Abstract Class - java.lang.AbstractStringBuilder
Created by : Mr Dk.
2020 / 06 / 05 14:29
Nanjing, Jiangsu, China
Definition
内容和长度都可以被修改的字符序列。与 C 语言的字符串类似，这里实现了所有对于内存空间的维护。从实现的接口可以看出，字符序列是可以被 append 的，同时也需要满足字符序列的所有特性。
/**
* A mutable sequence of characters.
* <p>
* Implements a modifiable string. At any point in time it contains some
* particular sequence of characters, but the length and content of the
* sequence can be changed through certain method calls.
*
* <p>Unless otherwise noted, passing a {@code null} argument to a constructor
* or method in this class will cause a {@link NullPointerException} to be
* thrown.
*
* @author Michael McCloskey
* @author Martin Buchholz
* @author Ulf Zibis
* @since 1.5
*/
abstract class AbstractStringBuilder implements Appendable, CharSequence {
}
维护的内部变量：字符数组内存 + 字符使用的长度 (不一定就是字符数组的内存长度)。
/**
* The value is used for character storage.
*/
char[] value;
/**
* The count is the number of characters used.
*/
int count;
/**
* Needed by {@code String} for the contentEquals method.
*/
final char[] getValue() {
return value;
}
构造函数可以指定内部 char[] value 的初始容量。
/**
* 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];
}
字符串的 长度 与 容量 的区别：
/**
* Returns the length (character count).
*
* @return the length of the sequence of characters currently
* represented by this object
*/
@Override
public int length() {
return count;
}
/**
* Returns the current capacity. The capacity is the amount of storage
* available for newly inserted characters, beyond which an allocation
* will occur.
*
* @return the current capacity
*/
public int capacity() {
return value.length;
}
以下是与开辟 char[] 相关的函数，主要是为了保证这个数组只要要达到指定的长度。首先尝试将数组的长度扩展到 2n+2，如果还没有达到要求，就直接设置为指定的容量，并对一些特殊的大容量进行处理。
/**
* Ensures that the capacity is at least equal to the specified minimum.
* If the current capacity is less than the argument, then a new internal
* array is allocated with greater capacity. The new capacity is the
* larger of:
* <ul>
* <li>The {@code minimumCapacity} argument.
* <li>Twice the old capacity, plus {@code 2}.
* </ul>
* If the {@code minimumCapacity} argument is nonpositive, this
* method takes no action and simply returns.
* Note that subsequent operations on this object can reduce the
* actual capacity below that requested here.
*
* @param minimumCapacity the minimum desired capacity.
*/
public void ensureCapacity(int minimumCapacity) {
if (minimumCapacity > 0)
ensureCapacityInternal(minimumCapacity);
}
/**
* For positive values of {@code minimumCapacity}, this method
* behaves like {@code ensureCapacity}, however it is never
* synchronized.
* If {@code minimumCapacity} is non positive due to numeric
* overflow, this method throws {@code OutOfMemoryError}.
*/
private void ensureCapacityInternal(int minimumCapacity) {
// overflow-conscious code
if (minimumCapacity - value.length > 0) {
value = Arrays.copyOf(value,
newCapacity(minimumCapacity));
}
}
/**
* The maximum size of array to allocate (unless necessary).
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* Returns a capacity at least as large as the given minimum capacity.
* Returns the current capacity increased by the same amount + 2 if
* that suffices.
* Will not return a capacity greater than {@code MAX_ARRAY_SIZE}
* unless the given minimum capacity is greater than that.
*
* @param minCapacity the desired minimum capacity
* @throws OutOfMemoryError if minCapacity is less than zero or
* greater than Integer.MAX_VALUE
*/
private int newCapacity(int minCapacity) {
// overflow-conscious code
int newCapacity = (value.length << 1) + 2;
if (newCapacity - minCapacity < 0) {
newCapacity = minCapacity;
}
return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
? hugeCapacity(minCapacity)
: newCapacity;
}
private int hugeCapacity(int minCapacity) {
if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
throw new OutOfMemoryError();
}
return (minCapacity > MAX_ARRAY_SIZE)
? minCapacity : MAX_ARRAY_SIZE;
}
以下函数收缩 char[] value 的容量，使得 count 的值正好等于 value 的容量。在内部是通过复制数组的方式实现的。
/**
* Attempts to reduce storage used for the character sequence.
* If the buffer is larger than necessary to hold its current sequence of
* characters, then it may be resized to become more space efficient.
* Calling this method may, but is not required to, affect the value
* returned by a subsequent call to the {@link #capacity()} method.
*/
public void trimToSize() {
if (count < value.length) {
value = Arrays.copyOf(value, count);
}
}
以下函数调整 value 的容量。如果目标容量比当前容量小，那么结尾的一些内容将消失；如果目标容量比当前容量大，那么先将 value 扩容，然后将多出的空间用 \0 填充。
/**
* Sets the length of the character sequence.
* The sequence is changed to a new character sequence
* whose length is specified by the argument. For every nonnegative
* index <i>k</i> less than {@code newLength}, the character at
* index <i>k</i> in the new character sequence is the same as the
* character at index <i>k</i> in the old sequence if <i>k</i> is less
* than the length of the old character sequence; otherwise, it is the
* null character {@code '\u005Cu0000'}.
*
* In other words, if the {@code newLength} argument is less than
* the current length, the length is changed to the specified length.
* <p>
* If the {@code newLength} argument is greater than or equal
* to the current length, sufficient null characters
* ({@code '\u005Cu0000'}) are appended so that
* length becomes the {@code newLength} argument.
* <p>
* The {@code newLength} argument must be greater than or equal
* to {@code 0}.
*
* @param newLength the new length
* @throws IndexOutOfBoundsException if the
* {@code newLength} argument is negative.
*/
public void setLength(int newLength) {
if (newLength < 0)
throw new StringIndexOutOfBoundsException(newLength);
ensureCapacityInternal(newLength);
if (count < newLength) {
Arrays.fill(value, count, newLength, '\0');
}
count = newLength;
}
以下函数将 value 中指定范围的内容拷贝到目标字符数组中。
/**
* Characters are copied from this sequence into the
* destination character array {@code dst}. The first character to
* be copied is at index {@code srcBegin}; the last character to
* be copied is at index {@code srcEnd-1}. The total number of
* characters to be copied is {@code srcEnd-srcBegin}. The
* characters are copied into the subarray of {@code dst} starting
* at index {@code dstBegin} and ending at index:
* <pre>{@code
* dstbegin + (srcEnd-srcBegin) - 1
* }</pre>
*
* @param srcBegin start copying at this offset.
* @param srcEnd stop copying at this offset.
* @param dst the array to copy the data into.
* @param dstBegin offset into {@code dst}.
* @throws IndexOutOfBoundsException if any of the following is true:
* <ul>
* <li>{@code srcBegin} is negative
* <li>{@code dstBegin} is negative
* <li>the {@code srcBegin} argument is greater than
* the {@code srcEnd} argument.
* <li>{@code srcEnd} is greater than
* {@code this.length()}.
* <li>{@code dstBegin+srcEnd-srcBegin} is greater than
* {@code dst.length}
* </ul>
*/
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);
}
以下代码将value 中指定位置的字符设置为指定字符。
/**
* The character at the specified index is set to {@code ch}. This
* sequence is altered to represent a new character sequence that is
* identical to the old character sequence, except that it contains the
* character {@code ch} at position {@code index}.
* <p>
* The index argument must be greater than or equal to
* {@code 0}, and less than the length of this sequence.
*
* @param index the index of the character to modify.
* @param ch the new character.
* @throws IndexOutOfBoundsException if {@code index} is
* negative or greater than or equal to {@code length()}.
*/
public void setCharAt(int index, char ch) {
if ((index < 0) || (index >= count))
throw new StringIndexOutOfBoundsException(index);
value[index] = ch;
}
以下函数向 value 中进行追加。首先获取输入的长度，并保证 value 的空间足够；然后将待追加的内容复制到 value 数组中，并更新 count 的数值为字符串长度。
/**
* Appends the string representation of the {@code Object} argument.
* <p>
* The overall effect is exactly as if the argument were converted
* to a string by the method {@link String#valueOf(Object)},
* and the characters of that string were then
* {@link #append(String) appended} to this character sequence.
*
* @param obj an {@code Object}.
* @return a reference to this object.
*/
public AbstractStringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
/**
* 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);
str.getChars(0, len, value, count);
count += len;
return this;
}
/**
* @since 1.8
*/
AbstractStringBuilder append(AbstractStringBuilder asb) {
if (asb == null)
return appendNull();
int len = asb.length();
ensureCapacityInternal(count + len);
asb.getChars(0, len, value, count);
count += len;
return this;
}
private AbstractStringBuilder appendNull() {
int c = count;
ensureCapacityInternal(c + 4);
final char[] value = this.value;
value[c++] = 'n';
value[c++] = 'u';
value[c++] = 'l';
value[c++] = 'l';
count = c;
return this;
}
/**
* Appends a subsequence of the specified {@code CharSequence} to this
* sequence.
* <p>
* Characters of the argument {@code s}, starting at
* index {@code start}, are appended, in order, to the contents of
* this sequence up to the (exclusive) index {@code end}. The length
* of this sequence is increased by the value of {@code end - start}.
* <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 this character sequence becomes equal to the
* character at index <i>k</i> in this sequence, if <i>k</i> is less than
* <i>n</i>; otherwise, it is equal to the character at index
* <i>k+start-n</i> in the argument {@code s}.
* <p>
* If {@code s} is {@code null}, then this method appends
* characters as if the s parameter was a sequence containing the four
* characters {@code "null"}.
*
* @param s the sequence to append.
* @param start the starting index of the subsequence to be appended.
* @param end the end index of the subsequence to be appended.
* @return a reference to this object.
* @throws IndexOutOfBoundsException if
* {@code start} is negative, or
* {@code start} is greater than {@code end} or
* {@code end} is greater than {@code s.length()}
*/
@Override
public AbstractStringBuilder append(CharSequence s, int start, int end) {
if (s == null)
s = "null";
if ((start < 0) || (start > end) || (end > s.length()))
throw new IndexOutOfBoundsException(
"start " + start + ", end " + end + ", s.length() "
+ s.length());
int len = end - start;
ensureCapacityInternal(count + len);
for (int i = start, j = count; i < end; i++, j++)
value[j] = s.charAt(i);
count += len;
return this;
}
/**
* Appends the string representation of the {@code char} array
* argument to this sequence.
* <p>
* The characters of the array argument are appended, in order, to
* the contents of this sequence. The length of this sequence
* increases by the length of the argument.
* <p>
* The overall effect is exactly as if the argument were converted
* to a string by the method {@link String#valueOf(char[])},
* and the characters of that string were then
* {@link #append(String) appended} to this character sequence.
*
* @param str the characters to be appended.
* @return a reference to this object.
*/
public AbstractStringBuilder append(char[] str) {
int len = str.length;
ensureCapacityInternal(count + len);
System.arraycopy(str, 0, value, count, len);
count += len;
return this;
}
/**
* Appends the string representation of a subarray of the
* {@code char} array argument to this sequence.
* <p>
* Characters of the {@code char} array {@code str}, starting at
* index {@code offset}, are appended, in order, to the contents
* of this sequence. The length of this sequence increases
* by the value of {@code len}.
* <p>
* The overall effect is exactly as if the arguments were converted
* to a string by the method {@link String#valueOf(char[],int,int)},
* and the characters of that string were then
* {@link #append(String) appended} to this character sequence.
*
* @param str the characters to be appended.
* @param offset the index of the first {@code char} to append.
* @param len the number of {@code char}s to append.
* @return a reference to this object.
* @throws IndexOutOfBoundsException
* if {@code offset < 0} or {@code len < 0}
* or {@code offset+len > str.length}
*/
public AbstractStringBuilder append(char str[], int offset, int len) {
if (len > 0) // let arraycopy report AIOOBE for len < 0
ensureCapacityInternal(count + len);
System.arraycopy(str, offset, value, count, len);
count += len;
return this;
}
另外，还有支持将各种数据类型 append 到 value 的函数。
以下函数删除 value 中指定范围内的内容。不回收空间，只删除内容，将指定范围之后的内容向前拷贝。
/**
* 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) {
System.arraycopy(value, start+len, value, start, count-end);
count -= len;
}
return this;
}
/**
* Removes the {@code char} at the specified position in this
* sequence. This sequence is shortened by one {@code char}.
*
* <p>Note: If the character at the given index is a supplementary
* character, this method does not remove the entire character. If
* correct handling of supplementary characters is required,
* determine the number of {@code char}s to remove by calling
* {@code Character.charCount(thisSequence.codePointAt(index))},
* where {@code thisSequence} is this sequence.
*
* @param index Index of {@code char} to remove
* @return This object.
* @throws StringIndexOutOfBoundsException if the {@code index}
* is negative or greater than or equal to
* {@code length()}.
*/
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;
}
以下函数将 value 中指定范围内的字符替换为输入的字符串。首先将指定范围之后的字符向前或向后移，中间留出输入字符串的长度；同时，如果字符串长度增加，需要保证 value 的空间足够。然后将输入字符串复制到 value 的指定范围中。
/**
* 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);
ensureCapacityInternal(newCount);
System.arraycopy(value, end, value, start + len, count - end);
str.getChars(value, start);
count = newCount;
return this;
}
以下函数返回当前 value 中字符串的子串。首先进行边界检查，然后构造新的 String 对象：
/**
* Returns a new {@code String} that contains a subsequence of
* characters currently contained in this character sequence. The
* substring begins at the specified index and extends to the end of
* this sequence.
*
* @param start The beginning index, inclusive.
* @return The new string.
* @throws StringIndexOutOfBoundsException if {@code start} is
* less than zero, or greater than the length of this object.
*/
public String substring(int start) {
return substring(start, count);
}
/**
* Returns a new character sequence that is a subsequence of this sequence.
*
* <p> An invocation of this method of the form
*
* <pre>{@code
* sb.subSequence(begin, end)}</pre>
*
* behaves in exactly the same way as the invocation
*
* <pre>{@code
* sb.substring(begin, end)}</pre>
*
* This method is provided so that this class can
* implement the {@link CharSequence} interface.
*
* @param start the start index, inclusive.
* @param end the end index, exclusive.
* @return the specified subsequence.
*
* @throws IndexOutOfBoundsException
* if {@code start} or {@code end} are negative,
* if {@code end} is greater than {@code length()},
* or if {@code start} is greater than {@code end}
* @spec JSR-51
*/
@Override
public CharSequence subSequence(int start, int end) {
return substring(start, end);
}
/**
* Returns a new {@code String} that contains a subsequence of
* characters currently contained in this sequence. The
* substring begins at the specified {@code start} and
* extends to the character at index {@code end - 1}.
*
* @param start The beginning index, inclusive.
* @param end The ending index, exclusive.
* @return The new string.
* @throws StringIndexOutOfBoundsException if {@code start}
* or {@code end} are negative or greater than
* {@code length()}, or {@code start} is
* greater than {@code end}.
*/
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);
}
以下函数将字符序列插入到 value 的某个偏移开始的位置，在实现上与 replace() 有些类似。先将指定偏移之后的内容向后复制，然后将待插入的字符序列复制到空出来的空间中。
/**
* 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);
System.arraycopy(value, index, value, index + len, count - index);
System.arraycopy(str, offset, value, index, len);
count += len;
return this;
}
类似地，还有 insert() 针对各种数据类型的重载。
以下函数用于对字符串进行颠倒。其中需要特殊处理的一个情况是占据两个 char 长度的字符 - 在颠倒过后，字符内部的两个 char 的顺序不应当被颠倒。函数先按独立 char 的方式颠倒字符串，如果在其中发现了占据两个 char 的字符，则再扫描一遍字符串将代表高低位的两个颠倒的 char 的顺序复原。
/**
* Causes this character sequence to be replaced by the reverse of
* the sequence. If there are any surrogate pairs included in the
* sequence, these are treated as single characters for the
* reverse operation. Thus, the order of the high-low surrogates
* is never reversed.
*
* Let <i>n</i> be the character length of this character sequence
* (not the length in {@code char} values) just prior to
* execution of the {@code reverse} method. Then the
* character at index <i>k</i> in the new character sequence is
* equal to the character at index <i>n-k-1</i> in the old
* character sequence.
*
* <p>Note that the reverse operation may result in producing
* surrogate pairs that were unpaired low-surrogates and
* high-surrogates before the operation. For example, reversing
* "\u005CuDC00\u005CuD800" produces "\u005CuD800\u005CuDC00" which is
* a valid surrogate pair.
*
* @return a reference to this object.
*/
public AbstractStringBuilder reverse() {
boolean hasSurrogates = false;
int n = count - 1;
for (int j = (n-1) >> 1; j >= 0; j--) {
int k = n - j;
char cj = value[j];
char ck = value[k];
value[j] = ck;
value[k] = cj;
if (Character.isSurrogate(cj) ||
Character.isSurrogate(ck)) {
hasSurrogates = true;
}
}
if (hasSurrogates) {
reverseAllValidSurrogatePairs();
}
return this;
}
/** Outlined helper method for reverse() */
private void reverseAllValidSurrogatePairs() {
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;
}
}
}
}
java.lang.StringBuilder 和 java.lang.StringBuffer 全部继承自这个类，其中的函数基本都在这个函数中实现。StringBuffer 在每个函数上套了一层 synchronized 关键字以实现线程安全。