Collections源码
介绍
Collections 是java集合框架中的一个工具类，主要用于Collectiont提供的通用算法，比如：排序(sort)、二分查找(binarySearch)、洗牌(shuffle)、旋转(rotate)
常量&变量
/*
* Tuning parameters for algorithms - Many of the List algorithms have
* two implementations, one of which is appropriate for RandomAccess
* lists, the other for "sequential." Often, the random access variant
* yields better performance on small sequential access lists. The
* tuning parameters below determine the cutoff point for what constitutes
* a "small" sequential access list for each algorithm. The values below
* were empirically determined to work well for LinkedList. Hopefully
* they should be reasonable for other sequential access List
* implementations. Those doing performance work on this code would
* do well to validate the values of these parameters from time to time.
* (The first word of each tuning parameter name is the algorithm to which
* it applies.)
*/
/** 一些算法所用到的常量，主要是些阈值 **/
private static final int BINARYSEARCH_THRESHOLD = 5000;
private static final int REVERSE_THRESHOLD = 18;
private static final int SHUFFLE_THRESHOLD = 5;
private static final int FILL_THRESHOLD = 25;
private static final int ROTATE_THRESHOLD = 100;
private static final int COPY_THRESHOLD = 10;
private static final int REPLACEALL_THRESHOLD = 11;
private static final int INDEXOFSUBLIST_THRESHOLD = 35;
构造方法
私有无参构造
// Suppresses default constructor, ensuring non-instantiability.
private Collections() {
}
内部类
Collections中提供了大量的集合类，以代理的方式对现有的集合进行了功能上的修改。
总共有以下几类集合：
Unmodifiable：不可修改的集合类
Synchronized：同步的集合类
Checked：类型安全的集合类
Empty：空集合类
Empty：只有单个元素的集合类
Collections中的内部集合类为传入的类作为代理，以改变传入集合的行为。
UnmodifiableCollection
UnmodifiableCollection是所有Unmodifiable类的父类
UnmodifiableCollection用一个成员变量保存传入对象，然后为该对象提供代理
UnmodifiableCollection将传入的集合包装成一个不可变的集合。
/**
* @serial include
*/
static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
//序列化版本号
private static final long serialVersionUID = 1820017752578914078L;
//存储的集合类
final Collection<? extends E> c;
UnmodifiableCollection(Collection<? extends E> c) {
if (c==null)
throw new NullPointerException();
this.c = c;
}
// 实现行为相同的方法，以代理的方式访问传入的集合类
public int size() {return c.size();}
public boolean isEmpty() {return c.isEmpty();}
public boolean contains(Object o) {return c.contains(o);}
public Object[] toArray() {return c.toArray();}
public <T> T[] toArray(T[] a) {return c.toArray(a);}
public String toString() {return c.toString();}
public Iterator<E> iterator() {
return new Iterator<E>() {
private final Iterator<? extends E> i = c.iterator();
public boolean hasNext() {return i.hasNext();}
public E next() {return i.next();}
public void remove() {
throw new UnsupportedOperationException();
}
@Override
public void forEachRemaining(Consumer<? super E> action) {
// Use backing collection version
i.forEachRemaining(action);
}
};
}
public boolean add(E e) {
throw new UnsupportedOperationException();
}
public boolean remove(Object o) {
throw new UnsupportedOperationException();
}
public boolean containsAll(Collection<?> coll) {
return c.containsAll(coll);
}
public boolean addAll(Collection<? extends E> coll) {
throw new UnsupportedOperationException();
}
public boolean removeAll(Collection<?> coll) {
throw new UnsupportedOperationException();
}
public boolean retainAll(Collection<?> coll) {
throw new UnsupportedOperationException();
}
public void clear() {
throw new UnsupportedOperationException();
}
// Override default methods in Collection
@Override
public void forEach(Consumer<? super E> action) {
c.forEach(action);
}
@Override
public boolean removeIf(Predicate<? super E> filter) {
throw new UnsupportedOperationException();
}
@SuppressWarnings("unchecked")
@Override
public Spliterator<E> spliterator() {
return (Spliterator<E>)c.spliterator();
}
@SuppressWarnings("unchecked")
@Override
public Stream<E> stream() {
return (Stream<E>)c.stream();
}
@SuppressWarnings("unchecked")
@Override
public Stream<E> parallelStream() {
return (Stream<E>)c.parallelStream();
}
}
SynchronizedCollection
SynchronizedCollection应该算是一个比较重的的集合，他会把传入的集合包装成一个线程安全的集合类
/**
* Returns a synchronized (thread-safe) collection backed by the specified
* collection. In order to guarantee serial access, it is critical that
* all access to the backing collection is accomplished
* through the returned collection.
*
* It is imperative that the user manually synchronize on the returned
* collection when traversing it via {@link Iterator}, {@link Spliterator}
* or {@link Stream}:
*
* Collection c = Collections.synchronizedCollection(myCollection);
* ...
* synchronized (c) {
* Iterator i = c.iterator(); // Must be in the synchronized block
* while (i.hasNext())
* foo(i.next());
* }
*
* Failure to follow this advice may result in non-deterministic behavior.
*
* The returned collection does not pass the {@code hashCode}
* and {@code equals} operations through to the backing collection, but
* relies on {@code Object}'s equals and hashCode methods. This is
* necessary to preserve the contracts of these operations in the case
* that the backing collection is a set or a list.
*
* The returned collection will be serializable if the specified collection
* is serializable.
*
* @param the class of the objects in the collection
* @param c the collection to be "wrapped" in a synchronized collection.
* @return a synchronized view of the specified collection.
* 返回一个被synchronized包装的集合，方法全部使用synchronized怼代码块加锁
*/
public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
return new SynchronizedCollection<>(c);
}
CheckedCollection
**CheckedCollection必须指定类型，将传入对象包装成一个类型安全的集合，**CheckedCollection插入时会检查类型，确保集合是类型安全的
/**
* @serial include
*/
static class CheckedCollection<E> implements Collection<E>, Serializable {
private static final long serialVersionUID = 1578914078182001775L;
final Collection<E> c;
final Class<E> type;
@SuppressWarnings("unchecked")
E typeCheck(Object o) {
if (o != null && !type.isInstance(o))
throw new ClassCastException(badElementMsg(o));
return (E) o;
}
private String badElementMsg(Object o) {
return "Attempt to insert " + o.getClass() +
" element into collection with element type " + type;
}
//必须指定类型
CheckedCollection(Collection<E> c, Class<E> type) {
this.c = Objects.requireNonNull(c, "c");
this.type = Objects.requireNonNull(type, "type");
}
public int size() { return c.size(); }
public boolean isEmpty() { return c.isEmpty(); }
public boolean contains(Object o) { return c.contains(o); }
public Object[] toArray() { return c.toArray(); }
public <T> T[] toArray(T[] a) { return c.toArray(a); }
public String toString() { return c.toString(); }
public boolean remove(Object o) { return c.remove(o); }
public void clear() { c.clear(); }
public boolean containsAll(Collection<?> coll) {
return c.containsAll(coll);
}
public boolean removeAll(Collection<?> coll) {
return c.removeAll(coll);
}
public boolean retainAll(Collection<?> coll) {
return c.retainAll(coll);
}
public Iterator<E> iterator() {
// JDK-6363904 - unwrapped iterator could be typecast to
// ListIterator with unsafe set()
final Iterator<E> it = c.iterator();
return new Iterator<E>() {
public boolean hasNext() { return it.hasNext(); }
public E next() { return it.next(); }
public void remove() { it.remove(); }};
}
public boolean add(E e) { return c.add(typeCheck(e)); }
private E[] zeroLengthElementArray; // Lazily initialized
private E[] zeroLengthElementArray() {
return zeroLengthElementArray != null ? zeroLengthElementArray :
(zeroLengthElementArray = zeroLengthArray(type));
}
@SuppressWarnings("unchecked")
Collection<E> checkedCopyOf(Collection<? extends E> coll) {
Object[] a;
try {
E[] z = zeroLengthElementArray();
a = coll.toArray(z);
// Defend against coll violating the toArray contract
if (a.getClass() != z.getClass())
a = Arrays.copyOf(a, a.length, z.getClass());
} catch (ArrayStoreException ignore) {
// To get better and consistent diagnostics,
// we call typeCheck explicitly on each element.
// We call clone() to defend against coll retaining a
// reference to the returned array and storing a bad
// element into it after it has been type checked.
a = coll.toArray().clone();
for (Object o : a)
typeCheck(o);
}
// A slight abuse of the type system, but safe here.
return (Collection<E>) Arrays.asList(a);
}
public boolean addAll(Collection<? extends E> coll) {
// Doing things this way insulates us from concurrent changes
// in the contents of coll and provides all-or-nothing
// semantics (which we wouldn't get if we type-checked each
// element as we added it)
return c.addAll(checkedCopyOf(coll));
}
// Override default methods in Collection
@Override
public void forEach(Consumer<? super E> action) {c.forEach(action);}
@Override
public boolean removeIf(Predicate<? super E> filter) {
return c.removeIf(filter);
}
@Override
public Spliterator<E> spliterator() {return c.spliterator();}
@Override
public Stream<E> stream() {return c.stream();}
@Override
public Stream<E> parallelStream() {return c.parallelStream();}
}
EmptyList
EmptyList提供一个空的对象，对象不可修改。
Collections中有一个EMPTY_LIST成员变量，该变量为static
例如：某个函数返回一个List，如果返回的List不存在。那么我们不应该返回null，而是使用一个空对象代替。这时候我们可以使用return Collections.EMPTY_LIST;
/**
* @serial include
*/
private static class EmptyList<E>
extends AbstractList<E>
implements RandomAccess, Serializable {
private static final long serialVersionUID = 8842843931221139166L;
public Iterator<E> iterator() {
return emptyIterator();
}
public ListIterator<E> listIterator() {
return emptyListIterator();
}
public int size() {return 0;}
public boolean isEmpty() {return true;}
public boolean contains(Object obj) {return false;}
public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
public Object[] toArray() { return new Object[0]; }
public <T> T[] toArray(T[] a) {
if (a.length > 0)
a[0] = null;
return a;
}
public E get(int index) {
throw new IndexOutOfBoundsException("Index: "+index);
}
public boolean equals(Object o) {
return (o instanceof List) && ((List<?>)o).isEmpty();
}
public int hashCode() { return 1; }
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);
return false;
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
}
@Override
public void sort(Comparator<? super E> c) {
}
// Override default methods in Collection
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
}
@Override
public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
// Preserves singleton property
private Object readResolve() {
return EMPTY_LIST;
}
}
SingletonList
SingletonList内只允许存储一个元素，该元素在List初始化时确定。
/**
* @serial include
*/
private static class SingletonList<E>
extends AbstractList<E>
implements RandomAccess, Serializable {
private static final long serialVersionUID = 3093736618740652951L;
private final E element;
// 初始化时传入一个元素
SingletonList(E obj) {element = obj;}
public Iterator<E> iterator() {
return singletonIterator(element);
}
public int size() {return 1;}
public boolean contains(Object obj) {return eq(obj, element);}
// 只允许返回第一个元素
public E get(int index) {
if (index != 0)
throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
return element;
}
// Override default methods for Collection
@Override
public void forEach(Consumer<? super E> action) {
action.accept(element);
}
@Override
public boolean removeIf(Predicate<? super E> filter) {
throw new UnsupportedOperationException();
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
throw new UnsupportedOperationException();
}
@Override
public void sort(Comparator<? super E> c) {
}
@Override
public Spliterator<E> spliterator() {
return singletonSpliterator(element);
}
}
常用方法
sort
Collections.sort集合排序最终调用的是java.util包下Arrays类的 Arrays.sort(T[] a, Comparator c)，这个方法根据有没有传入Compararot c 调用快速排序(sort())和优化的归并排序(TimSort.sort())
/**
* Sorts the specified list into ascending order, according to the
* {@linkplain Comparable natural ordering} of its elements.
* All elements in the list must implement the {@link Comparable}
* interface. Furthermore, all elements in the list must be
* mutually comparable (that is, {@code e1.compareTo(e2)}
* must not throw a {@code ClassCastException} for any elements
* {@code e1} and {@code e2} in the list).
*
* This sort is guaranteed to be stable: equal elements will
* not be reordered as a result of the sort.
*
*
The specified list must be modifiable, but need not be resizable.
*
* @implNote
* This implementation defers to the {@link List#sort(Comparator)}
* method using the specified list and a {@code null} comparator.
*
* @param the class of the objects in the list
* @param list the list to be sorted.
* @throws ClassCastException if the list contains elements that are not
* mutually comparable (for example, strings and integers).
* @throws UnsupportedOperationException if the specified list's
* list-iterator does not support the {@code set} operation.
* @throws IllegalArgumentException (optional) if the implementation
* detects that the natural ordering of the list elements is
* found to violate the {@link Comparable} contract
* @see List#sort(Comparator)
*/
@SuppressWarnings("unchecked")
public static <T extends Comparable<? super T>> void sort(List<T> list) {
// 直接调用list的排序方法
list.sort(null);
}
/**
* Sorts the specified list according to the order induced by the
* specified comparator. All elements in the list must be mutually
* comparable using the specified comparator (that is,
* {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
* for any elements {@code e1} and {@code e2} in the list).
*
* This sort is guaranteed to be stable: equal elements will
* not be reordered as a result of the sort.
*
*
The specified list must be modifiable, but need not be resizable.
*
* @implNote
* This implementation defers to the {@link List#sort(Comparator)}
* method using the specified list and comparator.
*
* @param the class of the objects in the list
* @param list the list to be sorted.
* @param c the comparator to determine the order of the list. A
* {@code null} value indicates that the elements' natural
* ordering should be used.
* @throws ClassCastException if the list contains elements that are not
* mutually comparable using the specified comparator.
* @throws UnsupportedOperationException if the specified list's
* list-iterator does not support the {@code set} operation.
* @throws IllegalArgumentException (optional) if the comparator is
* found to violate the {@link Comparator} contract
* @see List#sort(Comparator)
* 排序，传入集合和比较器
*/
@SuppressWarnings({"unchecked", "rawtypes"})
public static <T> void sort(List<T> list, Comparator<? super T> c) {
// 直接调用ArrayList的排序方法
list.sort(c);
}
ArrayList的sort()方法调用Arrays的sort(),请参考Arrays源码
binarySearch
二分查找的前提是集合有序，否则不能满足二分算法的查找过程。
/**
* Searches the specified list for the specified object using the binary
* search algorithm. The list must be sorted into ascending order
* according to the {@linkplain Comparable natural ordering} of its
* elements (as by the {@link #sort(List)} method) prior to making this
* call. If it is not sorted, the results are undefined. If the list
* contains multiple elements equal to the specified object, there is no
* guarantee which one will be found.
*
* This method runs in log(n) time for a "random access" list (which
* provides near-constant-time positional access). If the specified list
* does not implement the {@link RandomAccess} interface and is large,
* this method will do an iterator-based binary search that performs
* O(n) link traversals and O(log n) element comparisons.
*
* @param the class of the objects in the list
* @param list the list to be searched.
* @param key the key to be searched for.
* @return the index of the search key, if it is contained in the list;
* otherwise, (-(insertion point) - 1). The
* insertion point is defined as the point at which the
* key would be inserted into the list: the index of the first
* element greater than the key, or list.size() if all
* elements in the list are less than the specified key. Note
* that this guarantees that the return value will be >= 0 if
* and only if the key is found.
* @throws ClassCastException if the list contains elements that are not
* mutually comparable (for example, strings and
* integers), or the search key is not mutually comparable
* with the elements of the list.
*/
public static <T>
int binarySearch(List<? extends Comparable<? super T>> list, T key) {
// 当list为随机访问列表或者长度小于阈值5000时，使用索引二叉搜索
if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
return Collections.indexedBinarySearch(list, key);
else
// 否则使用迭代二叉搜索
return Collections.iteratorBinarySearch(list, key);
}
//索引二叉搜索
private static <T>
int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
//初始低位为0
int low = 0;
//初始高位为 list.size()-1
int high = list.size()-1;
while (low <= high) {
// 位运算获取中间下标
int mid = (low + high) >>> 1;
//获取中间下标元素
Comparable<? super T> midVal = list.get(mid);
// 调用compareTo方法, 中间元素和寻找元素做对比 大于返回大于0, 小于返回小于0, 相等返回0
int cmp = midVal.compareTo(key);
// 更新上界和下界，缩小查找范围
if (cmp < 0)
// 小于0说明中间元素小于寻找元素, 说明寻找元素在右边, 低位 = 中间下标 + 1
low = mid + 1;
else if (cmp > 0)
// 大于0说明中间元素大于寻找元素, 说明寻找元素在左边, 高位 = 中间下标 - 1
high = mid - 1;
else
// 相等 找到该值 返回中间下标
return mid; // key found
}
// 当为查找到时，返回负的最小下标+1
return -(low + 1); // key not found
}
//迭代器二叉搜索 迭代器二分查找通过迭代器获取元素, 时间复杂度为O(n)
private static <T>
int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
{
// 初始低位为0
int low = 0;
// 初始高位为集合大小 - 1
int high = list.size()-1;
// 与索引搜索不同，使用迭代器
ListIterator<? extends Comparable<? super T>> i = list.listIterator();
while (low <= high) {
// 通过位运算获取中间下标
int mid = (low + high) >>> 1;
// 通过迭代器获取中间下标元素值
Comparable<? super T> midVal = get(i, mid);
// 中间值和寻找值做对于 大于返回大于0, 小于返回小于0, 相等返回等于0
int cmp = midVal.compareTo(key);
if (cmp < 0)
// 中间值小于寻找值, 说明寻找值在右边, 低位 = 中间下标 + 1
low = mid + 1;
else if (cmp > 0)
// 中间值大于寻找值, 说明寻找值在左边, 高位 = 中间下标 - 1
high = mid - 1;
else
// 找到该值 返回下标
return mid; // key found
}
// 没有找到返回 -(low + 1)
return -(low + 1); // key not found
}
shuffle
洗牌算法就是将List集合中的元素打乱，一般可以用于抽奖、摇号、洗牌等场景。
/**
* Randomly permutes the specified list using a default source of
* randomness. All permutations occur with approximately equal
* likelihood.
*
* The hedge "approximately" is used in the foregoing description because
* default source of randomness is only approximately an unbiased source
* of independently chosen bits. If it were a perfect source of randomly
* chosen bits, then the algorithm would choose permutations with perfect
* uniformity.
*
*
This implementation traverses the list backwards, from the last
* element up to the second, repeatedly swapping a randomly selected element
* into the "current position". Elements are randomly selected from the
* portion of the list that runs from the first element to the current
* position, inclusive.
*
*
This method runs in linear time. If the specified list does not
* implement the {@link RandomAccess} interface and is large, this
* implementation dumps the specified list into an array before shuffling
* it, and dumps the shuffled array back into the list. This avoids the
* quadratic behavior that would result from shuffling a "sequential
* access" list in place.
*
* @param list the list to be shuffled.
* @throws UnsupportedOperationException if the specified list or
* its list-iterator does not support the set operation.
*/
public static void shuffle(List<?> list) {
Random rnd = r;
// 没有传入固定随机种子则初始化
if (rnd == null)
r = rnd = new Random(); // harmless race.
// 洗牌算法
shuffle(list, rnd);
}
//随机值
private static Random r;
/**
* Randomly permute the specified list using the specified source of
* randomness. All permutations occur with equal likelihood
* assuming that the source of randomness is fair.
*
* This implementation traverses the list backwards, from the last element
* up to the second, repeatedly swapping a randomly selected element into
* the "current position". Elements are randomly selected from the
* portion of the list that runs from the first element to the current
* position, inclusive.
*
* This method runs in linear time. If the specified list does not
* implement the {@link RandomAccess} interface and is large, this
* implementation dumps the specified list into an array before shuffling
* it, and dumps the shuffled array back into the list. This avoids the
* quadratic behavior that would result from shuffling a "sequential
* access" list in place.
*
* @param list the list to be shuffled.
* @param rnd the source of randomness to use to shuffle the list.
* @throws UnsupportedOperationException if the specified list or its
* list-iterator does not support the set operation.
*/
@SuppressWarnings({"rawtypes", "unchecked"})
public static void shuffle(List<?> list, Random rnd) {
//集合容量
int size = list.size();
// 集合容量小于洗牌阙值5 或者 是RandomAccess
if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
//调用交换算法
for (int i=size; i>1; i--)
swap(list, i-1, rnd.nextInt(i));
} else {
// 集合容量大于洗牌阙值5, 且不是RandomAccess, 比如LindedList
// 转成数组,对数组做调换操作, 不调用list的set方法主要是因为LindedList的set方法要获取传入下标的元素,获取元素
// 时间复杂度为O(n)性能低
Object[] arr = list.toArray();
// Shuffle array
//对数组做交换操作
for (int i=size; i>1; i--)
swap(arr, i-1, rnd.nextInt(i));
// Dump array back into list
// instead of using a raw type here, it's possible to capture
// the wildcard but it will require a call to a supplementary
// private method
// 通过迭代器进行元素的调换, 使用迭代器的set方法性能就是很高,因为可以通过迭代器获取元素
ListIterator it = list.listIterator();
for (int i=0; i<arr.length; i++) {
it.next();
it.set(arr[i]);
}
}
}
swap
/**
* Swaps the elements at the specified positions in the specified list.
* (If the specified positions are equal, invoking this method leaves
* the list unchanged.)
*
* @param list The list in which to swap elements.
* @param i the index of one element to be swapped.
* @param j the index of the other element to be swapped.
* @throws IndexOutOfBoundsException if either i or j
* is out of range (i < 0 || i >= list.size()
* || j < 0 || j >= list.size()).
* @since 1.4
*/
@SuppressWarnings({"rawtypes", "unchecked"})
public static void swap(List<?> list, int i, int j) {
// instead of using a raw type here, it's possible to capture
// the wildcard but it will require a call to a supplementary
// private method
final List l = list;
// 通过list.set(int index, E e)方法进行元素调换, 该方法返回原元素
// 比如list(1, 2, 3, 4, 5, 6, 7, 8) i = 7, j = 0
// 首先 l.set(j, l.get(i)) 将下标为7的元素调到下位为0的位置,并返回元素1, 此时list为(8,2,3,4,5,6,7,8)
// l.set(i, 1), 将元素1设置到下标7, 此时list为(8,2,3,4,5,6,7,1)
// 这样子就完成了元素调换
l.set(i, l.set(j, l.get(i)));
}
/**
* Swaps the two specified elements in the specified array.
*/
private static void swap(Object[] arr, int i, int j) {
//完成下标i,j元素的调换
Object tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
rotate
rotate旋转算法可以把ArrayList、LinkedList从指定的位置开始，进行顺时针或者逆时针旋转操作。
/**
* Rotates the elements in the specified list by the specified distance.
* After calling this method, the element at index i will be
* the element previously at index (i - distance) mod
* list.size(), for all values of i between 0
* and list.size()-1, inclusive. (This method has no effect on
* the size of the list.)
*
* For example, suppose list comprises [t, a, n, k, s].
* After invoking Collections.rotate(list, 1) (or
* Collections.rotate(list, -4)), list will comprise
* [s, t, a, n, k].
*
*
Note that this method can usefully be applied to sublists to
* move one or more elements within a list while preserving the
* order of the remaining elements. For example, the following idiom
* moves the element at index j forward to position
* k (which must be greater than or equal to j):
*
* Collections.rotate(list.subList(j, k+1), -1);
*
* To make this concrete, suppose list comprises
* [a, b, c, d, e]. To move the element at index 1
* (b) forward two positions, perform the following invocation:
*
* Collections.rotate(l.subList(1, 4), -1);
*
* The resulting list is [a, c, d, b, e].
*
* To move more than one element forward, increase the absolute value
* of the rotation distance. To move elements backward, use a positive
* shift distance.
*
*
If the specified list is small or implements the {@link
* RandomAccess} interface, this implementation exchanges the first
* element into the location it should go, and then repeatedly exchanges
* the displaced element into the location it should go until a displaced
* element is swapped into the first element. If necessary, the process
* is repeated on the second and successive elements, until the rotation
* is complete. If the specified list is large and doesn't implement the
* RandomAccess interface, this implementation breaks the
* list into two sublist views around index -distance mod size.
* Then the {@link #reverse(List)} method is invoked on each sublist view,
* and finally it is invoked on the entire list. For a more complete
* description of both algorithms, see Section 2.3 of Jon Bentley's
* Programming Pearls (Addison-Wesley, 1986).
*
* @param list the list to be rotated.
* @param distance the distance to rotate the list. There are no
* constraints on this value; it may be zero, negative, or
* greater than list.size().
* @throws UnsupportedOperationException if the specified list or
* its list-iterator does not support the set operation.
* @since 1.4
* 翻转一定距离
*/
public static void rotate(List<?> list, int distance) {
// 为随机访问列表或者数量小于100时
if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)
rotate1(list, distance);
else
rotate2(list, distance);
}
private static <T> void rotate1(List<T> list, int distance) {
//集合容量
int size = list.size();
if (size == 0)
return;
// 计算旋转的距离
distance = distance % size;
// 将过短的距离增加
if (distance < 0)
distance += size;
if (distance == 0)
return;
// 将元素进行移动distance距离
for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
//旋转的元素
T displaced = list.get(cycleStart);
// 旋转的元素下标
int i = cycleStart;
do {
i += distance;
// 将后半部分元素前移动
if (i >= size)
i -= size;
// 通过list.set方法旋转元素
displaced = list.set(i, displaced);
nMoved ++;
} while (i != cycleStart);
}
}
private static void rotate2(List<?> list, int distance) {
int size = list.size();
if (size == 0)
return;
int mid = -distance % size;
// 防止越界
if (mid < 0)
mid += size;
if (mid == 0)
return;
// 通过mid进行划分，分别逆置
reverse(list.subList(0, mid));
reverse(list.subList(mid, size));
// 逆置所有元素
reverse(list);
}
reverse
/**
* Reverses the order of the elements in the specified list.
*
* This method runs in linear time.
*
* @param list the list whose elements are to be reversed.
* @throws UnsupportedOperationException if the specified list or
* its list-iterator does not support the set operation.
*/
@SuppressWarnings({"rawtypes", "unchecked"})
public static void reverse(List<?> list) {
int size = list.size();
// 当数量小于18或者为随机访问列表时，直接对头尾依次交换
if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
swap(list, i, j);
} else {
// instead of using a raw type here, it's possible to capture
// the wildcard but it will require a call to a supplementary
// private method
// 使用迭代器进行交换
ListIterator fwd = list.listIterator();
ListIterator rev = list.listIterator(size);
for (int i=0, mid=list.size()>>1; i<mid; i++) {
Object tmp = fwd.next();
fwd.set(rev.previous());
rev.set(tmp);
}
}
}
fill
/**
* Replaces all of the elements of the specified list with the specified
* element.
*
* This method runs in linear time.
*
* @param the class of the objects in the list
* @param list the list to be filled with the specified element.
* @param obj The element with which to fill the specified list.
* @throws UnsupportedOperationException if the specified list or its
* list-iterator does not support the set operation.
* 将元素填入list每一个位置
*/
public static <T> void fill(List<? super T> list, T obj) {
int size = list.size();
// 当数量小于25并且时是随机访问列表时，直接使用set进行赋值
if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
for (int i=0; i<size; i++)
list.set(i, obj);
} else {
// 使用迭代器进行赋值
ListIterator<? super T> itr = list.listIterator();
for (int i=0; i<size; i++) {
itr.next();
itr.set(obj);
}
}
}
min
/**
* Returns the minimum element of the given collection, according to the
* natural ordering of its elements. All elements in the
* collection must implement the Comparable interface.
* Furthermore, all elements in the collection must be mutually
* comparable (that is, e1.compareTo(e2) must not throw a
* ClassCastException for any elements e1 and
* e2 in the collection).
*
* This method iterates over the entire collection, hence it requires
* time proportional to the size of the collection.
*
* @param the class of the objects in the collection
* @param coll the collection whose minimum element is to be determined.
* @return the minimum element of the given collection, according
* to the natural ordering of its elements.
* @throws ClassCastException if the collection contains elements that are
* not mutually comparable (for example, strings and
* integers).
* @throws NoSuchElementException if the collection is empty.
* @see Comparable
* 求最小值，最大值类似
*/
public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
Iterator<? extends T> i = coll.iterator();
T candidate = i.next();
// 依次进行比较，选出最小的元素
while (i.hasNext()) {
T next = i.next();
if (next.compareTo(candidate) < 0)
candidate = next;
}
return candidate;
}
replaceAll
/**
* Replaces all occurrences of one specified value in a list with another.
* More formally, replaces with newVal each element e
* in list such that
* (oldVal==null ? e==null : oldVal.equals(e)).
* (This method has no effect on the size of the list.)
*
* @param the class of the objects in the list
* @param list the list in which replacement is to occur.
* @param oldVal the old value to be replaced.
* @param newVal the new value with which oldVal is to be
* replaced.
* @return true if list contained one or more elements
* e such that
* (oldVal==null ? e==null : oldVal.equals(e)).
* @throws UnsupportedOperationException if the specified list or
* its list-iterator does not support the set operation.
* @since 1.4
*/
public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
boolean result = false;
int size = list.size();
// 当大小小于11或者为随机访问列表时
if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
// 老的值为空时
if (oldVal==null) {
// 遍历修改值
for (int i=0; i<size; i++) {
if (list.get(i)==null) {
list.set(i, newVal);
result = true;
}
}
} else {
// 当不为空时，通过equals判断是否相等，然后遍历赋值
for (int i=0; i<size; i++) {
if (oldVal.equals(list.get(i))) {
list.set(i, newVal);
result = true;
}
}
}
} else {
ListIterator<T> itr=list.listIterator();
if (oldVal==null) {
for (int i=0; i<size; i++) {
if (itr.next()==null) {
itr.set(newVal);
result = true;
}
}
} else {
for (int i=0; i<size; i++) {
if (oldVal.equals(itr.next())) {
itr.set(newVal);
result = true;
}
}
}
}
return result;
}
indexOfSubList
/**
* Returns the starting position of the first occurrence of the specified
* target list within the specified source list, or -1 if there is no
* such occurrence. More formally, returns the lowest index i
* such that {@code source.subList(i, i+target.size()).equals(target)},
* or -1 if there is no such index. (Returns -1 if
* {@code target.size() > source.size()})
*
* This implementation uses the "brute force" technique of scanning
* over the source list, looking for a match with the target at each
* location in turn.
*
* @param source the list in which to search for the first occurrence
* of target.
* @param target the list to search for as a subList of source.
* @return the starting position of the first occurrence of the specified
* target list within the specified source list, or -1 if there
* is no such occurrence.
* @since 1.4
* 查找子字符串的下标
*/
public static int indexOfSubList(List<?> source, List<?> target) {
int sourceSize = source.size();
int targetSize = target.size();
// 最大下标
int maxCandidate = sourceSize - targetSize;
// 当源list长度小于35或源list和目标list都是随机访问列表时
if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
(source instanceof RandomAccess&&target instanceof RandomAccess)) {
// 当元素不相同时直接跳出，继续判断下一个元素，当满足条件时，则返回maxCandidate
nextCand:
for (int candidate = 0; candidate <= maxCandidate; candidate++) {
for (int i=0, j=candidate; i<targetSize; i++, j++)
if (!eq(target.get(i), source.get(j)))
continue nextCand; // Element mismatch, try next cand
return candidate; // All elements of candidate matched target
}
} else { // Iterator version of above algorithm
ListIterator<?> si = source.listIterator();
nextCand:
for (int candidate = 0; candidate <= maxCandidate; candidate++) {
ListIterator<?> ti = target.listIterator();
for (int i=0; i<targetSize; i++) {
if (!eq(ti.next(), si.next())) {
// Back up source iterator to next candidate
for (int j=0; j<i; j++)
si.previous();
continue nextCand;
}
}
return candidate;
}
}
// 未匹配时返回-1
return -1; // No candidate matched the target
}
copy
/**
* Copies all of the elements from one list into another. After the
* operation, the index of each copied element in the destination list
* will be identical to its index in the source list. The destination
* list must be at least as long as the source list. If it is longer, the
* remaining elements in the destination list are unaffected.
*
* This method runs in linear time.
*
* @param the class of the objects in the lists
* @param dest The destination list.
* @param src The source list.
* @throws IndexOutOfBoundsException if the destination list is too small
* to contain the entire source List.
* @throws UnsupportedOperationException if the destination list's
* list-iterator does not support the set operation.
*/
public static <T> void copy(List<? super T> dest, List<? extends T> src) {
// 源list长度
int srcSize = src.size();
// 当源list长度大于目标list长度时，直接抛出
if (srcSize > dest.size())
throw new IndexOutOfBoundsException("Source does not fit in dest");
// 当源list长度小于10或者两个list都是随机访问列表时
if (srcSize < COPY_THRESHOLD ||
(src instanceof RandomAccess && dest instanceof RandomAccess)) {
// 遍历赋值
for (int i=0; i<srcSize; i++)
dest.set(i, src.get(i));
} else {
// 使用迭代器进行赋值
ListIterator<? super T> di=dest.listIterator();
ListIterator<? extends T> si=src.listIterator();
for (int i=0; i<srcSize; i++) {
di.next();
di.set(si.next());
}
}
}
nCopies
/**
* Returns an immutable list consisting of n copies of the
* specified object. The newly allocated data object is tiny (it contains
* a single reference to the data object). This method is useful in
* combination with the List.addAll method to grow lists.
* The returned list is serializable.
*
* @param the class of the object to copy and of the objects
* in the returned list.
* @param n the number of elements in the returned list.
* @param o the element to appear repeatedly in the returned list.
* @return an immutable list consisting of n copies of the
* specified object.
* @throws IllegalArgumentException if {@code n < 0}
* @see List#addAll(Collection)
* @see List#addAll(int, Collection)
* 生成一个含有n个o的不可变list
*/
public static <T> List<T> nCopies(int n, T o) {
// n小于0时直接抛出
if (n < 0)
throw new IllegalArgumentException("List length = " + n);
// 生成一个复制list(无set方法)
return new CopiesList<>(n, o);
}
frequency
/**
* Returns the number of elements in the specified collection equal to the
* specified object. More formally, returns the number of elements
* e in the collection such that
* (o == null ? e == null : o.equals(e)).
*
* @param c the collection in which to determine the frequency
* of o
* @param o the object whose frequency is to be determined
* @return the number of elements in {@code c} equal to {@code o}
* @throws NullPointerException if c is null
* @since 1.5
* 求list中某一元素的出现频率
*/
public static int frequency(Collection<?> c, Object o) {
int result = 0;
// 当对象为null时直接判断null的数量，否则使用equals进行判断
if (o == null) {
for (Object e : c)
if (e == null)
result++;
} else {
for (Object e : c)
if (o.equals(e))
result++;
}
return result;
}