ArrayList常用API原始碼分析
阿新 • • 發佈：2020-06-24
3.1、ArrayList
簡介：
底層是可改變大小的陣列結構，實現了list介面，執行緒不安全，可以加入null元素，元素有序，允許重複。適合查詢，不適合指定位置的插入（adding n elements requires O(n) time）、刪除操作。
UML圖：
重要屬性
/**
* 預設初始容量.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* 最大可以分配的陣列大小.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/ 3.1.1新增元素
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return <tt>true</tt> (as specified by {@link Collection#add})
*/
public 3.1.2刪除元素
/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
*
* @param 3.1.3獲取指定位置的元素
/**
* Returns the element at the specified position in this list.
*
* @param index index of the element to return
* @return the element at the specified position in this list
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
//索引位置檢查
rangeCheck(index);
return elementData(index);
}
複製程式碼3.1.4獲取某個元素第一次出現的索引位置
/**
* 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) {
//正序查詢
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;
}
複製程式碼3.1.5獲取某個元素最後一次出現的索引位置
/**
* Returns the index of the last occurrence of the specified element
* in this list,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) {
//倒序查詢
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;
}
複製程式碼3.1.6更新某個索引位置的元素
/**
* Replaces the element at the specified position in this list with
* the specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index,E element) {
//索引檢查
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
複製程式碼3.1.7擴容
/**
* Increases the capacity to ensure that it can hold at least the
* number of elements specified by the minimum capacity argument.
*
* @param minCapacity the desired minimum capacity
*/
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
//新大小=原大小+原大小*0.5,即擴容到原來的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);
// minCapacity is usually close to size,so this is a win:
elementData = Arrays.copyOf(elementData,newCapacity);
}
複製程式碼3.1.8返回陣列大小
/**
* Returns the number of elements in this list.
*
* @return the number of elements in this list
*/
public int size() {
return size;
}
複製程式碼3.1.9陣列是否為空判斷
/**
* Returns <tt>true</tt> if this list contains no elements.
*
* @return <tt>true</tt> if this list contains no elements
*/
public boolean isEmpty() {
return size == 0;
}
複製程式碼3.1.10是否包含某個元素
/**
* 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>.
*
* @param o element whose presence in this list is to be tested
* @return <tt>true</tt> if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
複製程式碼3.1.11清空陣列
/**
* Removes all of the elements from this list. The list will
* be empty after this call returns.
*/
public void clear() {
modCount++;
// clear to let GC do its work
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
複製程式碼3.1.12交集操作
/**
* Retains only the elements in this list that are contained in the
* specified collection (optional operation). In other words,removes
* from this list all of its elements that are not contained in the
* specified collection.
*
* @param c collection containing elements to be retained in this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
* is not supported by this list
* @throws ClassCastException if the class of an element of this list
* is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this list contains a null element and the
* specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
boolean retainAll(Collection<?> c);
複製程式碼3.1.13新增某個集合的所有元素
/**
* 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 (optional operation). The behavior of this
* operation is undefined if the specified collection is modified while
* the operation is in progress. (Note that this will occur if the
* specified collection is this list,and it's 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 UnsupportedOperationException if the <tt>addAll</tt> operation
* is not supported by this list
* @throws ClassCastException if the class of an element of the specified
* collection prevents it from being added to this list
* @throws NullPointerException if the specified collection contains one
* or more null elements and this list does not permit null
* elements,or if the specified collection is null
* @throws IllegalArgumentException if some property of an element of the
* specified collection prevents it from being added to this list
* @see #add(Object)
*/
boolean addAll(Collection<? extends E> c);
複製程式碼3.1.13刪除陣列中所有在特定集合中出現的元素
/**
* Removes from this list all of its elements that are contained in the
* specified collection (optional operation).
*
* @param c collection containing elements to be removed from this list
* @return <tt>true</tt> if this list changed as a result of the call
* @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
* is not supported by this list
* @throws ClassCastException if the class of an element of this list
* is incompatible with the specified collection
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if this list contains a null element and the
* specified collection does not permit null elements
* (<a href="Collection.html#optional-restrictions">optional</a>),* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
boolean removeAll(Collection<?> c);
複製程式碼示例：
public class ArrayListTest {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(null);
list.add(1);
list.add(2);
System.out.println(list);
System.out.println("list 第一次出現元素的索引值：" + list.indexOf(1));
System.out.println("list 最後一次出現元素的索引值：" + list.lastIndexOf(1));
list.set(1,6);
System.out.println(list);
System.out.println("list 是否為空？" + list.isEmpty());
System.out.println("list 是否包含元素6？" + list.contains(6));
list.clear();
System.out.println("list 大小：" + list.size());
//Arrays 中的 ArrayList 與 util 包下的 ArrayList 不同，要進行轉換
List<Integer> temp2 = Arrays.asList(1,2,3);
List<Integer> temp3 = Arrays.asList(2,3,4);
List<Integer> list2 = new ArrayList<>(temp2);
List<Integer> list3 = new ArrayList<>(temp3);
list.addAll(list2);
list.addAll(list3);
System.out.println(list);
list2.retainAll(list3);
System.out.println("list2 與list3 的交集：" + list2);
list.removeAll(list2);
System.out.println("list3 與 list2 的差集：" + list);
list.addAll(list2);
System.out.println("list3 與 list2 的並集：" + list);
}
}
複製程式碼執行結果：
[1,null,1,2]
list 第一次出現元素的索引值：0
list 最後一次出現元素的索引值：2
[1,6,2]
list 是否為空？false
list 是否包含元素6？true
list 大小：0
[1,4]
list2 與list3 的交集：[2,3]
list3 與 list2 的差集：[1,4]
list3 與 list2 的並集：[1,4,3]複製程式碼