List和Set都是接口,它們繼承與Collection。List是有序的隊列,可以用重複的元素;而Set是數學概念中的集合,不能有重複的元素。List和Set都有它們各自的實現類。
爲了方便,我們抽象出AbstractCollection類來讓其他類繼承,該類實現類Collection中的絕大部分方法。AbstractList和AbstractSet都繼承與AbstractCollection,具體的List實現類繼承與AbstractList,而Set的實現類則繼承與AbstractSet。
另外,Collection中有個iterator()方法,它的作用是返回一個Iterator接口。通常,我們通過Iterator迭代器來遍歷集合。ListIterator是List接口所特有的,在List接口中,通過ListIterator()返回一個ListIterator對象。
Collection的定義如下:
public interface Collection<E> extends Iterable<E> {}
從它的定義中可以看出,Collection是一個接口。它是一個高度抽象出來的集合,包含了集合的基本操作:添加、刪除、清空、遍歷、是否爲空、獲取大小等。
Collection接口的所有子類(直接子類和簡介子類)都必須實現2種構造函數:不帶參數的構造函數和參數爲Collection的構造函數。帶參數的構造函數可以用來轉換Collection的類型。下面是Collection接口中定義的API:
-
// Collection的API
-
abstract boolean add(E object)
-
abstract boolean addAll(Collection<? extends E> collection)
-
abstract void clear()
-
abstract boolean contains(Object object)
-
abstract boolean containsAll(Collection<?> collection)
-
abstract boolean equals(Object object)
-
abstract int hashCode()
-
abstract boolean isEmpty()
-
abstract Iterator<E> iterator()
-
abstract boolean remove(Object object)
-
abstract boolean removeAll(Collection<?> collection)
-
abstract boolean retainAll(Collection<?> collection)
-
abstract int size()
-
abstract <T> T[] toArray(T[] array)
-
abstract Object[] toArray()
. List
List的定義如下
從List定義中可以看出,它繼承與Collection接口,即List是集合的一種。List是有序的隊列,List中的每一個元素都有一個索引,第一個元素的索引值爲0,往後的元素的索引值依次+1.,List中允許有重複的元素。
List繼承Collection自然包含了Collection的所有接口,由於List是有序隊列,所以它也有自己額外的API接口。API如下:
-
// Collection的API
-
abstract boolean add(E object)
-
abstract boolean addAll(Collection<? extends E> collection)
-
abstract void clear()
-
abstract boolean contains(Object object)
-
abstract boolean containsAll(Collection<?> collection)
-
abstract boolean equals(Object object)
-
abstract int hashCode()
-
abstract boolean isEmpty()
-
abstract Iterator<E> iterator()
-
abstract boolean remove(Object object)
-
abstract boolean removeAll(Collection<?> collection)
-
abstract boolean retainAll(Collection<?> collection)
-
abstract int size()
-
abstract <T> T[] toArray(T[] array)
-
abstract Object[] toArray()
-
// 相比與Collection,List新增的API:
-
abstract void add(int location, E object) //在指定位置添加元素
-
abstract boolean addAll(int location, Collection<? extends E> collection) //在指定位置添加其他集合中的元素
-
abstract E get(int location) //獲取指定位置的元素
-
abstract int indexOf(Object object) //獲得指定元素的索引
-
abstract int lastIndexOf(Object object) //從右邊的索引
-
abstract ListIterator<E> listIterator(int location) //獲得iterator
-
abstract ListIterator<E> listIterator()
-
abstract E remove(int location) //刪除指定位置的元素
-
abstract E set(int location, E object) //修改指定位置的元素
-
abstract List<E> subList(int start, int end) //獲取子list
3. Set
Set的定義如下:
public interface Set<E> extends Collection<E> {}
Set也繼承與Collection接口,且裏面不能有重複元素。關於API,Set與Collection的API完全一樣,不在贅述。
public abstract class AbstractCollection<E> implements Collection<E> {}
AbstractCollection是一個抽象類,它實現了Collection中除了iterator()和size()之外的所有方法。AbstractCollection的主要作用是方便其他類實現Collection.,比如ArrayList、LinkedList等。它們想要實現Collection接口,通過集成AbstractCollection就已經實現大部分方法了,再實現一下iterator()和size()即可。
下面看一下AbstractCollection實現的部分方法的源碼:
-
public abstract class AbstractCollection<E> implements Collection<E> {
-
protected AbstractCollection() {
-
}
-
public abstract Iterator<E> iterator();//iterator()方法沒有實現
-
public abstract int size(); //size()方法也沒有實現
-
public boolean isEmpty() { //檢測集合是否爲空
-
return size() == 0;
-
}
-
/*檢查集合中是否包含特定對象*/
-
public boolean contains(Object o) {
-
Iterator<E> it = iterator();
-
if (o==null) {
-
while (it.hasNext()) //從這裏可以看出,任何非空集合都包含null
-
if (it.next()==null)
-
return true;
-
} else {
-
while (it.hasNext())
-
if (o.equals(it.next()))
-
return true;
-
}
-
return false;
-
}
-
/*將集合轉變成數組*/
-
public Object[] toArray() {
-
// Estimate size of array; be prepared to see more or fewer elements
-
Object[] r = new Object[size()]; //創建與集合大小相同的數組
-
Iterator<E> it = iterator();
-
for (int i = 0; i < r.length; i++) {
-
if (! it.hasNext()) // fewer elements than expected
-
//Arrays.copy(**,**)的第二個參數是待copy的長度,如果這個長度大於r,則保留r的長度
-
return Arrays.copyOf(r, i);
-
r[i] = it.next();
-
}
-
return it.hasNext() ? finishToArray(r, it) : r;
-
}
-
public <T> T[] toArray(T[] a) {
-
// Estimate size of array; be prepared to see more or fewer elements
-
int size = size();
-
T[] r = a.length >= size ? a :
-
(T[])java.lang.reflect.Array
-
.newInstance(a.getClass().getComponentType(), size);
-
Iterator<E> it = iterator();
-
for (int i = 0; i < r.length; i++) {
-
if (! it.hasNext()) { // fewer elements than expected
-
if (a == r) {
-
r[i] = null; // null-terminate
-
} else if (a.length < i) {
-
return Arrays.copyOf(r, i);
-
} else {
-
System.arraycopy(r, 0, a, 0, i);
-
if (a.length > i) {
-
a[i] = null;
-
}
-
}
-
return a;
-
}
-
r[i] = (T)it.next();
-
}
-
// more elements than expected
-
return it.hasNext() ? finishToArray(r, it) : r;
-
}
-
private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
-
int i = r.length;
-
while (it.hasNext()) {
-
int cap = r.length;
-
if (i == cap) {
-
int newCap = cap + (cap >> 1) + 1;
-
// overflow-conscious code
-
if (newCap - MAX_ARRAY_SIZE > 0)
-
newCap = hugeCapacity(cap + 1);
-
r = Arrays.copyOf(r, newCap);
-
}
-
r[i++] = (T)it.next();
-
}
-
// trim if overallocated
-
return (i == r.length) ? r : Arrays.copyOf(r, i);
-
}
-
private static int hugeCapacity(int minCapacity) {
-
if (minCapacity < 0) // overflow
-
throw new OutOfMemoryError
-
("Required array size too large");
-
return (minCapacity > MAX_ARRAY_SIZE) ?
-
Integer.MAX_VALUE :
-
MAX_ARRAY_SIZE;
-
}
-
// 刪除對象o
-
public boolean remove(Object o) {
-
Iterator<E> it = iterator();
-
if (o==null) {
-
while (it.hasNext()) {
-
if (it.next()==null) {
-
it.remove();
-
return true;
-
}
-
}
-
} else {
-
while (it.hasNext()) {
-
if (o.equals(it.next())) {
-
it.remove();
-
return true;
-
}
-
}
-
}
-
return false;
-
}
-
<pre name="code" class="java"> // 判斷是否包含集合c中所有元素
-
public boolean containsAll(Collection<?> c) {
-
for (Object e : c)
-
if (!contains(e))
-
return false;
-
return true;
-
}
-
//添加集合c中所有元素
-
public boolean addAll(Collection<? extends E> c) {
-
boolean modified = false;
-
for (E e : c)
-
if (add(e))
-
modified = true;
-
return modified;
-
}
-
//刪除集合c中所有元素(如果存在的話)
-
public boolean removeAll(Collection<?> c) {
-
boolean modified = false;
-
Iterator<?> it = iterator();
-
while (it.hasNext()) {
-
if (c.contains(it.next())) {
-
it.remove();
-
modified = true;
-
}
-
}
-
return modified;
-
}
-
//清空
-
public void clear() {
-
Iterator<E> it = iterator();
-
while (it.hasNext()) {
-
it.next();
-
it.remove();
-
}
-
}
-
//將集合元素顯示成[String]
-
public String toString() {
-
Iterator<E> it = iterator();
-
if (! it.hasNext())
-
return "[]";
-
StringBuilder sb = new StringBuilder();
-
sb.append('[');
-
for (;;) {
-
E e = it.next();
-
sb.append(e == this ? "(this Collection)" : e);
-
if (! it.hasNext())
-
return sb.append(']').toString();
-
sb.append(',').append(' ');
-
}
-
}
-
}
5. AbstractList
AbstractList的定義如下:
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {}
從定義中可以看出,AbstractList是一個繼承AbstractCollection,並且實現了List接口的抽象類。它實現了List中除了size()、get(int location)之外的方法。
AbstractList的主要作用:它實現了List接口中的大部分函數,從而方便其它類繼承List。另外,和AbstractCollection相比,AbstractList抽象類中,實現了iterator()方法。
AbstractList抽象類的源碼如下:
-
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
-
protected AbstractList() {
-
}
-
public boolean add(E e) {
-
add(size(), e);
-
return true;
-
}
-
abstract public E get(int index);
-
public E set(int index, E element) {
-
throw new UnsupportedOperationException();
-
}
-
public void add(int index, E element) {
-
throw new UnsupportedOperationException();
-
}
-
public E remove(int index) {
-
throw new UnsupportedOperationException();
-
}
-
/***************************** Search Operations**********************************/
-
public int indexOf(Object o) { //搜索對象o的索引
-
ListIterator<E> it = listIterator();
-
if (o==null) {
-
while (it.hasNext())
-
if (it.next()==null) //執行it.next(),會先返回it指向位置的值,然後it會移到下一個位置
-
return it.previousIndex(); //所以要返回it.previousIndex(); 關於it幾個方法的源碼在下面
-
} else {
-
while (it.hasNext())
-
if (o.equals(it.next()))
-
return it.previousIndex();
-
}
-
return -1;
-
}
-
public int lastIndexOf(Object o) {
-
ListIterator<E> it = listIterator(size());
-
if (o==null) {
-
while (it.hasPrevious())
-
if (it.previous()==null)
-
return it.nextIndex();
-
} else {
-
while (it.hasPrevious())
-
if (o.equals(it.previous()))
-
return it.nextIndex();
-
}
-
return -1;
-
}
-
/**********************************************************************************/
-
/****************************** Bulk Operations ***********************************/
-
public void clear() {
-
removeRange(0, size());
-
}
-
public boolean addAll(int index, Collection<? extends E> c) {
-
rangeCheckForAdd(index);
-
boolean modified = false;
-
for (E e : c) {
-
add(index++, e);
-
modified = true;
-
}
-
return modified;
-
}
-
protected void removeRange(int fromIndex, int toIndex) {
-
ListIterator<E> it = listIterator(fromIndex);
-
for (int i=0, n=toIndex-fromIndex; i<n; i++) {
-
it.next();
-
it.remove();
-
}
-
}
-
/**********************************************************************************/
-
/********************************* Iterators **************************************/
-
public Iterator<E> iterator() {
-
return new Itr();
-
}
-
public ListIterator<E> listIterator() {
-
return listIterator(0); //返回的iterator索引從0開始
-
}
-
public ListIterator<E> listIterator(final int index) {
-
rangeCheckForAdd(index); //首先檢查index範圍是否正確
-
return new ListItr(index); //ListItr繼承與Itr且實現了ListIterator接口,Itr實現了Iterator接口,往下看
-
}
-
private class Itr implements Iterator<E> {
-
int cursor = 0; //元素的索引,當調用next()方法時,返回當前索引的值
-
int lastRet = -1; //lastRet也是元素的索引,但如果刪掉此元素,該值置爲-1
-
/*
-
*迭代器都有個modCount值,在使用迭代器的時候,如果使用remove,add等方法的時候都會修改modCount,
-
*在迭代的時候需要保持單線程的唯一操作,如果期間進行了插入或者刪除,modCount就會被修改,迭代器就會檢測到被併發修改,從而出現運行時異常。
-
*舉個簡單的例子,現在某個線程正在遍歷一個List,另一個線程對List中的某個值做了刪除,那原來的線程用原來的迭代器當然無法正常遍歷了
-
*/
-
int expectedModCount = modCount;
-
public boolean hasNext() {
-
return cursor != size(); //當索引值和元素個數相同時表示沒有下一個元素了,索引是從0到size-1
-
}
-
public E next() {
-
checkForComodification(); //檢查modCount是否改變
-
try {
-
int i = cursor; //next()方法主要做了兩件事:
-
E next = get(i);
-
lastRet = i;
-
cursor = i + 1; //1.將索引指向了下一個位置
-
return next; //2. 返回當前索引的值
-
} catch (IndexOutOfBoundsException e) {
-
checkForComodification();
-
throw new NoSuchElementException();
-
}
-
}
-
public void remove() {
-
if (lastRet < 0) //lastRet<0表示已經不存在了
-
throw new IllegalStateException();
-
checkForComodification();
-
try {
-
AbstractList.this.remove(lastRet);
-
if (lastRet < cursor)
-
cursor--; //原位置的索引值減小了1,但是實際位置沒變
-
lastRet = -1; //置爲-1表示已刪除
-
expectedModCount = modCount;
-
} catch (IndexOutOfBoundsException e) {
-
throw new ConcurrentModificationException();
-
}
-
}
-
final void checkForComodification() {
-
if (modCount != expectedModCount)
-
throw new ConcurrentModificationException();
-
}
-
}
-
private class ListItr extends Itr implements ListIterator<E> {
-
ListItr(int index) {
-
cursor = index;
-
}
-
public boolean hasPrevious() {
-
return cursor != 0;
-
}
-
public E previous() {
-
checkForComodification();
-
try {
-
int i = cursor - 1; //previous()方法中也做了兩件事:
-
E previous = get(i); //1. 將索引向前移動一位
-
lastRet = cursor = i; //2. 返回索引處的值
-
return previous;
-
} catch (IndexOutOfBoundsException e) {
-
checkForComodification();
-
throw new NoSuchElementException();
-
}
-
}
-
public int nextIndex() { //iterator中的index本來就是下一個位置,在next()方法中可以看出
-
return cursor;
-
}
-
public int previousIndex() {
-
return cursor-1;
-
}
-
public void set(E e) { //修改當前位置的元素
-
if (lastRet < 0)
-
throw new IllegalStateException();
-
checkForComodification();
-
try {
-
AbstractList.this.set(lastRet, e);
-
expectedModCount = modCount;
-
} catch (IndexOutOfBoundsException ex) {
-
throw new ConcurrentModificationException();
-
}
-
}
-
public void add(E e) { //在當前位置添加元素
-
checkForComodification();
-
try {
-
int i = cursor;
-
AbstractList.this.add(i, e);
-
lastRet = -1;
-
cursor = i + 1;
-
expectedModCount = modCount;
-
} catch (IndexOutOfBoundsException ex) {
-
throw new ConcurrentModificationException();
-
}
-
}
-
}
-
/**********************************************************************************/
-
//獲得子List,詳細源碼往下看SubList類
-
public List<E> subList(int fromIndex, int toIndex) {
-
return (this instanceof RandomAccess ?
-
new RandomAccessSubList<>(this, fromIndex, toIndex) :
-
new SubList<>(this, fromIndex, toIndex));
-
}
-
/*************************** Comparison and hashing *******************************/
-
public boolean equals(Object o) {
-
if (o == this)
-
return true;
-
if (!(o instanceof List))
-
return false;
-
ListIterator<E> e1 = listIterator();
-
ListIterator e2 = ((List) o).listIterator();
-
while (e1.hasNext() && e2.hasNext()) {
-
E o1 = e1.next();
-
Object o2 = e2.next();
-
if (!(o1==null ? o2==null : o1.equals(o2)))
-
return false;
-
}
-
return !(e1.hasNext() || e2.hasNext());
-
}
-
public int hashCode() { //hashcode
-
int hashCode = 1;
-
for (E e : this)
-
hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
-
return hashCode;
-
}
-
/**********************************************************************************/
-
protected transient int modCount = 0;
-
private void rangeCheckForAdd(int index) {
-
if (index < 0 || index > size())
-
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
-
}
-
private String outOfBoundsMsg(int index) {
-
return "Index: "+index+", Size: "+size();
-
}
-
}
-
class SubList<E> extends AbstractList<E> {
-
private final AbstractList<E> l;
-
private final int offset;
-
private int size;
-
/* 從SubList源碼可以看出,當需要獲得一個子List時,底層並不是真正的返回一個子List,還是原來的List,只不過
-
* 在操作的時候,索引全部限定在用戶所需要的子List部分而已
-
*/
-
SubList(AbstractList<E> list, int fromIndex, int toIndex) {
-
if (fromIndex < 0)
-
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
-
if (toIndex > list.size())
-
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
-
if (fromIndex > toIndex)
-
throw new IllegalArgumentException("fromIndex(" + fromIndex +
-
") > toIndex(" + toIndex + ")");
-
l = list; //原封不動的將原來的list賦給l
-
offset = fromIndex; //偏移量,用在操作新的子List中
-
size = toIndex - fromIndex; //子List的大小,所以子List中不包括toIndex處的值,即子List中包括左邊不包括右邊
-
this.modCount = l.modCount;
-
}
-
//注意下面所有的操作都在索引上加上偏移量offset,相當於在原來List的副本上操作子List
-
public E set(int index, E element) {
-
rangeCheck(index);
-
checkForComodification();
-
return l.set(index+offset, element);
-
}
-
public E get(int index) {
-
rangeCheck(index);
-
checkForComodification();
-
return l.get(index+offset);
-
}
-
public int size() {
-
checkForComodification();
-
return size;
-
}
-
public void add(int index, E element) {
-
rangeCheckForAdd(index);
-
checkForComodification();
-
l.add(index+offset, element);
-
this.modCount = l.modCount;
-
size++;
-
}
-
public E remove(int index) {
-
rangeCheck(index);
-
checkForComodification();
-
E result = l.remove(index+offset);
-
this.modCount = l.modCount;
-
size--;
-
return result;
-
}
-
protected void removeRange(int fromIndex, int toIndex) {
-
checkForComodification();
-
l.removeRange(fromIndex+offset, toIndex+offset);
-
this.modCount = l.modCount;
-
size -= (toIndex-fromIndex);
-
}
-
public boolean addAll(Collection<? extends E> c) {
-
return addAll(size, c);
-
}
-
public boolean addAll(int index, Collection<? extends E> c) {
-
rangeCheckForAdd(index);
-
int cSize = c.size();
-
if (cSize==0)
-
return false;
-
checkForComodification();
-
l.addAll(offset+index, c);
-
this.modCount = l.modCount;
-
size += cSize;
-
return true;
-
}
-
public Iterator<E> iterator() {
-
return listIterator();
-
}
-
public ListIterator<E> listIterator(final int index) {
-
checkForComodification();
-
rangeCheckForAdd(index);
-
return new ListIterator<E>() {
-
private final ListIterator<E> i = l.listIterator(index+offset); //相當子List的索引0
-
public boolean hasNext() {
-
return nextIndex() < size;
-
}
-
public E next() {
-
if (hasNext())
-
return i.next();
-
else
-
throw new NoSuchElementException();
-
}
-
public boolean hasPrevious() {
-
return previousIndex() >= 0;
-
}
-
public E previous() {
-
if (hasPrevious())
-
return i.previous();
-
else
-
throw new NoSuchElementException();
-
}
-
public int nextIndex() {
-
return i.nextIndex() - offset;
-
}
-
public int previousIndex() {
-
return i.previousIndex() - offset;
-
}
-
public void remove() {
-
i.remove();
-
SubList.this.modCount = l.modCount;
-
size--;
-
}
-
public void set(E e) {
-
i.set(e);
-
}
-
public void add(E e) {
-
i.add(e);
-
SubList.this.modCount = l.modCount;
-
size++;
-
}
-
};
-
}
-
public List<E> subList(int fromIndex, int toIndex) {
-
return new SubList<>(this, fromIndex, toIndex);
-
}
-
private void rangeCheck(int index) {
-
if (index < 0 || index >= size)
-
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
-
}
-
private void rangeCheckForAdd(int index) {
-
if (index < 0 || index > size)
-
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
-
}
-
private String outOfBoundsMsg(int index) {
-
return "Index: "+index+", Size: "+size;
-
}
-
private void checkForComodification() {
-
if (this.modCount != l.modCount)
-
throw new ConcurrentModificationException();
-
}
-
}
-
class RandomAccessSubList<E> extends SubList<E> implements RandomAccess {
-
RandomAccessSubList(AbstractList<E> list, int fromIndex, int toIndex) {
-
super(list, fromIndex, toIndex);
-
}
-
public List<E> subList(int fromIndex, int toIndex) {
-
return new RandomAccessSubList<>(this, fromIndex, toIndex);
-
}
-
}
6. AbstractSet
AbstractSet的定義如下:
public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {}
AbstractSet是一個繼承與AbstractCollection,並且實現了Set接口的抽象類。由於Set接口和Collection接口中的API完全一樣,所以Set也就沒有自己單獨的API。和AbstractCollection一樣,它實現了List中除iterator()和size()外的方法。所以源碼和AbstractCollection的一樣。
AbstractSet的主要作用:它實現了Set接口總的大部分函數,從而方便其他類實現Set接口。
AbstractSet是一個繼承與AbstractCollection,並且實現了Set接口的抽象類。由於Set接口和Collection接口中的API完全一樣,所以Set也就沒有自己單獨的API。和AbstractCollection一樣,它實現了List中除iterator()和size()外的方法。所以源碼和AbstractCollection的一樣。
AbstractSet的主要作用:它實現了Set接口總的大部分函數,從而方便其他類實現Set接口。
Iterator的定義如下:
public interface Iterator<E> {}
Iterator是一個接口,它是集合的迭代器。集合可以通過Iterator去遍歷其中的元素。Iterator提供的API接口包括:是否存在下一個元素,獲取下一個元素和刪除當前元素。
注意:Iterator遍歷Collection時,是fail-fast機制的。即,當某一個線程A通過iterator去遍歷某集合的過程中,若該集合的內容被其他線程所改變了,那麼線程A訪問集合時,就會拋出CurrentModificationException異常,產生fail-fast事件。下面是Iterator的幾個API。
-
// Iterator的API
-
abstract boolean hasNext()
-
abstract E next()
-
abstract void remove()
8. ListIterator
ListIterator的定義如下:
public interface ListIterator<E> extends Iterator<E> {}
ListIterator是一個繼承Iterator的接口,它是隊列迭代器。專門用於遍歷List,能提供向前和向後遍歷。相比於Iterator,它新增了添加、是否存在上一個元素、獲取上一個元素等API接口:
-
// 繼承於Iterator的接口
-
abstract boolean hasNext()
-
abstract E next()
-
abstract void remove()
-
// 新增API接口
-
abstract void add(E object)
-
abstract boolean hasPrevious()
-
abstract int nextIndex()
-
abstract E previous()
-
abstract int previousIndex()
-
abstract void set(E object)
Collection的架構就討論到這吧,如果有問題歡迎留言指正~