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);
// 数据拷贝
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
List<String> list = new ArrayList<>();
list.add("c");
list.add("6");
list.add("4");
list.add("2");
for (int i = 0; i< list.size(); i++){
if (list.get(i).equals("c")){
list.remove("c");
}
}
System.out.println(list);
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: new #2 // class java/util/ArrayList
3: dup
4: invokespecial #3 // Method java/util/ArrayList."<init>":()V
7: astore_1
8: aload_1
9: ldc #4 // String c
11: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
16: pop
17: aload_1
18: ldc #6 // String 6
20: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
25: pop
26: aload_1
27: ldc #7 // String 4
29: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
34: pop
35: aload_1
36: ldc #8 // String 2
38: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
43: pop
44: iconst_0
45: istore_2
46: iload_2
47: aload_1
48: invokeinterface #9, 1 // InterfaceMethod java/util/List.size:()I
53: if_icmpge 89
56: aload_1
57: iload_2
58: invokeinterface #10, 2 // InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
63: checkcast #11 // class java/lang/String
66: ldc #4 // String c
68: invokevirtual #12 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
71: ifeq 83
74: aload_1
75: ldc #4 // String c
77: invokeinterface #13, 2 // InterfaceMethod java/util/List.remove:(Ljava/lang/Object;)Z
82: pop
83: iinc 2, 1
86: goto 46
89: getstatic #14 // Field java/lang/System.out:Ljava/io/PrintStream;
92: aload_1
93: invokevirtual #15 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
96: return
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
List<String> list = new ArrayList<>();
list.add("c");
list.add("6");
list.add("4");
list.add("2");
for (String s : list){
if (s.equals("c")){
list.remove("c");
}
}
System.out.println(list);
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=4, args_size=1
0: new #2 // class java/util/ArrayList
3: dup
4: invokespecial #3 // Method java/util/ArrayList."<init>":()V
7: astore_1
8: aload_1
9: ldc #4 // String c
11: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
16: pop
17: aload_1
18: ldc #6 // String 6
20: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
25: pop
26: aload_1
27: ldc #7 // String 4
29: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
34: pop
35: aload_1
36: ldc #8 // String 2
38: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
43: pop
44: aload_1
45: invokeinterface #9, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator; 增强for本质使用的是java/util/List.iterator
50: astore_2
51: aload_2
52: invokeinterface #10, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z
57: ifeq 91
60: aload_2
61: invokeinterface #11, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
66: checkcast #12 // class java/lang/String
69: astore_3
70: aload_3
71: ldc #4 // String c
73: invokevirtual #13 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
76: ifeq 88
79: aload_1
80: ldc #4 // String c
82: invokeinterface #14, 2 // InterfaceMethod java/util/List.remove:(Ljava/lang/Object;)Z
87: pop
88: goto 51
91: getstatic #15 // Field java/lang/System.out:Ljava/io/PrintStream;
94: aload_1
95: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
98: return
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);
}
}
List<String> list = new ArrayList<>();
list.add("c");
list.add("6");
list.add("4");
list.add("2");
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()){
if (iterator.next().equals("c")){
iterator.remove();
}
}
System.out.println(list);
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: new #2 // class java/util/ArrayList
3: dup
4: invokespecial #3 // Method java/util/ArrayList."<init>":()V
7: astore_1
8: aload_1
9: ldc #4 // String c
11: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
16: pop
17: aload_1
18: ldc #6 // String 6
20: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
25: pop
26: aload_1
27: ldc #7 // String 4
29: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
34: pop
35: aload_1
36: ldc #8 // String 2
38: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
43: pop
44: aload_1
45: invokeinterface #9, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
50: astore_2
51: aload_2
52: invokeinterface #10, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z
57: ifeq 86
60: aload_2
61: invokeinterface #11, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
66: checkcast #12 // class java/lang/String
69: ldc #4 // String c
71: invokevirtual #13 // Method java/lang/String.equals:(Ljava/lang/Object;)Z
74: ifeq 51
77: aload_2
78: invokeinterface #14, 1 // InterfaceMethod java/util/Iterator.remove:()V
83: goto 51
86: getstatic #15 // Field java/lang/System.out:Ljava/io/PrintStream;
89: aload_1
90: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
93: return
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