ArrayList方法初探
程序员文章站
2022-07-13 16:02:31
...
1、继承的类以及实现的接口：
继承自：AbstractList
实现了：List，RandomAccess，Cloneable，java.io.Serializable；
定义的成员变量：
transient Object[] elementData;
是一个数组缓冲区，从下面可以看到所有的操作似乎都与之有关。（transient 表示其不可序列化）
size: The size of the ArrayList (the number of elements it contains).
我们可以在创建ArrayList的时候指定其大小，相应的其实是实例化了前面提到的对象Object类型的数组。
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
this.elementData = new Object[initialCapacity];
}
但如果我们没有指定其大小的话，他就会默认创建一个容量为10的数组。
public ArrayList() {
this(10);
}
/**
* Constructs a list 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 list
* @throws NullPointerException if the specified collection is null
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}
上面的这个构造方法允许你传入一个Collection或者其子类。
仔细看他的处理，首先将这个传进来的Collection转换成数组，并指向ArrayList中定义的Object类型的数组。然后获取其长度;
注：6260652
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength] : (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength));
return copy;
}
/**
* Trims the capacity of this <tt>ArrayList</tt> instance to be the
* list's current size. An application can use this operation to minimize
* the storage of an <tt>ArrayList</tt> instance.
*/
public void trimToSize() {
modCount++;
int oldCapacity = elementData.length;
if (size < oldCapacity) {
elementData = Arrays.copyOf(elementData, size);
}
}
注：modCount是其父类AbstractList中的一个成员变量, 其代表的含义是:
The number of times this list has been structurally modified, 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.
该方法的作用是修整此ArrayList实例的是列表的当前大小的容量。应用程序可以使用此操作，以尽量减少一个ArrayList实例的存储。
/**
* Increases the capacity of this <tt>ArrayList</tt> instance, if necessary, to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
* @param minCapacity the desired minimum capacity
*/
public void ensureCapacity(int minCapacity) {
modCount++;
int oldCapacity = elementData.length;
if (minCapacity > oldCapacity) {
Object oldData[] = elementData;
int newCapacity = (oldCapacity * 3) / 2 + 1;
if (newCapacity < minCapacity)
newCapacity = minCapacity;
// minCapacity is usually close to size, so this is a win:
elementData = Arrays.copyOf(elementData, newCapacity);
}
}
这个方法可以实现手动扩大容量，如果你申请的容量大于原来容量的1.5倍+1，那么就使用你申请的容量大小作为之后的List的容量。
return true if this list contains no elements.
public boolean isEmpty() {
return size == 0;
}
/**
* Returns <tt>true</tt> if this list contains the specified element. More formally, returns <tt>true</tt> if and
* only if this list contains at least one element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
/**
* Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not
* contain the element. More formally, returns the lowest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, or -1 if there is no such index.
*/
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i] == null)
return i;
}
else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
他会遍历List中的元素，然后将每个元素都与源元素比较，如果相同，就返回这个元素的下标。否则返回-1;
/**
* Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not
* contain the element. More formally, returns the highest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, or -1 if there is no such index.
*/
public int lastIndexOf(Object o) {
if (o == null) {
for (int i = size - 1; i >= 0; i--)
if (elementData[i] == null)
return i;
}
else {
for (int i = size - 1; i >= 0; i--)
if (o.equals(elementData[i]))
return i;
}
return -1;
}
从最后一个元素遍历，逐个比较，如果相等，则返回元素的下标，否则返回-1;
/**
* Returns a shallow copy of this <tt>ArrayList</tt> instance. (The elements themselves are not copied.)
*
* @return a clone of this <tt>ArrayList</tt> instance
*/
public Object clone() {
try {
ArrayList<E> v = (ArrayList<E>) super.clone();
v.elementData = Arrays.copyOf(elementData, size);
v.modCount = 0;
return v;
}
catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
加粗标红的那个语句表名，在进行克隆的时候会将所有的元素都拷贝过去，两个对象各自独立，不会相互影响。注意和使用"="进行传值的方式的区别，使用"="传递的是对象的引用，他们指向同一块内存地址，即指向同一个对象，因此他们修改的也是同一个对象。
ArrayList<String> list = new ArrayList<String>(5);
list.add("welcome");
list.add("to");
list.add("Java");
@SuppressWarnings("unchecked")
ArrayList<String> listCopy = (ArrayList<String>) list.clone();
listCopy.add("world");
System.out.println(list.toString());
System.out.println(listCopy.toString());
ArrayList<String> listCopy2 = list;
listCopy2.add("Hello");
System.out.println(list.toString());
System.out.println(listCopy2.toString());
输出结果：
[welcome, to, Java]
[welcome, to, Java, world]
[welcome, to, Java, Hello]
[welcome, to, Java, Hello]
/**
* Returns an array containing all of the elements in this list in proper sequence (from first to last element).
* <p>
* The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this
* method must allocate a new array). The caller is thus free to modify the returned array.
* <p>
* This method acts as bridge between array-based and collection-based APIs.
*
* @return an array containing all of the elements in this list in proper sequence
*/
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}
注：请看下面一段代码：
List<String> list = new ArrayList<String>();
// ...省略添加元素的细节
// 意将list转换为数组
String[] str = list.toArray();// 会报类型不匹配的错误
String[] str = (String[]) list.toArray();// 会报ClassCastException异常
主要是因为上面这个方法返回的是一个Object类型的一个数组，就像数组的初始化一样，必须一个一个的赋值，而不能将整个数组转换为另外一种类型。
下面这个方法为上面的问题提供了解决方案：
可以改为：
String[] str = (String[]) list.toArray(new String[list.size()]);
/**
* Returns an array containing all of the elements in this list in proper sequence (from first to last element); the
* runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is
* returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size
* of this list.
* <p>
* If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the
* element in the array immediately following the end of the collection is set to <tt>null</tt>. (This is useful in
* determining the length of the list <i>only</i> if the caller knows that the list does not contain any null
* elements.)
*
* @param a the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new
* array of the same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @throws ArrayStoreException if the runtime type of the specified array is not a supertype of the runtime type of
* every element in this list
* @throws NullPointerException if the specified array is null
*/
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;【疑问：为何此处还要置null？】
return a;
}
/**
* Appends the specified element to the end of this list.
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacity(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
如果只是普通的增加一个元素，则会直接增加到下一个元素的位置。但是如果想是替换某个位置的元素，则需要指定要替换元素的位置，并给出待替换值，下面是这个功能的实现：
/**
* Inserts the specified element at the specified position in this list. Shifts the element currently at that
* position (if any) and any subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
*/
public void add(int index, E element) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
ensureCapacity(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1, size - index);
elementData[index] = element;
size++;
}
下面这个方法可以指定待移除的元素的位置。
/**
* Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts
* one from their indices).
*
* @param index the index of the element to be removed
* @return the element that was removed from the list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
RangeCheck(index);
modCount++;
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index, numMoved);
elementData[--size] = null; // Let gc do its work
return oldValue;
}
/**
* Removes the first occurrence of the specified element from this list, if it is present. If the list does not
* contain the element, it is unchanged. More formally, removes the element with the lowest index <tt>i</tt> such
* that <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> (if such an element exists).
* Returns <tt>true</tt> if this list contained the specified element (or equivalently, if this list changed as a
* result of the call).
*
* @param o element to be removed from this list, if present
* @return <tt>true</tt> if this list contained the specified element
*/
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
fastRemove(index);
return true;
}
}
else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}
/*
* Private remove method that skips bounds checking and does not return the value removed.
*/
private void fastRemove(int index) {
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index, numMoved);
elementData[--size] = null; // Let gc do its work
}
/**
* Removes all of the elements from this list. The list will be empty after this call returns.
*/
public void clear() {
modCount++;
// Let gc do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
/**
* Appends all of the elements in the specified collection to the end of this list, in the order that they are
* returned by the specified collection's Iterator. The behavior of this operation is undefined if the specified
* collection is modified while the operation is in progress. (This implies that the behavior of this call is
* undefined if the specified collection is this list, and this list is nonempty.)
*
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacity(size + numNew); // Increments modCount
System.arraycopy(a, 0, elementData, size, numNew);
size += numNew;
return numNew != 0;
}
/**
* Inserts all of the elements in the specified collection into this list, starting at the specified position.
* Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their
* indices). The new elements will appear in the list in the order that they are returned by the specified
* collection's iterator.
*
* @param index index at which to insert the first element from the specified collection
* @param c collection containing elements to be added to this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacity(size + numNew); // Increments modCount
int numMoved = size - index;
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
System.arraycopy(a, 0, elementData, index, numNew);
size += numNew;
return numNew != 0;
}
/**
* Removes from this list all of the elements whose index is between <tt>fromIndex</tt>, inclusive, and
* <tt>toIndex</tt>, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens
* the list by <tt>(toIndex - fromIndex)</tt> elements. (If <tt>toIndex==fromIndex</tt>, this operation has no
* effect.)
*
* @param fromIndex index of first element to be removed
* @param toIndex index after last element to be removed
* @throws IndexOutOfBoundsException if fromIndex or toIndex out of range (fromIndex < 0 || fromIndex >=
* size() || toIndex > size() || toIndex < fromIndex)
*/
protected void removeRange(int fromIndex, int toIndex) {
modCount++;
int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved);
// Let gc do its work
int newSize = size - (toIndex - fromIndex);
while (size != newSize)
elementData[--size] = null;
}
/**
* Checks if the given index is in range. If not, throws an appropriate runtime exception. This method does *not*
* check if the index is negative: It is always used immediately prior to an array access, which throws an
* ArrayIndexOutOfBoundsException if index is negative.
*/
private void RangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
}
/**
* Save the state of the <tt>ArrayList</tt> instance to a stream (that is, serialize it).
* @serialData The length of the array backing the <tt>ArrayList</tt> instance is emitted (int), followed by all of
* its elements (each an <tt>Object</tt>) in the proper order.
*/
private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
// Write out element count, and any hidden stuff
int expectedModCount = modCount;
s.defaultWriteObject();
// Write out array length
s.writeInt(elementData.length);
// Write out all elements in the proper order.
for (int i = 0; i < size; i++)
s.writeObject(elementData[i]);
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}
/**
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is, deserialize it).
*/
private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
// Read in size, and any hidden stuff
s.defaultReadObject();
// Read in array length and allocate array
int arrayLength = s.readInt();
Object[] a = elementData = new Object[arrayLength];
// Read in all elements in the proper order.
for (int i = 0; i < size; i++)
a[i] = s.readObject();
}
上一篇： 红黑树
下一篇： web前端-Bootstrop框架