读Collections源码

//一个操作集合的工具类,里面全部都是静态方法。

//根据元素的自然顺序进行排序
 public static <T extends Comparable<? super T>> void sort(List<T> list) {
        //转为array
        Object[] a = list.toArray();
	//排序
        Arrays.sort(a);
        ListIterator<T> i = list.listIterator();
	//设置值
        for (int j=0; j<a.length; j++) {
            i.next();
            i.set((T)a[j]);
        }
    }

//根据传入的比较器对元素进行排序
public static <T> void sort(List<T> list, Comparator<? super T> c) {
        Object[] a = list.toArray();
        Arrays.sort(a, (Comparator)c);
        ListIterator i = list.listIterator();
        for (int j=0; j<a.length; j++) {
            i.next();
            i.set(a[j]);
        }
    }


//使用二分查找法根据指定key查询对象
public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
        //没有传入比较器
        if (c==null)
            return binarySearch((List) list, key);

        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
            return Collections.indexedBinarySearch(list, key, c);
        else
            return Collections.iteratorBinarySearch(list, key, c);
    }

public static <T>
    int binarySearch(List<? extends Comparable<? super T>> list, T key) {
        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
            return Collections.indexedBinarySearch(list, key);
        else
            return Collections.iteratorBinarySearch(list, key);
    }

private static <T>
    int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key)
    {
        int low = 0;
        int high = list.size()-1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            Comparable<? super T> midVal = list.get(mid);
            int cmp = midVal.compareTo(key);
            //比他小往右边找
            if (cmp < 0)
                low = mid + 1;
            //比他大往左边找
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

//将指定元素添加到collection中
 public static <T> boolean addAll(Collection<? super T> c, T... elements) {
        boolean result = false;
        for (T element : elements)
            result |= c.add(element);
        return result;
    }

//以后进先出的形式返回某个deque视图
public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
        return new AsLIFOQueue<>(deque);
    }

//返回一个动态类型安全的collection插入错误的类型会直接抛出ClassCastException异常
//其实现就是在添加元素的时候加入类型检查
public static <E> Collection<E> checkedCollection(Collection<E> c,
                                                      Class<E> type) {
        return new CheckedCollection<>(c, type);
    }

//返回一个动态类型安全的list插入错误的类型会直接抛出ClassCastException异常
//其实现就是在添加元素的时候加入类型检查
public static <E> List<E> checkedList(List<E> list, Class<E> type) {
        return (list instanceof RandomAccess ?
                new CheckedRandomAccessList<>(list, type) :
                new CheckedList<>(list, type));
    }

//返回一个动态类型安全的map插入错误的类型会直接抛出ClassCastException异常
//其实现就是在添加元素的时候加入类型检查
public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
                                              Class<K> keyType,
                                              Class<V> valueType) {
        return new CheckedMap<>(m, keyType, valueType);
    }

//将元素从一个列表复制到另一个列表
public static <T> void copy(List<? super T> dest, List<? extends T> src) {
        int srcSize = src.size();
        if (srcSize > dest.size())
            throw new IndexOutOfBoundsException("Source does not fit in dest");

        if (srcSize < COPY_THRESHOLD ||
            (src instanceof RandomAccess && dest instanceof RandomAccess)) {
            for (int i=0; i<srcSize; i++)
                dest.set(i, src.get(i));
        } else {
            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());
            }
        }
    }

//如果两个集合中没有相同的元素则返回true
 public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
       
        Collection<?> contains = c2;
        
        Collection<?> iterate = c1;

        //注释上写的这样效率更高
        if (c1 instanceof Set) {
            
            iterate = c2;
            contains = c1;
        } else if (!(c2 instanceof Set)) {
           
            int c1size = c1.size();
            int c2size = c2.size();
            if (c1size == 0 || c2size == 0) {
                
                return true;
            }
            //大的包含小的效率更高
            if (c1size > c2size) {
                iterate = c2;
                contains = c1;
            }
        }

        for (Object e : iterate) {
            if (contains.contains(e)) {
               // Found a common element. Collections are not disjoint.
                return false;
            }
        }

        return true;
    }


//用指定的元素替换列表中的所有元素
public static <T> void fill(List<? super T> list, T obj) {
        int size = list.size();
        //小于25或者支持随机访问
        if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
            for (int i=0; i<size; i++)
                list.set(i, obj);
        } else {
            ListIterator<? super T> itr = list.listIterator();
            for (int i=0; i<size; i++) {
                itr.next();
                itr.set(obj);
            }
        }
    }


//返回集合中等于元素的个数
public static int frequency(Collection<?> c, Object o) {
        int result = 0;
        if (o == null) {
            for (Object e : c)
                if (e == null)
                    result++;
        } else {
            for (Object e : c)
                if (o.equals(e))
                    result++;
        }
        return result;
    }


//返回source中第一次出现在target的位置
public static int indexOfSubList(List<?> source, List<?> target) {
        int sourceSize = source.size();
        int targetSize = target.size();
	//如果源列表比目标列表短则直接返回-1
        int maxCandidate = sourceSize - targetSize;

        if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
            (source instanceof RandomAccess&&target instanceof RandomAccess)) {
        nextCand:
            for (int candidate = 0; candidate <= maxCandidate; candidate++) {
                for (int i=0, j=candidate; i<targetSize; i++, j++)
		    //如果不相等则重新循环
                    if (!eq(target.get(i), source.get(j)))
                        continue nextCand;  
                return candidate;  
            }
        } 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())) {
                        //回溯si的指针
                        for (int j=0; j<i; j++)
                            si.previous();
                        continue nextCand;
                    }
                }
                return candidate;
            }
        }
        return -1;  // No candidate matched the target
    }

//返回source中最后一次出现在target的位置
public static int lastIndexOfSubList(List<?> source, List<?> target) {
        int sourceSize = source.size();
        int targetSize = target.size();
        int maxCandidate = sourceSize - targetSize;

        if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
            source instanceof RandomAccess) {   // Index access version
        nextCand:
            for (int candidate = maxCandidate; candidate >= 0; 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
            if (maxCandidate < 0)
                return -1;
            ListIterator<?> si = source.listIterator(maxCandidate);
        nextCand:
            for (int candidate = maxCandidate; candidate >= 0; candidate--) {
                ListIterator<?> ti = target.listIterator();
                for (int i=0; i<targetSize; i++) {
                    if (!eq(ti.next(), si.next())) {
                        if (candidate != 0) {
                            // Back up source iterator to next candidate
                            for (int j=0; j<=i+1; j++)
                                si.previous();
                        }
                        continue nextCand;
                    }
                }
                return candidate;
            }
        }
        return -1;  // No candidate matched the target
    }


//根据元素的自然顺序返回最大值
public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (next.compareTo(candidate) > 0)
                candidate = next;
        }
        return candidate;
    }

//根据比较器比较返回Collection中的最大值
 public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
        if (comp==null)
            return (T)max((Collection<SelfComparable>) (Collection) coll);

        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (comp.compare(next, candidate) > 0)
                candidate = next;
        }
        return candidate;
    }

//根据自然顺序返回集合中的最小值
public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (next.compareTo(candidate) < 0)
                candidate = next;
        }
        return candidate;
    }

//根据比较器比较返回Collection中的最小值
public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
        if (comp==null)
            return (T)min((Collection<SelfComparable>) (Collection) coll);

        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (comp.compare(next, candidate) < 0)
                candidate = next;
        }
        return candidate;
    }

//返回由n个o组成的不可变集合
 public static <T> List<T> nCopies(int n, T o) {
        if (n < 0)
            throw new IllegalArgumentException("List length = " + n);
        return new CopiesList<>(n, o);
    }

//替换旧值
 public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
        boolean result = false;
        int size = list.size();
        if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
            if (oldVal==null) {
                for (int i=0; i<size; i++) {
                    if (list.get(i)==null) {
                        list.set(i, newVal);
                        result = true;
                    }
                }
            } else {
                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;
    }

//反转整个list
 public static void reverse(List<?> list) {
        int size = list.size();
        if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
            for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
	        //交换i和j位置的值
                swap(list, i, j);
        } else {
            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);
            }
        }
    }

public static void swap(List<?> list, int i, int j) {
        final List l = list;
        l.set(i, l.set(j, l.get(i)));
    }

//随机交换list元素的位置
 public static void shuffle(List<?> list) {
        Random rnd = r;
        if (rnd == null)
            r = rnd = new Random();
        shuffle(list, rnd);
    }

 public static void shuffle(List<?> list, Random rnd) {
        int size = list.size();
        if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
            for (int i=size; i>1; i--)
                swap(list, i-1, rnd.nextInt(i));
        } else {
            Object arr[] = list.toArray();

            // Shuffle array
            for (int i=size; i>1; i--)
                swap(arr, i-1, rnd.nextInt(i));

            // Dump array back into list
            ListIterator it = list.listIterator();
            for (int i=0; i<arr.length; i++) {
                it.next();
                it.set(arr[i]);
            }
        }
    }


//返回一个逆转顺序的比较器
 public static <T> Comparator<T> reverseOrder() {
        return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
    }

//逆转比较器顺序
 public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
        if (cmp == null)
            return reverseOrder();

        if (cmp instanceof ReverseComparator2)
            return ((ReverseComparator2<T>)cmp).cmp;

        return new ReverseComparator2<>(cmp);
    }

//将list中的元素移动distance距离
public static void rotate(List<?> list, int distance) {
        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) {
        int size = list.size();
        if (size == 0)
            return;
        distance = distance % size;
        if (distance < 0)
            distance += size;
        if (distance == 0)
            return;

        for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
            T displaced = list.get(cycleStart);
            int i = cycleStart;
            do {
                i += distance;
                if (i >= size)
                    i -= size;
                displaced = list.set(i, displaced);
                nMoved ++;
            } while (i != cycleStart);
        

//返回只包含一个元素的不可变set
public static <T> Set<T> singleton(T o) {
        return new SingletonSet<>(o);
    }


//将此collection转为同步的
public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
        return new SynchronizedCollection<>(c);
    }

//将集合转为不可变集合
public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
        return new UnmodifiableCollection<>(c);
    }