for (int i=start; i<end; i++)            value[dstOffset++] = s.charAt(i);    count = newCount;        return this;    }

31.插入基本类型：除了char是直接插入外，其他都是先转成String，然后调用编号为28的insert方法

 public AbstractStringBuilder insert(int offset, boolean b) {    return insert(offset, String.valueOf(b));    }    public AbstractStringBuilder insert(int offset, char c) {    int newCount = count + 1;    if (newCount > value.length)        expandCapacity(newCount);    System.arraycopy(value, offset, value, offset + 1, count - offset);    value[offset] = c;    count = newCount;    return this;    }    public AbstractStringBuilder insert(int offset, int i) {    return insert(offset, String.valueOf(i));    }    public AbstractStringBuilder insert(int offset, long l) {    return insert(offset, String.valueOf(l));    }    public AbstractStringBuilder insert(int offset, float f) {    return insert(offset, String.valueOf(f));    }    public AbstractStringBuilder insert(int offset, double d) {    return insert(offset, String.valueOf(d));    }

删:

21.delete(int start, int end):删掉value数组的[start,end)部分，并将end后面的数据移到start位置

 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) {            System.arraycopy(value, start+len, value, start, count-end);            count -= len;        }        return this;    }

22.deleteCharAt(int index):删除下标为index的数据，并将后面的数据前移一位

 public AbstractStringBuilder deleteCharAt(int index) {        if ((index < 0) || (index >= count))        throw new StringIndexOutOfBoundsException(index);    System.arraycopy(value, index+1, value, index, count-index-1);    count--;        return this;    }

查(其实CRUD前面那几个方法中有一两个也算查找)：

32.indexOf(String str)：在value[]中找字符串str，若能找到，返回第一个字符串的第一个字符的下标

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

33.从fromIndex开始，在value[]中找字符串str，若能找到，返回第一个字符串的第一个字符的下标

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

34.lastIndexOf(String str):从后往前找

  public int lastIndexOf(String str) {        return lastIndexOf(str, count);    }

35.lastIndexOf(String str, int fromIndex):从后往前到fromIndex，找子串str

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