数据结构---Java---ArrayList

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{

　　　//初始容量10
     private static final int DEFAULT_CAPACITY = 10;
       //空的Object[]
     private static final Object[] EMPTY_ELEMENTDATA = {};
       //默认的空数组容量
     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
　　　//底层使用数组实现
     transient Object[] elementData;
     private int size;
     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

       ++++++++++++++++++构造器+++++++++

      //无参构造器---Object[]的大小为0
     public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

     // 有参构造器
    public ArrayList(int initialCapacity) {
         //如果自定义容量大于0，创建一个自定义容量的Object数组
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        }
         //如果自定义容量=0，Object[]大小为0
        else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

     public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

   +++++++++++++++方法+++++++++++++++++

    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        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++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
　　//扩容
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
　　　　//新容量=旧容量+(旧容量>>1)　旧容量的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);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

}

1、ArrayList遍历时的删除问题：

　　　　　　1.1　　普通for

package com.an.cycle;

import java.util.ArrayList;
import java.util.List;

public class Test {

    public static void main(String[] args){

        List<String> list=new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("c");

        for(int i=0;i<list.size();i++){
            if(list.get(i).equals("a"))
                list.remove(i);
        }
    }
}

由于删除后集合大小、元素位置发生移动，导致数据索引出现问题

　　

　　　　　　1.2　　增强for　

package com.an.cycle;

import java.util.ArrayList;
import java.util.List;

public class Test {

    public static void main(String[] args){

        List<String> list=new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("c");

        for(String x:list){
            if(x.equals("a"))
                list.remove(x);
        }
    }
}

结果：使用增强for进行删除，引发异常
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
	at java.util.ArrayList$Itr.next(ArrayList.java:859)
	at com.an.cycle.Test.main(Test.java:20)

分析：进入异常信息查看
final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

1、ArrayList的父类AbstractList有一个属性modCount：
   protected transient int modCount = 0;   用于记录对ArrayList的操作次数

2、ArrayList有一个内部类Itr，实现了Iterator：
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{

    private class Itr implements Iterator<E> {
        int expectedModCount = modCount;

        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        //异常提示位置
         final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
}

3、 list.remove(x);会调用ArrayList的fastRemove方法：
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; // clear to let GC do its work
    }

    此时 modCount++，而expectedModCount没有变化，导致异常触发

　　　　　　1.3　　Iterator遍历

package com.an.cycle;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {

    public static void main(String[] args){

        List<String> list=new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("c");

        Iterator<String> it = list.iterator();
        while(it.hasNext()){
            String x = it.next();
            if(x.equals("a")){
                list.remove(x);
            }
        }
    }
}

结果：使用ArrayList的remove仍会抛出异常：
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
	at java.util.ArrayList$Itr.next(ArrayList.java:859)
	at com.an.cycle.Test.main(Test.java:28)

　　　　　　1.4　　解决ArrayList的删除问题：

使用Iterator的remove 方法

package com.an.cycle;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {

    public static void main(String[] args){

        List<String> list=new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("c");

        Iterator<String> it = list.iterator();
        while(it.hasNext()){
            String x = it.next();
            if(x.equals("a")){
                it.remove();
            }
        }

        System.out.println(list);
    }
}