小白学集合之看ArrayList和LinkedList源码
List实现类【重要】
ArrayList：
底层是数据，查询快，增删慢
JDK1.2版本，运行效率快，线程不安全
LinkedList：
底层是链表，查询慢，增删快
Vector：
底层是数据，查询快，增删慢
JDK1.0版本，运行效率慢，线程安全
ArrayList
ArrayListDemo类是ArrayList的一些方法的使用。
package com.list;
import com.collection.Student;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.ListIterator;
/**
* ArrayList的使用
* 存储结构是数组，查找快，增删慢
*/
public class ArrayListDemo {
public static void main(String[] args) {
ArrayList arrayList = new ArrayList<>();
Student s1 = new Student("张三", 18);
Student s2 = new Student("李四", 20);
Student s3 = new Student("王二", 22);
//添加元素
arrayList.add(s1);
arrayList.add(s2);
arrayList.add(s3);
System.out.println("元素个数："+arrayList.size());
//删除
// arrayList.remove(s1);
// System.out.println("删除之后个数："+arrayList.size());
//元素遍历
Iterator it = arrayList.iterator();
while (it.hasNext()) {
Student s =(Student) it.next();
System.out.println("迭代器遍历："+s.toString());
}
ListIterator listIterator = arrayList.listIterator();
while (listIterator.hasNext()) {
Student stu = (Student) listIterator.next();
System.out.println("列表迭代器遍历："+stu.toString());
}
//判断
System.out.println("是否包含s1：" +arrayList.contains(s1));
System.out.println("是否为空："+arrayList.isEmpty());
//查找
System.out.println("s2的下标："+arrayList.indexOf(s2));
}
}
ArrarList源码分析【重点】
ArrayList实现了List接口，有序容器，就是存进去元素的顺序和存的顺序一致，允许null元素，底层是数组，除了未实现同步，其他和Vector大致相同。
每个ArrayList都有一个容量（capacity），表示数组的实际大小。当像容器添加元素的时候，如果容量不够，容器会自动扩容，
变量名：
DEFAULT_CAPACITY：默认容量
EMPTY_ELEMENTDATA：空元素数据
DEFAULTCAPACITY_EMPTY_ELEMENTDATA：默认容量空元素数据
elementData：元素数据
size：元素个数
/**
* Default initial capacity.
*/
private static final int DEFAULT_CAPACITY = 10;
/**
* Shared empty array instance used for empty instances.
*/
private static final Object[] EMPTY_ELEMENTDATA = {
};
/**
* Shared empty array instance used for default sized empty instances. We
* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
* first element is added.
*/
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {
};
/**
* The array buffer into which the elements of the ArrayList are stored.
* The capacity of the ArrayList is the length of this array buffer. Any
* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
* will be expanded to DEFAULT_CAPACITY when the first element is added.
*/
transient Object[] elementData; // non-private to simplify nested class access
/**
* The size of the ArrayList (the number of elements it contains).
*
* @serial
*/
private int size;
构造函数
可以看到有3个构造函数，如果ArrayList arrayList = new ArrayList<>();创建一个对象，就会调用无参构造，生成一个容量为0的arryList集合。
/**
* Constructs an empty list with the specified initial capacity.
*
* @param initialCapacity the initial capacity of the list
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
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);
}
}
/**
* Constructs an empty list with an initial capacity of ten.
*/
public ArrayList() {
this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
/**
* 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) {
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;
}
}
add(E e) 方法
当添加第一个元素的时候，因为初始为0，所以执行：ensureCapacityInternal(1)
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return true (as specified by {@link Collection#add})
*/
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
自动扩容
每当向数组中添加元素的时候，都会先检查添加后元素个数是否超过数组容量，如果超出，进行扩容。扩容通过 ensureCapacity(int minCapacity)方法实现。
数组扩容的时候会将老数组中的元素复制一份到新的数组中，第一次扩容是从0到10，然后每次扩容大概是原容量的1.5倍。扩容代价很高，我们应尽量避免扩容。
/**
* Increases the capacity of this ArrayList 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) {
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
// any size if not default element table
? 0
// larger than default for default empty table. It's already
// supposed to be at default size.
: DEFAULT_CAPACITY;
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}
private static int calculateCapacity(Object[] elementData, int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
return Math.max(DEFAULT_CAPACITY, minCapacity);
}
return minCapacity;
}
private void ensureCapacityInternal(int minCapacity) {
ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// 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;
/**
* 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;
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);
}
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // overflow
throw new OutOfMemoryError();
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}
LinkedList源码分析【重点】
LinkedList底层是双向链表
变量名
size：链表大小
first：第一个节点
last：最后一个节点
transient int size = 0;
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node<E> first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node<E> last;
Node是私有内部类
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;
}
}
构造函数
/**
* Constructs an empty list.
* 构造一个空列表。
*/
public LinkedList() {
}
/**
* 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 LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}
add(E)方法
/**
* Appends the specified element to the end of this list.
* 将指定的元素追加到此列表的末尾。
* This method is equivalent to {@link #addLast}.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
linkLast(e);
return true;
}
/**
* Links e as last element.
* 将e链接为最后一个元素。
*/
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ArrayList的中间插入或删除一个元素，那列表中该元素往后的元素都会后移；而在LinkedList的中间插入或删除一个元素的开销是固定的。
LinkedList不能随机访问。