【JAVA】BitSet的源码研究
这几天看Bloom Filter，因为在java中，并不能像C/C++一样直接操纵bit级别的数据，所以只能另想办法替代：
1）使用整数数组来替代；
2）使用BitSet；
BitSet实际是由“二进制位”构成的一个Vector。如果希望高效率地保存大量“开－关”信息，就应使用BitSet。它只有从尺寸的角度看才有意义；如果希望的高效率的访问，那么它的速度会比使用一些固有类型的数组慢一些。
BitSet的大小与实际申请的大小并不一定一样，BitSet的size方法打印出的大小一定是64的倍数，这与它的实际申请代码有关，假设以下面的代码实例化一个BitSet:
BitSet set = new BitSet(129);
我们来看看实际是如何申请的：申请源码如下：
/**
* Creates a bit set whose initial size is large enough to explicitly
* represent bits with indices in the range <code>0</code> through
* <code>nbits-1</code>. All bits are initially <code>false</code>.
*
* @param nbits the initial size of the bit set.
* @exception NegativeArraySizeException if the specified initial size
* is negative.
*/
public BitSet(int nbits) {
// nbits can't be negative; size 0 is OK
if (nbits < 0)
throw new NegativeArraySizeException("nbits < 0: " + nbits);
initWords(nbits);
sizeIsSticky = true;
}
private void initWords(int nbits) {
words = new long[wordIndex(nbits-1) + 1];
}
实际的空间是由initWords方法控制的，在这个方法里面，我们实例化了一个long型数组，那么wordIndex又是干嘛的呢？其源码如下：
/**
* Given a bit index, return word index containing it.
*/
private static int wordIndex(int bitIndex) {
return bitIndex >> ADDRESS_BITS_PER_WORD;
}
这里涉及到一个常量ADDRESS_BITS_PER_WORD，先解释一下，源码中的定义如下：
private final static int ADDRESS_BITS_PER_WORD = 6;
那么很明显2^6=64,所以，当我们传进129作为参数的时候，我们会申请一个long[(129-1)>>6+1]也就是long[3]的数组，到此就很明白了，实际上替代办法的1）和2）是很相似的：都是通过一个整数（4个byte或者8个byte）来表示一定的bit位，之后，通过与十六位进制的数进行and,or,~等等操作进行Bit位的操作。
接下来讲讲其他比较重要的方法
1）set方法，源码如下：
/**
* Sets the bit at the specified index to <code>true</code>.
*
* @param bitIndex a bit index.
* @exception IndexOutOfBoundsException if the specified index is negative.
* @since JDK1.0
*/
public void set(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
int wordIndex = wordIndex(bitIndex);
expandTo(wordIndex);
words[wordIndex] |= (1L << bitIndex); // Restores invariants
checkInvariants();
}
这个方法将bitIndex位上的值由false设置为true,解释如下：
我们设置的时候很明显是在改变long数组的某一个元素的值，首先需要确定的是改变哪一个元素，其次需要使用与或操作改变这个元素，在上面的代码中，首先将bitIndex>>6，这样就确定了是修改哪一个元素的值，其次这里涉及到一个expandTo方法，我们先跳过去，直接看代码：
words[wordIndex] |= (1L << bitIndex); // Restores invariants
这里不是很好理解，要注意：需要注意的是java中的移位操作会模除位数，也就是说，long类型的移位会模除64。例如对long类型的值左移65位，实际是左移了65%64=1位。所以这行代码就等于：
int transderBits = bitIndex % 64; words[wordsIndex] |= (1L << transferBits);
上面这样写就很清楚了。
与之相对的一个方法是：
/**
* Sets the bit specified by the index to <code>false</code>.
*
* @param bitIndex the index of the bit to be cleared.
* @exception IndexOutOfBoundsException if the specified index is negative.
* @since JDK1.0
*/
public void clear(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
int wordIndex = wordIndex(bitIndex);
if (wordIndex >= wordsInUse)
return;
words[wordIndex] &= ~(1L << bitIndex);
recalculateWordsInUse();
checkInvariants();
}
这段代码理解上与set大同小异,主要是用来设置某一位上的值为false的。
上面有个方法，顺带着解释一下：
expandTo方法：
/**
* Ensures that the BitSet can accommodate a given wordIndex,
* temporarily violating the invariants. The caller must
* restore the invariants before returning to the user,
* possibly using recalculateWordsInUse().
* @param wordIndex the index to be accommodated.
*/
private void expandTo(int wordIndex) {
int wordsRequired = wordIndex+1;
if (wordsInUse < wordsRequired) {
ensureCapacity(wordsRequired);
wordsInUse = wordsRequired;
}
}
这里面又有个参数wordsInUse,定义如下：
/**
* The number of words in the logical size of this BitSet.
*/
private transient int wordsInUse = 0;
根据其定义解释，这个参数表示的是BitSet中的words的逻辑大小。当我们传进一个wordIndex的时候，首先需要判断这个逻辑大小与wordIndex的大小关系，如果小于它，我们就调用方法ensureCapacity:
private void ensureCapacity(int wordsRequired) {
if (words.length < wordsRequired) {
// Allocate larger of doubled size or required size
int request = Math.max(2 * words.length, wordsRequired);
words = Arrays.copyOf(words, request);
sizeIsSticky = false;
}
}
也就是说将words的大小变为原来的两倍，复制数组，标志sizeIsSticky为false,这个参数的定义如下：
/**
* Whether the size of "words" is user-specified. If so, we assume
* the user knows what he's doing and try harder to preserve it.
*/
private transient boolean sizeIsSticky = false;
执行完这个方法后，我们可以将wordsInUse设置为wordsRequired。（换句话说，BitSet具有自动扩充的功能）
2）get方法：
/**
* Returns the value of the bit with the specified index. The value
* is <code>true</code> if the bit with the index <code>bitIndex</code>
* is currently set in this <code>BitSet</code>; otherwise, the result
* is <code>false</code>.
*
* @param bitIndex the bit index.
* @return the value of the bit with the specified index.
* @exception IndexOutOfBoundsException if the specified index is negative.
*/
public boolean get(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
checkInvariants();
int wordIndex = wordIndex(bitIndex);
return (wordIndex < wordsInUse)
&& ((words[wordIndex] & (1L << bitIndex)) != 0);
}
这里主要是最后一个return语句，
return (wordIndex < wordsInUse) && ((words[wordIndex] & (1L << bitIndex)) != 0);
只有当wordIndex越界，并且wordIndex上的wordIndex上的bit不为0的时候，我们才说这一位是true.
3）size()方法：
/**
* Returns the number of bits of space actually in use by this
* <code>BitSet</code> to represent bit values.
* The maximum element in the set is the size - 1st element.
*
* @return the number of bits currently in this bit set.
*/
public int size() {
return words.length * BITS_PER_WORD;
}
这里也有一个常量，定义如下：
private final static int ADDRESS_BITS_PER_WORD = 6; private final static int BITS_PER_WORD = 1 << ADDRESS_BITS_PER_WORD;
很明显，BITS_PER_WORD = 64，这里很重要的一点就是，如果使用size来返回BitSet数组的大小，其值一定是64的倍数，原因就在这里
4）与size相似的一个方法：length()源码如下：
/**
* Returns the "logical size" of this <code>BitSet</code>: the index of
* the highest set bit in the <code>BitSet</code> plus one. Returns zero
* if the <code>BitSet</code> contains no set bits.
*
* @return the logical size of this <code>BitSet</code>.
* @since 1.2
*/
public int length() {
if (wordsInUse == 0)
return 0;
return BITS_PER_WORD * (wordsInUse - 1) +
(BITS_PER_WORD - Long.numberOfLeadingZeros(words[wordsInUse - 1]));
}
方法虽然短小，却比较难以理解，细细分析一下：根据注释，这个方法法返回的是BitSet的逻辑大小，比如说你声明了一个129位的BitSet,设置了第23，45，67位，那么其逻辑大小就是67，也就是说逻辑大小其实是的是在你设置的所有位里面最高位的Index。
这里有一个方法，Long.numberOfLeadingZeros，网上没有很好的解释，做实验如下：
long test = 1;
System.out.println(Long.numberOfLeadingZeros(test<<3));
System.out.println(Long.numberOfLeadingZeros(test<<40));
System.out.println(Long.numberOfLeadingZeros(test<<40 | test<<4));
打印结果如下：
60
23
23
也就是说，这个方法是输出一个64位二进制字符串前面0的个数的。
总结：
其实BitSet的源码并不复杂，只要理解其原理，对整数的移位等操作比较熟悉，细心阅读就可以理解。下面附上完整源码供研究：
* %W% %E%
*
* Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util;
import java.io.*;
/**
* This class implements a vector of bits that grows as needed. Each
* component of the bit set has a <code>boolean</code> value. The
* bits of a <code>BitSet</code> are indexed by nonnegative integers.
* Individual indexed bits can be examined, set, or cleared. One
* <code>BitSet</code> may be used to modify the contents of another
* <code>BitSet</code> through logical AND, logical inclusive OR, and
* logical exclusive OR operations.
* <p>
* By default, all bits in the set initially have the value
* <code>false</code>.
* <p>
* Every bit set has a current size, which is the number of bits
* of space currently in use by the bit set. Note that the size is
* related to the implementation of a bit set, so it may change with
* implementation. The length of a bit set relates to logical length
* of a bit set and is defined independently of implementation.
* <p>
* Unless otherwise noted, passing a null parameter to any of the
* methods in a <code>BitSet</code> will result in a
* <code>NullPointerException</code>.
*
* <p>A <code>BitSet</code> is not safe for multithreaded use without
* external synchronization.
*
* @author Arthur van Hoff
* @author Michael McCloskey
* @author Martin Buchholz
* @version %I%, %G%
* @since JDK1.0
*/
public class BitSet implements Cloneable, java.io.Serializable {
/*
* BitSets are packed into arrays of "words." Currently a word is
* a long, which consists of 64 bits, requiring 6 address bits.
* The choice of word size is determined purely by performance concerns.
*/
private final static int ADDRESS_BITS_PER_WORD = 6;
private final static int BITS_PER_WORD = 1 << ADDRESS_BITS_PER_WORD;
private final static int BIT_INDEX_MASK = BITS_PER_WORD - 1;
/* Used to shift left or right for a partial word mask */
private static final long WORD_MASK = 0xffffffffffffffffL;
/**
* @serialField bits long[]
*
* The bits in this BitSet. The ith bit is stored in bits[i/64] at
* bit position i % 64 (where bit position 0 refers to the least
* significant bit and 63 refers to the most significant bit).
*/
private static final ObjectStreamField[] serialPersistentFields = {
new ObjectStreamField("bits", long[].class),
};
/**
* The internal field corresponding to the serialField "bits".
*/
private long[] words;
/**
* The number of words in the logical size of this BitSet.
*/
private transient int wordsInUse = 0;
/**
* Whether the size of "words" is user-specified. If so, we assume
* the user knows what he's doing and try harder to preserve it.
*/
private transient boolean sizeIsSticky = false;
/* use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 7997698588986878753L;
/**
* Given a bit index, return word index containing it.
*/
private static int wordIndex(int bitIndex) {
return bitIndex >> ADDRESS_BITS_PER_WORD;
}
/**
* Every public method must preserve these invariants.
*/
private void checkInvariants() {
assert(wordsInUse == 0 || words[wordsInUse - 1] != 0);
assert(wordsInUse >= 0 && wordsInUse <= words.length);
assert(wordsInUse == words.length || words[wordsInUse] == 0);
}
/**
* Set the field wordsInUse with the logical size in words of the bit
* set. WARNING:This method assumes that the number of words actually
* in use is less than or equal to the current value of wordsInUse!
*/
private void recalculateWordsInUse() {
// Traverse the bitset until a used word is found
int i;
for (i = wordsInUse-1; i >= 0; i--)
if (words[i] != 0)
break;
wordsInUse = i+1; // The new logical size
}
/**
* Creates a new bit set. All bits are initially <code>false</code>.
*/
public BitSet() {
initWords(BITS_PER_WORD);
sizeIsSticky = false;
}
/**
* Creates a bit set whose initial size is large enough to explicitly
* represent bits with indices in the range <code>0</code> through
* <code>nbits-1</code>. All bits are initially <code>false</code>.
*
* @param nbits the initial size of the bit set.
* @exception NegativeArraySizeException if the specified initial size
* is negative.
*/
public BitSet(int nbits) {
// nbits can't be negative; size 0 is OK
if (nbits < 0)
throw new NegativeArraySizeException("nbits < 0: " + nbits);
initWords(nbits);
sizeIsSticky = true;
}
private void initWords(int nbits) {
words = new long[wordIndex(nbits-1) + 1];
}
/**
* Ensures that the BitSet can hold enough words.
* @param wordsRequired the minimum acceptable number of words.
*/
private void ensureCapacity(int wordsRequired) {
if (words.length < wordsRequired) {
// Allocate larger of doubled size or required size
int request = Math.max(2 * words.length, wordsRequired);
words = Arrays.copyOf(words, request);
sizeIsSticky = false;
}
}
/**
* Ensures that the BitSet can accommodate a given wordIndex,
* temporarily violating the invariants. The caller must
* restore the invariants before returning to the user,
* possibly using recalculateWordsInUse().
* @param wordIndex the index to be accommodated.
*/
private void expandTo(int wordIndex) {
int wordsRequired = wordIndex+1;
if (wordsInUse < wordsRequired) {
ensureCapacity(wordsRequired);
wordsInUse = wordsRequired;
}
}
/**
* Checks that fromIndex ... toIndex is a valid range of bit indices.
*/
private static void checkRange(int fromIndex, int toIndex) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex < 0: " + fromIndex);
if (toIndex < 0)
throw new IndexOutOfBoundsException("toIndex < 0: " + toIndex);
if (fromIndex > toIndex)
throw new IndexOutOfBoundsException("fromIndex: " + fromIndex +
" > toIndex: " + toIndex);
}
/**
* Sets the bit at the specified index to the complement of its
* current value.
*
* @param bitIndex the index of the bit to flip.
* @exception IndexOutOfBoundsException if the specified index is negative.
* @since 1.4
*/
public void flip(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
int wordIndex = wordIndex(bitIndex);
expandTo(wordIndex);
words[wordIndex] ^= (1L << bitIndex);
recalculateWordsInUse();
checkInvariants();
}
/**
* Sets each bit from the specified <tt>fromIndex</tt> (inclusive) to the
* specified <tt>toIndex</tt> (exclusive) to the complement of its current
* value.
*
* @param fromIndex index of the first bit to flip.
* @param toIndex index after the last bit to flip.
* @exception IndexOutOfBoundsException if <tt>fromIndex</tt> is negative,
* or <tt>toIndex</tt> is negative, or <tt>fromIndex</tt> is
* larger than <tt>toIndex</tt>.
* @since 1.4
*/
public void flip(int fromIndex, int toIndex) {
checkRange(fromIndex, toIndex);
if (fromIndex == toIndex)
return;
int startWordIndex = wordIndex(fromIndex);
int endWordIndex = wordIndex(toIndex - 1);
expandTo(endWordIndex);
long firstWordMask = WORD_MASK << fromIndex;
long lastWordMask = WORD_MASK >>> -toIndex;
if (startWordIndex == endWordIndex) {
// Case 1: One word
words[startWordIndex] ^= (firstWordMask & lastWordMask);
} else {
// Case 2: Multiple words
// Handle first word
words[startWordIndex] ^= firstWordMask;
// Handle intermediate words, if any
for (int i = startWordIndex+1; i < endWordIndex; i++)
words[i] ^= WORD_MASK;
// Handle last word
words[endWordIndex] ^= lastWordMask;
}
recalculateWordsInUse();
checkInvariants();
}
/**
* Sets the bit at the specified index to <code>true</code>.
*
* @param bitIndex a bit index.
* @exception IndexOutOfBoundsException if the specified index is negative.
* @since JDK1.0
*/
public void set(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
int wordIndex = wordIndex(bitIndex);
expandTo(wordIndex);
words[wordIndex] |= (1L << bitIndex); // Restores invariants
checkInvariants();
}
/**
* Sets the bit at the specified index to the specified value.
*
* @param bitIndex a bit index.
* @param value a boolean value to set.
* @exception IndexOutOfBoundsException if the specified index is negative.
* @since 1.4
*/
public void set(int bitIndex, boolean value) {
if (value)
set(bitIndex);
else
clear(bitIndex);
}
/**
* Sets the bits from the specified <tt>fromIndex</tt> (inclusive) to the
* specified <tt>toIndex</tt> (exclusive) to <code>true</code>.
*
* @param fromIndex index of the first bit to be set.
* @param toIndex index after the last bit to be set.
* @exception IndexOutOfBoundsException if <tt>fromIndex</tt> is negative,
* or <tt>toIndex</tt> is negative, or <tt>fromIndex</tt> is
* larger than <tt>toIndex</tt>.
* @since 1.4
*/
public void set(int fromIndex, int toIndex) {
checkRange(fromIndex, toIndex);
if (fromIndex == toIndex)
return;
// Increase capacity if necessary
int startWordIndex = wordIndex(fromIndex);
int endWordIndex = wordIndex(toIndex - 1);
expandTo(endWordIndex);
long firstWordMask = WORD_MASK << fromIndex;
long lastWordMask = WORD_MASK >>> -toIndex;
if (startWordIndex == endWordIndex) {
// Case 1: One word
words[startWordIndex] |= (firstWordMask & lastWordMask);
} else {
// Case 2: Multiple words
// Handle first word
words[startWordIndex] |= firstWordMask;
// Handle intermediate words, if any
for (int i = startWordIndex+1; i < endWordIndex; i++)
words[i] = WORD_MASK;
// Handle last word (restores invariants)
words[endWordIndex] |= lastWordMask;
}
checkInvariants();
}
/**
* Sets the bits from the specified <tt>fromIndex</tt> (inclusive) to the
* specified <tt>toIndex</tt> (exclusive) to the specified value.
*
* @param fromIndex index of the first bit to be set.
* @param toIndex index after the last bit to be set
* @param value value to set the selected bits to
* @exception IndexOutOfBoundsException if <tt>fromIndex</tt> is negative,
* or <tt>toIndex</tt> is negative, or <tt>fromIndex</tt> is
* larger than <tt>toIndex</tt>.
* @since 1.4
*/
public void set(int fromIndex, int toIndex, boolean value) {
if (value)
set(fromIndex, toIndex);
else
clear(fromIndex, toIndex);
}
/**
* Sets the bit specified by the index to <code>false</code>.
*
* @param bitIndex the index of the bit to be cleared.
* @exception IndexOutOfBoundsException if the specified index is negative.
* @since JDK1.0
*/
public void clear(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
int wordIndex = wordIndex(bitIndex);
if (wordIndex >= wordsInUse)
return;
words[wordIndex] &= ~(1L << bitIndex);
recalculateWordsInUse();
checkInvariants();
}
/**
* Sets the bits from the specified <tt>fromIndex</tt> (inclusive) to the
* specified <tt>toIndex</tt> (exclusive) to <code>false</code>.
*
* @param fromIndex index of the first bit to be cleared.
* @param toIndex index after the last bit to be cleared.
* @exception IndexOutOfBoundsException if <tt>fromIndex</tt> is negative,
* or <tt>toIndex</tt> is negative, or <tt>fromIndex</tt> is
* larger than <tt>toIndex</tt>.
* @since 1.4
*/
public void clear(int fromIndex, int toIndex) {
checkRange(fromIndex, toIndex);
if (fromIndex == toIndex)
return;
int startWordIndex = wordIndex(fromIndex);
if (startWordIndex >= wordsInUse)
return;
int endWordIndex = wordIndex(toIndex - 1);
if (endWordIndex >= wordsInUse) {
toIndex = length();
endWordIndex = wordsInUse - 1;
}
long firstWordMask = WORD_MASK << fromIndex;
long lastWordMask = WORD_MASK >>> -toIndex;
if (startWordIndex == endWordIndex) {
// Case 1: One word
words[startWordIndex] &= ~(firstWordMask & lastWordMask);
} else {
// Case 2: Multiple words
// Handle first word
words[startWordIndex] &= ~firstWordMask;
// Handle intermediate words, if any
for (int i = startWordIndex+1; i < endWordIndex; i++)
words[i] = 0;
// Handle last word
words[endWordIndex] &= ~lastWordMask;
}
recalculateWordsInUse();
checkInvariants();
}
/**
* Sets all of the bits in this BitSet to <code>false</code>.
*
* @since 1.4
*/
public void clear() {
while (wordsInUse > 0)
words[--wordsInUse] = 0;
}
/**
* Returns the value of the bit with the specified index. The value
* is <code>true</code> if the bit with the index <code>bitIndex</code>
* is currently set in this <code>BitSet</code>; otherwise, the result
* is <code>false</code>.
*
* @param bitIndex the bit index.
* @return the value of the bit with the specified index.
* @exception IndexOutOfBoundsException if the specified index is negative.
*/
public boolean get(int bitIndex) {
if (bitIndex < 0)
throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);
checkInvariants();
int wordIndex = wordIndex(bitIndex);
return (wordIndex < wordsInUse)
&& ((words[wordIndex] & (1L << bitIndex)) != 0);
}
/**
* Returns a new <tt>BitSet</tt> composed of bits from this <tt>BitSet</tt>
* from <tt>fromIndex</tt> (inclusive) to <tt>toIndex</tt> (exclusive).
*
* @param fromIndex index of the first bit to include.
* @param toIndex index after the last bit to include.
* @return a new <tt>BitSet</tt> from a range of this <tt>BitSet</tt>.
* @exception IndexOutOfBoundsException if <tt>fromIndex</tt> is negative,
* or <tt>toIndex</tt> is negative, or <tt>fromIndex</tt> is
* larger than <tt>toIndex</tt>.
* @since 1.4
*/
public BitSet get(int fromIndex, int toIndex) {
checkRange(fromIndex, toIndex);
checkInvariants();
int len = length();
// If no set bits in range return empty bitset
if (len <= fromIndex || fromIndex == toIndex)
return new BitSet(0);
// An optimization
if (toIndex > len)
toIndex = len;
BitSet result = new BitSet(toIndex - fromIndex);
int targetWords = wordIndex(toIndex - fromIndex - 1) + 1;
int sourceIndex = wordIndex(fromIndex);
boolean wordAligned = ((fromIndex & BIT_INDEX_MASK) == 0);
// Process all words but the last word
for (int i = 0; i < targetWords - 1; i++, sourceIndex++)
result.words[i] = wordAligned ? words[sourceIndex] :
(words[sourceIndex] >>> fromIndex) |
(words[sourceIndex+1] << -fromIndex);
// Process the last word
long lastWordMask = WORD_MASK >>> -toIndex;
result.words[targetWords - 1] =
((toIndex-1) & BIT_INDEX_MASK) < (fromIndex & BIT_INDEX_MASK)
? /* straddles source words */
((words[sourceIndex] >>> fromIndex) |
(words[sourceIndex+1] & lastWordMask) << -fromIndex)
:
((words[sourceIndex] & lastWordMask) >>> fromIndex);
// Set wordsInUse correctly
result.wordsInUse = targetWords;
result.recalculateWordsInUse();
result.checkInvariants();
return result;
}
/**
* Returns the index of the first bit that is set to <code>true</code>
* that occurs on or after the specified starting index. If no such
* bit exists then -1 is returned.
*
* To iterate over the <code>true</code> bits in a <code>BitSet</code>,
* use the following loop:
*
* <pre>
* for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1)) {
* // operate on index i here
* }</pre>
*
* @param fromIndex the index to start checking from (inclusive).
* @return the index of the next set bit.
* @throws IndexOutOfBoundsException if the specified index is negative.
* @since 1.4
*/
public int nextSetBit(int fromIndex) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex < 0: " + fromIndex);
checkInvariants();
int u = wordIndex(fromIndex);
if (u >= wordsInUse)
return -1;
long word = words[u] & (WORD_MASK << fromIndex);
while (true) {
if (word != 0)
return (u * BITS_PER_WORD) + Long.numberOfTrailingZeros(word);
if (++u == wordsInUse)
return -1;
word = words[u];
}
}
/**
* Returns the index of the first bit that is set to <code>false</code>
* that occurs on or after the specified starting index.
*
* @param fromIndex the index to start checking from (inclusive).
* @return the index of the next clear bit.
* @throws IndexOutOfBoundsException if the specified index is negative.
* @since 1.4
*/
public int nextClearBit(int fromIndex) {
// Neither spec nor implementation handle bitsets of maximal length.
// See 4816253.
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex < 0: " + fromIndex);
checkInvariants();
int u = wordIndex(fromIndex);
if (u >= wordsInUse)
return fromIndex;
long word = ~words[u] & (WORD_MASK << fromIndex);
while (true) {
if (word != 0)
return (u * BITS_PER_WORD) + Long.numberOfTrailingZeros(word);
if (++u == wordsInUse)
return wordsInUse * BITS_PER_WORD;
word = ~words[u];
}
}
/**
* Returns the "logical size" of this <code>BitSet</code>: the index of
* the highest set bit in the <code>BitSet</code> plus one. Returns zero
* if the <code>BitSet</code> contains no set bits.
*
* @return the logical size of this <code>BitSet</code>.
* @since 1.2
*/
public int length() {
if (wordsInUse == 0)
return 0;
return BITS_PER_WORD * (wordsInUse - 1) +
(BITS_PER_WORD - Long.numberOfLeadingZeros(words[wordsInUse - 1]));
}
/**
* Returns true if this <code>BitSet</code> contains no bits that are set
* to <code>true</code>.
*
* @return boolean indicating whether this <code>BitSet</code> is empty.
* @since 1.4
*/
public boolean isEmpty() {
return wordsInUse == 0;
}
/**
* Returns true if the specified <code>BitSet</code> has any bits set to
* <code>true</code> that are also set to <code>true</code> in this
* <code>BitSet</code>.
*
* @param set <code>BitSet</code> to intersect with
* @return boolean indicating whether this <code>BitSet</code> intersects
* the specified <code>BitSet</code>.
* @since 1.4
*/
public boolean intersects(BitSet set) {
for (int i = Math.min(wordsInUse, set.wordsInUse) - 1; i >= 0; i--)
if ((words[i] & set.words[i]) != 0)
return true;
return false;
}
/**
* Returns the number of bits set to <tt>true</tt> in this
* <code>BitSet</code>.
*
* @return the number of bits set to <tt>true</tt> in this
* <code>BitSet</code>.
* @since 1.4
*/
public int cardinality() {
int sum = 0;
for (int i = 0; i < wordsInUse; i++)
sum += Long.bitCount(words[i]);
return sum;
}
/**
* Performs a logical <b>AND</b> of this target bit set with the
* argument bit set. This bit set is modified so that each bit in it
* has the value <code>true</code> if and only if it both initially
* had the value <code>true</code> and the corresponding bit in the
* bit set argument also had the value <code>true</code>.
*
* @param set a bit set.
*/
public void and(BitSet set) {
if (this == set)
return;
while (wordsInUse > set.wordsInUse)
words[--wordsInUse] = 0;
// Perform logical AND on words in common
for (int i = 0; i < wordsInUse; i++)
words[i] &= set.words[i];
recalculateWordsInUse();
checkInvariants();
}
/**
* Performs a logical <b>OR</b> of this bit set with the bit set
* argument. This bit set is modified so that a bit in it has the
* value <code>true</code> if and only if it either already had the
* value <code>true</code> or the corresponding bit in the bit set
* argument has the value <code>true</code>.
*
* @param set a bit set.
*/
public void or(BitSet set) {
if (this == set)
return;
int wordsInCommon = Math.min(wordsInUse, set.wordsInUse);
if (wordsInUse < set.wordsInUse) {
ensureCapacity(set.wordsInUse);
wordsInUse = set.wordsInUse;
}
// Perform logical OR on words in common
for (int i = 0; i < wordsInCommon; i++)
words[i] |= set.words[i];
// Copy any remaining words
if (wordsInCommon < set.wordsInUse)
System.arraycopy(set.words, wordsInCommon,
words, wordsInCommon,
wordsInUse - wordsInCommon);
// recalculateWordsInUse() is unnecessary
checkInvariants();
}
/**
* Performs a logical <b>XOR</b> of this bit set with the bit set
* argument. This bit set is modified so that a bit in it has the
* value <code>true</code> if and only if one of the following
* statements holds:
* <ul>
* <li>The bit initially has the value <code>true</code>, and the
* corresponding bit in the argument has the value <code>false</code>.
* <li>The bit initially has the value <code>false</code>, and the
* corresponding bit in the argument has the value <code>true</code>.
* </ul>
*
* @param set a bit set.
*/
public void xor(BitSet set) {
int wordsInCommon = Math.min(wordsInUse, set.wordsInUse);
if (wordsInUse < set.wordsInUse) {
ensureCapacity(set.wordsInUse);
wordsInUse = set.wordsInUse;
}
// Perform logical XOR on words in common
for (int i = 0; i < wordsInCommon; i++)
words[i] ^= set.words[i];
// Copy any remaining words
if (wordsInCommon < set.wordsInUse)
System.arraycopy(set.words, wordsInCommon,
words, wordsInCommon,
set.wordsInUse - wordsInCommon);
recalculateWordsInUse();
checkInvariants();
}
/**
* Clears all of the bits in this <code>BitSet</code> whose corresponding
* bit is set in the specified <code>BitSet</code>.
*
* @param set the <code>BitSet</code> with which to mask this
* <code>BitSet</code>.
* @since 1.2
*/
public void andNot(BitSet set) {
// Perform logical (a & !b) on words in common
for (int i = Math.min(wordsInUse, set.wordsInUse) - 1; i >= 0; i--)
words[i] &= ~set.words[i];
recalculateWordsInUse();
checkInvariants();
}
/**
* Returns a hash code value for this bit set. The hash code
* depends only on which bits have been set within this
* <code>BitSet</code>. The algorithm used to compute it may
* be described as follows.<p>
* Suppose the bits in the <code>BitSet</code> were to be stored
* in an array of <code>long</code> integers called, say,
* <code>words</code>, in such a manner that bit <code>k</code> is
* set in the <code>BitSet</code> (for nonnegative values of
* <code>k</code>) if and only if the expression
* <pre>((k>>6) < words.length) && ((words[k>>6] & (1L << (bit & 0x3F))) != 0)</pre>
* is true. Then the following definition of the <code>hashCode</code>
* method would be a correct implementation of the actual algorithm:
* <pre>
* public int hashCode() {
* long h = 1234;
* for (int i = words.length; --i >= 0; ) {
* h ^= words[i] * (i + 1);
* }
* return (int)((h >> 32) ^ h);
* }</pre>
* Note that the hash code values change if the set of bits is altered.
* <p>Overrides the <code>hashCode</code> method of <code>Object</code>.
*
* @return a hash code value for this bit set.
*/
public int hashCode() {
long h = 1234;
for (int i = wordsInUse; --i >= 0; )
h ^= words[i] * (i + 1);
return (int)((h >> 32) ^ h);
}
/**
* Returns the number of bits of space actually in use by this
* <code>BitSet</code> to represent bit values.
* The maximum element in the set is the size - 1st element.
*
* @return the number of bits currently in this bit set.
*/
public int size() {
return words.length * BITS_PER_WORD;
}
/**
* Compares this object against the specified object.
* The result is <code>true</code> if and only if the argument is
* not <code>null</code> and is a <code>Bitset</code> object that has
* exactly the same set of bits set to <code>true</code> as this bit
* set. That is, for every nonnegative <code>int</code> index <code>k</code>,
* <pre>((BitSet)obj).get(k) == this.get(k)</pre>
* must be true. The current sizes of the two bit sets are not compared.
* <p>Overrides the <code>equals</code> method of <code>Object</code>.
*
* @param obj the object to compare with.
* @return <code>true</code> if the objects are the same;
* <code>false</code> otherwise.
* @see java.util.BitSet#size()
*/
public boolean equals(Object obj) {
if (!(obj instanceof BitSet))
return false;
if (this == obj)
return true;
BitSet set = (BitSet) obj;
checkInvariants();
set.checkInvariants();
if (wordsInUse != set.wordsInUse)
return false;
// Check words in use by both BitSets
for (int i = 0; i < wordsInUse; i++)
if (words[i] != set.words[i])
return false;
return true;
}
/**
* Cloning this <code>BitSet</code> produces a new <code>BitSet</code>
* that is equal to it.
* The clone of the bit set is another bit set that has exactly the
* same bits set to <code>true</code> as this bit set.
*
* <p>Overrides the <code>clone</code> method of <code>Object</code>.
*
* @return a clone of this bit set.
* @see java.util.BitSet#size()
*/
public Object clone() {
if (! sizeIsSticky)
trimToSize();
try {
BitSet result = (BitSet) super.clone();
result.words = words.clone();
result.checkInvariants();
return result;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}
/**
* Attempts to reduce internal storage used for the bits in this bit set.
* Calling this method may, but is not required to, affect the value
* returned by a subsequent call to the {@link #size()} method.
*/
private void trimToSize() {
if (wordsInUse != words.length) {
words = Arrays.copyOf(words, wordsInUse);
checkInvariants();
}
}
/**
* Save the state of the <tt>BitSet</tt> instance to a stream (i.e.,
* serialize it).
*/
private void writeObject(ObjectOutputStream s)
throws IOException {
checkInvariants();
if (! sizeIsSticky)
trimToSize();
ObjectOutputStream.PutField fields = s.putFields();
fields.put("bits", words);
s.writeFields();
}
/**
* Reconstitute the <tt>BitSet</tt> instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException {
ObjectInputStream.GetField fields = s.readFields();
words = (long[]) fields.get("bits", null);
// Assume maximum length then find real length
// because recalculateWordsInUse assumes maintenance
// or reduction in logical size
wordsInUse = words.length;
recalculateWordsInUse();
sizeIsSticky = (words.length > 0 && words[words.length-1] == 0L); // heuristic
checkInvariants();
}
/**
* Returns a string representation of this bit set. For every index
* for which this <code>BitSet</code> contains a bit in the set
* state, the decimal representation of that index is included in
* the result. Such indices are listed in order from lowest to
* highest, separated by ", " (a comma and a space) and
* surrounded by braces, resulting in the usual mathematical
* notation for a set of integers.<p>
* Overrides the <code>toString</code> method of <code>Object</code>.
* <p>Example:
* <pre>
* BitSet drPepper = new BitSet();</pre>
* Now <code>drPepper.toString()</code> returns "<code>{}</code>".<p>
* <pre>
* drPepper.set(2);</pre>
* Now <code>drPepper.toString()</code> returns "<code>{2}</code>".<p>
* <pre>
* drPepper.set(4);
* drPepper.set(10);</pre>
* Now <code>drPepper.toString()</code> returns "<code>{2, 4, 10}</code>".
*
* @return a string representation of this bit set.
*/
public String toString() {
checkInvariants();
int numBits = (wordsInUse > 128) ?
cardinality() : wordsInUse * BITS_PER_WORD;
StringBuilder b = new StringBuilder(6*numBits + 2);
b.append('{');
int i = nextSetBit(0);
if (i != -1) {
b.append(i);
for (i = nextSetBit(i+1); i >= 0; i = nextSetBit(i+1)) {
int endOfRun = nextClearBit(i);
do { b.append(", ").append(i); }
while (++i < endOfRun);
}
}
b.append('}');
return b.toString();
}
}
（java的类真是大啊！！）
