java-2.容器之List
1.java容器有哪些？
2.Collection和Collections有什么区别？
java.util.Collection是一个集合接口(集合类的一个顶级接口)。
Collections是集合类的一个工具类/帮助类，其中提供了一系列静态方法。
3.List,Set,Map之间的区别是什么？
| 比较 | List | Set | Map |
|---|---|---|---|
| 继承接口 | Collection | Collection | |
| 常见实现类 | AbstractList(其常用子类有ArrayList,LinkedList,Vector) | Abstract(其常用子类有HashSet,LinkedHashSet,TreeSet) | HashMap,HashTable |
| 常见方法 | add(),remove(),clear(),get(),contains(),size() | add(),remove(),clear(),contains(),size() | put(),get(),remove(),clear(),containsKey(),containsValue(),keySet(),values(),size() |
| 元素 | 可重复 | 不可重复(用equals()判断) | 不可重复 |
| 顺序 | 有序 | 无序(实际上由HashCode决定) | |
| 线程安全 | Vector线程安全 | Hashtable线程安全 |
4.ArrayList源码分析？
底层：数组
源码：
// 定义的变量
transient Object[] elementData;
private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
// 构造方法
// 无参构造方法
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
// 一个参数的构造方法
public ArrayList(int initialCapacity) {
if (initialCapacity > 0) {
this.elementData = new Object[initialCapacity];
} else if (initialCapacity == 0) {
this.elementData = EMPTY_ELEMENTDATA;
} else {
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
}
}
// 一个参数的构造方法
public ArrayList(Collection<? extends E> c) {
Object[] a = c.toArray();
if ((size = a.length) != 0) {
if (c.getClass() == ArrayList.class) {
elementData = a;
} else {
elementData = Arrays.copyOf(a, size, Object[].class);
}
} else {
// replace with empty array.
elementData = EMPTY_ELEMENTDATA;
}
}
// get方法
public E get(int index) {
rangeCheck(index);
return elementData(index);
}
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
E elementData(int index) {
return (E) elementData[index];
}
// add方法
public boolean add(E e) {
ensureCapacityInternal(size + 1);
elementData[size++] = e;
return true;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
// 从size+1和当前长度的1.5倍中取一个大的值
private void grow(int minCapacity) {
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);
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)
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
}
public static <T> T[] copyOf(T[] original, int newLength) {
return (T[]) copyOf(original, newLength, original.getClass());
}
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class) ? (T[]) new Object[newLength] : (T[]) Array.newInstance(newType.getComponentType(), newLength);
// 拷贝后的数组的内容：将原数组的长度拷贝到copy新数组中
System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength));
return copy;
}
// set方法
public E set(int index, E element) {
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
E elementData(int index) {
return (E) elementData[index];
}
// remove方法
// clear方法
public void clear() {
modCount++;
for (int i = 0; i < size; i++)
elementData[i] = null;
size = 0;
}
// contains方法
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
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;
}
// contains
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
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;
}
5.LinkedList源码分析？
底层：双向链表
源码：
transient int size = 0;
transient Node<E> first;
transient Node<E> last;
// 无参构造函数
public LinkedList() {
}
// 有参构造函数
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}
// 在双向链表的头部插入一个节点
private static class Node<E> {
E item;
Node<E> next;
Node<E> prev;
Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
}
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
// 在双向链表的尾部插入一个节点
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
// 在某一个指定节点succ 前插入元素
void linkBefore(E e, Node<E> succ) {
final Node<E> pred = succ.prev;
final Node<E> newNode = new Node<>(pred, e, succ);
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
}
// 添加一个元素
// 添加到双向链表的尾部
public boolean add(E e) {
linkLast(e);
return true;
}
// 在指定节点前面添加一个节点
public void add(int index, E element) {
checkPositionIndex(index);
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
Node<E> node(int index) {
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
// 移除第一个元素 remove()
public E remove() {
return removeFirst();
}
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}
// 删除双向链表中的第一个元素
private E unlinkFirst(Node<E> f) {
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null;
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
return element; // 返回要删除的节点的值E
}
// 删除某个下标的元素 remove(int index)
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
E unlink(Node<E> x) {
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev;
if (prev == null) {
first = next;
} else {
prev.next = next;
x.prev = null;
}
if (next == null) {
last = prev;
} else {
next.prev = prev;
x.next = null;
}
x.item = null;
size--;
modCount++;
return element;
}
// 如果双向链表中包含指定的元素，就在链表中删除掉该元素第一次出现的位置
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}
public boolean remove(Object o) {
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}
// 清空元素 clear()
public void clear() {
for (Node<E> x = first; x != null; ) {
Node<E> next = x.next;
x.item = null;
x.next = null;
x.prev = null;
x = next;
}
first = last = null;
size = 0;
modCount++;
}
// 获得元素 get()
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
Node<E> node(int index) {
if (index < (size >> 1)) {
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}
// 包含什么元素 contains()
public boolean contains(Object o) {
return indexOf(o) != -1;
}
public int indexOf(Object o) {
int index = 0;
if (o == null) {
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
}
// 链表的长度 size()
public int size() {
return size;
}
6.LinkedList和ArrayList比较
总结：LinkedList的 get（index）方法效率比较低，因为需要遍历链表，虽然可通过头节点和尾节点选择从头遍历还是从尾遍历，但总归还是要遍历。
但是add（）方法要比ArrayList效率高，默认直接插入链表尾部，不用考虑扩容问题。
remove（index）和add（index，e）这两个方法虽然需要先遍历到index的位置，但是效率还是比ArrayList高的，不需要数组拷贝。
7.Vector源码分析？
底层：是用数组实现的List,相关的方法都加了同步检查，因此"线程安全，效率低"。
源码：
protected Object[] elementData;
protected int elementCount;
protected int capacityIncrement;
// 构造函数
public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}
// 构造函数
public Vector(int initialCapacity) {
this(initialCapacity, 0);
}
// 构造函数
public Vector() {
this(10);
}
// 构造函数
public Vector(Collection<? extends E> c) {
Object[] a = c.toArray();
elementCount = a.length;
if (c.getClass() == ArrayList.class) {
elementData = a;
} else {
elementData = Arrays.copyOf(a, elementCount, Object[].class);
}
}
// 添加元素 add方法
public synchronized boolean add(E e) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = e;
return true;
}
// 在某一个下标index插入元素
public void add(int index, E element) {
insertElementAt(element, index);
}
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++;
}
// 添加一个集合 addAll(Collection<> c)
public synchronized boolean addAll(Collection<? extends E> c) {
modCount++;
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityHelper(elementCount + numNew);
System.arraycopy(a, 0, elementData, elementCount, numNew);
elementCount += numNew;
return numNew != 0;
}
// 将集合c拷贝到从下标为index开始的位置
public synchronized boolean addAll(int index, Collection<? extends E> c) {
modCount++;
if (index < 0 || index > elementCount)
throw new ArrayIndexOutOfBoundsException(index);
Object[] a = c.toArray();
int numNew = a.length;
ensureCapacityHelper(elementCount + numNew);
int numMoved = elementCount - index;
// 先将elementData数组从下标index开始往后移动numMoved个单位
if (numMoved > 0)
System.arraycopy(elementData, index, elementData, index + numNew,
numMoved);
// 将集合c拷贝到下标从index开始的位置
System.arraycopy(a, 0, elementData, index, numNew);
elementCount += numNew;
return numNew != 0;
}
// 添加指定元素
public synchronized void addElement(E obj) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = obj;
}
private void ensureCapacityHelper(int minCapacity) {
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
private void grow(int minCapacity) {
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)
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
本文来自博客园，作者：jsqup，转载请注明原文链接：https://www.cnblogs.com/jsqup/p/15852268.html
