数据结构复习之Vector
/** * The number of times this list has been <i>structurally modified</i>. * Structural modifications are those that change the size of the * list, or otherwise perturb it in such a fashion that iterations in * progress may yield incorrect results. * * <p>This field is used by the iterator and list iterator implementation * returned by the {@code iterator} and {@code listIterator} methods. * If the value of this field changes unexpectedly, the iterator (or list * iterator) will throw a {@code ConcurrentModificationException} in * response to the {@code next}, {@code remove}, {@code previous}, * {@code set} or {@code add} operations. This provides * <i>fail-fast</i> behavior, rather than non-deterministic behavior in * the face of concurrent modification during iteration. * * <p><b>Use of this field by subclasses is optional.</b> If a subclass * wishes to provide fail-fast iterators (and list iterators), then it * merely has to increment this field in its {@code add(int, E)} and * {@code remove(int)} methods (and any other methods that it overrides * that result in structural modifications to the list). A single call to * {@code add(int, E)} or {@code remove(int)} must add no more than * one to this field, or the iterators (and list iterators) will throw * bogus {@code ConcurrentModificationExceptions}. If an implementation * does not wish to provide fail-fast iterators, this field may be * ignored. */ protected transient int modCount = 0;
迭代器要用到modCount属性
/** * The array buffer into which the components of the vector are * stored. The capacity of the vector is the length of this array buffer, * and is at least large enough to contain all the vector's elements. * * <p>Any array elements following the last element in the Vector are null. * * @serial */ protected Object[] elementData;
Vector的元素就存储在这个Object数组里,因而Vector的容量就是这个数组的长度,其值至少要大于Vector的要存储的元素的数量,该数组最后一个元素之后的元素都是null。
/** * The number of valid components in this {@code Vector} object. * Components {@code elementData[0]} through * {@code elementData[elementCount-1]} are the actual items. * * @serial */ protected int elementCount;
这个值代表Vector内有效数据的数量,而且起始元素的下标为0。
/** * The amount by which the capacity of the vector is automatically * incremented when its size becomes greater than its capacity. If * the capacity increment is less than or equal to zero, the capacity * of the vector is doubled each time it needs to grow. * * @serial */ protected int capacityIncrement;
有了这个值后,当Vector的大小超过了它的容量的时,Vector的容量才可以实现自动增长。当capacityIncrement小于等于0时,vector的容量就会在需要增长时直接翻倍。(该值可以在初始化时和初始容量一块指定,不指定时默认为0,初始容量默认为10)
/** * Constructs a vector containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. * * @param c the collection whose elements are to be placed into this * vector * @throws NullPointerException if the specified collection is null * @since 1.2 */ public Vector(Collection<? extends E> c) { elementData = c.toArray(); elementCount = elementData.length; // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, elementCount, Object[].class); }
也可以通过给定的集合来初始化一个Vector,通过该集合的迭代器的顺序来赋值。(c.toArray might (incorrectly) not return Object[] (see 6260652)不理解,待查资料)
/** * Copies the components of this vector into the specified array. * The item at index {@code k} in this vector is copied into * component {@code k} of {@code anArray}. * * @param anArray the array into which the components get copied * @throws NullPointerException if the given array is null * @throws IndexOutOfBoundsException if the specified array is not * large enough to hold all the components of this vector * @throws ArrayStoreException if a component of this vector is not of * a runtime type that can be stored in the specified array * @see #toArray(Object[]) */ public synchronized void copyInto(Object[] anArray) { System.arraycopy(elementData, 0, anArray, 0, elementCount); }
将vector的内容复制给另一指定数组。
/** * Trims the capacity of this vector to be the vector's current * size. If the capacity of this vector is larger than its current * size, then the capacity is changed to equal the size by replacing * its internal data array, kept in the field {@code elementData}, * with a smaller one. An application can use this operation to * minimize the storage of a vector. */ public synchronized void trimToSize() { modCount++; int oldCapacity = elementData.length; if (elementCount < oldCapacity) { elementData = Arrays.copyOf(elementData, elementCount); } }
如果vector的容量大于实际存储的元素数量,那么就将vector的容量调整为实际存储的元素数量,通过对内部数组重新赋值来实现,可以通过该操作最小化vector所占的内存。
/** * Increases the capacity of this vector, if necessary, to ensure * that it can hold at least the number of components specified by * the minimum capacity argument. * * <p>If the current capacity of this vector is less than * {@code minCapacity}, then its capacity is increased by replacing its * internal data array, kept in the field {@code elementData}, with a * larger one. The size of the new data array will be the old size plus * {@code capacityIncrement}, unless the value of * {@code capacityIncrement} is less than or equal to zero, in which case * the new capacity will be twice the old capacity; but if this new size * is still smaller than {@code minCapacity}, then the new capacity will * be {@code minCapacity}. * * @param minCapacity the desired minimum capacity */ public synchronized void ensureCapacity(int minCapacity) { if (minCapacity > 0) { modCount++; ensureCapacityHelper(minCapacity); } } /** * This implements the unsynchronized semantics of ensureCapacity. * Synchronized methods in this class can internally call this * method for ensuring capacity without incurring the cost of an * extra synchronization. * * @see #ensureCapacity(int) */ private void ensureCapacityHelper(int minCapacity) { // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * The maximum size of array to allocate. * 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; private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity); if (newCapacity - minCapacity < 0) newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); elementData = Arrays.copyOf(elementData, newCapacity); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; }
增加vector的容量,如果有必要,要保证增加后的容量由minCapacity给出的最小容量。如果vector当前的容量小于minCapacity,就通过对内部数组重新赋值一个更大的数组的方式来扩容。当capacityIncrement小于等于0时,新数组的大小是之前的2倍,大于0时新数组的大小等于旧数组的大小与capacityIncrement只和。如果增加后的数组容量仍然小于minCapacity,那么新数组的大小为minCapacity。(ensureCapacityHelper方法不加synchronized是因为ensureCapacity方法加了synchronized,而且ensureCapacityHelper方法是在ensureCapacity内部调用的,因而没必要增加额外的synchronized)
而设置MAX_ARRAY_SIZE是因为,vector内部元素的数量elementCount和容量都是int型因而不能超出int的范围,而减8是因为某些VM里设有头结点,所以要留出这部分的空间。
hugeCapacity方法中判断minCapacity是否小于零则是因为负数减MAX_ARRAY_SIZE也可能大于0,例如
但是这里又为什么返回了Integer.MAX_VALUE呢(待解决)
/** * Sets the size of this vector. If the new size is greater than the * current size, new {@code null} items are added to the end of * the vector. If the new size is less than the current size, all * components at index {@code newSize} and greater are discarded. * * @param newSize the new size of this vector * @throws ArrayIndexOutOfBoundsException if the new size is negative */ public synchronized void setSize(int newSize) { modCount++; if (newSize > elementCount) { ensureCapacityHelper(newSize); } else { for (int i = newSize ; i < elementCount ; i++) { elementData[i] = null; } } elementCount = newSize; }
setSize方法用来修改当前vector的大小,如果newSize大于当前元素数量elementCount那么调用ensureCapacityHelper方法进行扩容(此时没有经过ensureCapacity方法),当newSize小于elementCount时,将数组下标大于等于newSize的元素的值设为null,两种情况最后都要将elementCount大小改为newSize。
/** * Returns the current capacity of this vector. * * @return the current capacity (the length of its internal * data array, kept in the field {@code elementData} * of this vector) */ public synchronized int capacity() { return elementData.length; } /** * Returns the number of components in this vector. * * @return the number of components in this vector */ public synchronized int size() { return elementCount; } /** * Tests if this vector has no components. * * @return {@code true} if and only if this vector has * no components, that is, its size is zero; * {@code false} otherwise. */ public synchronized boolean isEmpty() { return elementCount == 0; }
以上三个方法分别返回vector容量、元素数量elementCount以及判断vector是否为空。
/** * Returns an enumeration of the components of this vector. The * returned {@code Enumeration} object will generate all items in * this vector. The first item generated is the item at index {@code 0}, * then the item at index {@code 1}, and so on. * * @return an enumeration of the components of this vector * @see Iterator */ public Enumeration<E> elements() { return new Enumeration<E>() { int count = 0; public boolean hasMoreElements() { return count < elementCount; } public E nextElement() { synchronized (Vector.this) { if (count < elementCount) { return elementData(count++); } } throw new NoSuchElementException("Vector Enumeration"); } }; }
该方法返回vector内部元素按下标顺序的Enumeration(类似枚举类型)
/** * Returns the index of the first occurrence of the specified element in * this vector, searching forwards from {@code index}, or returns -1 if * the element is not found. * More formally, returns the lowest index {@code i} such that * <tt>(i >= index && (o==null ? get(i)==null : o.equals(get(i))))</tt>, * or -1 if there is no such index. * * @param o element to search for * @param index index to start searching from * @return the index of the first occurrence of the element in * this vector at position {@code index} or later in the vector; * {@code -1} if the element is not found. * @throws IndexOutOfBoundsException if the specified index is negative * @see Object#equals(Object) */ public synchronized int indexOf(Object o, int index) { if (o == null) { for (int i = index ; i < elementCount ; i++) if (elementData[i]==null) return i; } else { for (int i = index ; i < elementCount ; i++) if (o.equals(elementData[i])) return i; } return -1; }
indexOf方法从给定index开始查找vector内是否存有Object o(可以为空),如果有则返回第一次出现的下标,如果不存在返回-1。
/** * Returns {@code true} if this vector contains the specified element. * More formally, returns {@code true} if and only if this vector * contains at least one element {@code e} such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this vector is to be tested * @return {@code true} if this vector contains the specified element */ public boolean contains(Object o) { return indexOf(o, 0) >= 0; } /** * Returns the index of the first occurrence of the specified element * in this vector, or -1 if this vector does not contain the element. * More formally, returns the lowest index {@code i} such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. * * @param o element to search for * @return the index of the first occurrence of the specified element in * this vector, or -1 if this vector does not contain the element */ public int indexOf(Object o) { return indexOf(o, 0); }
这两个方法都调用上边的indexOf方法,contains判断是否包含Object o,如果存在,返回true,否则返回false,indexOf也判断是否包含Object o,如果包含则返回第一次出现的下标,不存在返回-1。
/** * Returns the index of the last occurrence of the specified element in * this vector, searching backwards from {@code index}, or returns -1 if * the element is not found. * More formally, returns the highest index {@code i} such that * <tt>(i <= index && (o==null ? get(i)==null : o.equals(get(i))))</tt>, * or -1 if there is no such index. * * @param o element to search for * @param index index to start searching backwards from * @return the index of the last occurrence of the element at position * less than or equal to {@code index} in this vector; * -1 if the element is not found. * @throws IndexOutOfBoundsException if the specified index is greater * than or equal to the current size of this vector */ public synchronized int lastIndexOf(Object o, int index) { if (index >= elementCount) throw new IndexOutOfBoundsException(index + " >= "+ elementCount); if (o == null) { for (int i = index; i >= 0; i--) if (elementData[i]==null) return i; } else { for (int i = index; i >= 0; i--) if (o.equals(elementData[i])) return i; } return -1; }
lastIndexOf方法类似indexOf方法,区别在于lastIndexOf方法从后向前查找,因而返回的是最后一次出现的下标,不存在仍然返回-1。
/** * Returns the index of the last occurrence of the specified element * in this vector, or -1 if this vector does not contain the element. * More formally, returns the highest index {@code i} such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. * * @param o element to search for * @return the index of the last occurrence of the specified element in * this vector, or -1 if this vector does not contain the element */ public synchronized int lastIndexOf(Object o) { return lastIndexOf(o, elementCount-1); }
该方法调用lastIndexOf发法,返回Object o最后一次出现的下标,不存在仍然返回-1。
/** * Returns the component at the specified index. * * <p>This method is identical in functionality to the {@link #get(int)} * method (which is part of the {@link List} interface). * * @param index an index into this vector * @return the component at the specified index * @throws ArrayIndexOutOfBoundsException if the index is out of range * ({@code index < 0 || index >= size()}) */ public synchronized E elementAt(int index) { if (index >= elementCount) { throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount); } return elementData(index); } /** * Returns the first component (the item at index {@code 0}) of * this vector. * * @return the first component of this vector * @throws NoSuchElementException if this vector has no components */ public synchronized E firstElement() { if (elementCount == 0) { throw new NoSuchElementException(); } return elementData(0); } /** * Returns the last component of the vector. * * @return the last component of the vector, i.e., the component at index * <code>size() - 1</code>. * @throws NoSuchElementException if this vector is empty */ public synchronized E lastElement() { if (elementCount == 0) { throw new NoSuchElementException(); } return elementData(elementCount - 1); }
elementAt返回指定下标的元素,firstElement返回第一个元素,lastElement返回最后一个元素。
/** * Sets the component at the specified {@code index} of this * vector to be the specified object. The previous component at that * position is discarded. * * <p>The index must be a value greater than or equal to {@code 0} * and less than the current size of the vector. * * <p>This method is identical in functionality to the * {@link #set(int, Object) set(int, E)} * method (which is part of the {@link List} interface). Note that the * {@code set} method reverses the order of the parameters, to more closely * match array usage. Note also that the {@code set} method returns the * old value that was stored at the specified position. * * @param obj what the component is to be set to * @param index the specified index * @throws ArrayIndexOutOfBoundsException if the index is out of range * ({@code index < 0 || index >= size()}) */ public synchronized void setElementAt(E obj, int index) { if (index >= elementCount) { throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount); } elementData[index] = obj; }
该方法用来修改指定index的值(注意,不能添加index >= elementCount时会抛出异常),该方法与List接口中的set方法功能一样,区别在于set方法参数顺序更接近数组的用法,此外set还会返回旧元素的值。
/** * Deletes the component at the specified index. Each component in * this vector with an index greater or equal to the specified * {@code index} is shifted downward to have an index one * smaller than the value it had previously. The size of this vector * is decreased by {@code 1}. * * <p>The index must be a value greater than or equal to {@code 0} * and less than the current size of the vector. * * <p>This method is identical in functionality to the {@link #remove(int)} * method (which is part of the {@link List} interface). Note that the * {@code remove} method returns the old value that was stored at the * specified position. * * @param index the index of the object to remove * @throws ArrayIndexOutOfBoundsException if the index is out of range * ({@code index < 0 || index >= size()}) */ public synchronized void removeElementAt(int index) { modCount++; if (index >= elementCount) { throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount); } else if (index < 0) { throw new ArrayIndexOutOfBoundsException(index); } int j = elementCount - index - 1; if (j > 0) { System.arraycopy(elementData, index + 1, elementData, index, j); } elementCount--; elementData[elementCount] = null; /* to let gc do its work */ }
该方法通过将给定index(不包括index)后的所有有效元素前移的方式达到删除下标为index的元素的目的。
/** * Inserts the specified object as a component in this vector at the * specified {@code index}. Each component in this vector with * an index greater or equal to the specified {@code index} is * shifted upward to have an index one greater than the value it had * previously. * * <p>The index must be a value greater than or equal to {@code 0} * and less than or equal to the current size of the vector. (If the * index is equal to the current size of the vector, the new element * is appended to the Vector.) * * <p>This method is identical in functionality to the * {@link #add(int, Object) add(int, E)} * method (which is part of the {@link List} interface). Note that the * {@code add} method reverses the order of the parameters, to more closely * match array usage. * * @param obj the component to insert * @param index where to insert the new component * @throws ArrayIndexOutOfBoundsException if the index is out of range * ({@code index < 0 || index > size()}) */ public synchronized void insertElementAt(E obj, int index) { modCount++; if (index > elementCount) { throw new ArrayIndexOutOfBoundsException(index + " > " + elementCount); } ensureCapacityHelper(elementCount + 1); System.arraycopy(elementData, index, elementData, index + 1, elementCount - index); elementData[index] = obj; elementCount++; }
该方法将index及其后的所有有效元素后移一位,然后将obj对象赋值给下标为index元素,从而实现插入元素(当index=elementCount效果等同于在最后添加一个元素)
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