(基于JDK1.8源码分析)
一,List接口
1,继承Collection接口,实现了集合的有序存储; 对元素位置进行精确控制,根据索引对集合进行访问和遍历;
2,源码分析
public interface List<E> extends Collection<E> { int size(); boolean isEmpty(); boolean contains(Object o); Iterator<E> iterator(); Object[] toArray(); <T> T[] toArray(T[] a); boolean add(E e); boolean remove(Object o); boolean containsAll(Collection<?> c); boolean addAll(Collection<? extends E> c); boolean addAll(int index, Collection<? extends E> c); boolean removeAll(Collection<?> c); boolean retainAll(Collection<?> c); /***/ default void replaceAll(UnaryOperator<E> operator) { Objects.requireNonNull(operator); final ListIterator<E> li = this.listIterator(); while (li.hasNext()) { li.set(operator.apply(li.next())); } } /***/ @SuppressWarnings({"unchecked", "rawtypes"}) default void sort(Comparator<? super E> c) { Object[] a = this.toArray(); Arrays.sort(a, (Comparator) c); ListIterator<E> i = this.listIterator(); for (Object e : a) { i.next(); i.set((E) e); } } void clear(); boolean equals(Object o); int hashCode();
/** 返回指定索引位置元素*/ E get(int index); /** 设置指定索引位置元素*/ E set(int index, E element); /** 在指定索引位置新增元素*/ void add(int index, E element); /** 删除指定索引位置元素*/ E remove(int index); /** 返回指定对象最小索引位置*/ int indexOf(Object o); /** 返回指定对象最大索引位置,主要用于重复元素*/ int lastIndexOf(Object o); /** 返回迭代器默认索引位置为0; 支持前后遍历,原理是改变集合当前的索引位置*/ ListIterator<E> listIterator(); /** 返回迭代器并指定默认索引位置为index*/ ListIterator<E> listIterator(int index);
/** 返回指定索引间的元素集合*/ List<E> subList(int fromIndex, int toIndex);
/***/ @Override default Spliterator<E> spliterator() { return Spliterators.spliterator(this, Spliterator.ORDERED); }
二,ArrayList实现
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
RandomAccess: List实现使用的标记接口,用于提高连续或随机访问性能
Cloneable: 实现Cloneable可以对该类实例进行克隆
Serializable:用于表示可序列化
【1】底层实现
ArrayList底层通过数组实现
transient Object[] elementData; //transient避免Serializable持久化
【2】构造方法
/** 指定空间大小*/ 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); } } /** 默认空间大小为10*/ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } /** 将指定集合转为数组 1-集合转数组 2-数组大小为0使用空数组替换 3-数组大小>0进行数组拷贝*/ 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; } }
【3】新增
/** 设置指定索引位置元素 1-检查指定索引是否向前越界 2-获取指定索引位置原元素 3-将指定元素赋值到指定索引位置*/ public E set(int index, E element) { rangeCheck(index); //(index >= size)向前越界 E oldValue = elementData(index); elementData[index] = element; return oldValue; } /** 新增元素 1-容器扩容 2-将指定元素赋值到最大索引位置 */ public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; } /** 新增元素到指定索引位置 1-检查索引是否越界 2-容器扩容 3-将指定索引位置右端元素右移 4-设置指定元素到指定索引位置*/ public void add(int index, E element) { rangeCheckForAdd(index); //(index > size || index < 0)越界 ensureCapacityInternal(size + 1); // Increments modCount!! System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; }
/** 新增集合中所有元素 1-将指定集合转数组 2-容器扩容 3-将转后数组添加到扩容前最大索引位置 4-容器扩容,大小为指定集合大小*/ public boolean addAll(Collection<? extends E> c) { Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount System.arraycopy(a, 0, elementData, size, numNew); size += numNew; return numNew != 0; } /** 新增集合中所有元素到指定索引位置 1-检查指定索引越界 2-指定集合转数组 3-容器扩容 4-将扩容前最大索引位置右端元素右移 5-将换后数组添加到扩容前最大索引位置 6-容器扩容*/ public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); //(index > size || index < 0) 越界 Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); // Increments modCount int numMoved = size - index; if (numMoved > 0) System.arraycopy(elementData, index, elementData, index + numNew, numMoved); System.arraycopy(a, 0, elementData, index, numNew); size += numNew; return numNew != 0; }
【4】查询
/** 获取指定索引位置元素 1-检查索引向前越界 2-获取指定索引位置元素*/ public E get(int index) { rangeCheck(index); //(index >= size) 向前越界 return elementData(index); }
【5】删除 (①指定索引位置 ②指定元素)
/** 删除指定索引位置元素 1-检查索引向前越界 2-获取删除后需要左移元素总数 3-将指定索引+1位元素左移 4-将最大索引位置空*/ public E remove(int index) { rangeCheck(index); //(index >= size) 向前越界 modCount++; E oldValue = elementData(index); 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 return oldValue; } /** 删除指定元素在容器中首次出现的元素 */ public boolean remove(Object o) { if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; //只删除首次出现的元素 } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; //只删除首次出现的元素 } } return false; } /* 类似remove(int index)但是无索引越界检查 */ 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 } /** 容器清空 */ public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; }
【6】容器扩容
/** 容器扩容-1 */ 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); } } /** 容器扩容-2*/ private void ensureCapacityInternal(int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); } /** 新增元素集合之后的size > 集合容量,进行50%扩容 */ 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); }
【7】将底层数组大小调整为实际集合中元素数量
/** * Trims the capacity of this <tt>ArrayList</tt> instance to be the * list's current size. An application can use this operation to minimize * the storage of an <tt>ArrayList</tt> instance. */ public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } }
【8】Fail-fast(快速失败机制)
modCount++ :记录修改此列表的次数(包括:修改列表元素,大小,顺序)
定义:ArrayList是线程不安全的,如果使用迭代器过程中其他线程修改了List,会出现异常(ConcurrentModificationException)
原理:ArrayList使用迭代器时,只允许迭代器对List进行remove,add,如果其他线程对List的修改迭代器会立刻出现异常,快速失败
【9】迭代器
checkForComodification(): 用于检查快速失败机制
多线程同时操作同一个ArrayList时发现modCount不一致,出现快速失败ConcurrentModificationException异常
/** 返回迭代器 */ public Iterator<E> iterator() { return new Itr(); }
private class Itr implements Iterator<E> { int cursor; // index of next element to return int lastRet = -1; // index of last element returned; -1 if no such int expectedModCount = modCount; public boolean hasNext() { return cursor != size; } @SuppressWarnings("unchecked") 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]; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } @Override @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = ArrayList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[i++]); } // update once at end of iteration to reduce heap write traffic cursor = i; lastRet = i - 1; checkForComodification(); } final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } }
【10】子列表
subList(int fromIndex, int toIndex)对子列表的修改会同时修改原列表
List<String> a = new ArrayList<String>(); a.add("a"); a.add("b"); a.add("c"); a.add("d"); List<String> a1 = a.subList(1, 2); a1.add("e"); //此操作会同时修改a和a1列表 a列表: a,b,e,c,d a1列表:b,e a1.remove("b"); //此操作会同时修改a和a1列表 a列表: a,e,c,d a1列表:e
源码:
/** 获取指定索引段之间的元素并以列表形式返回*/ public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, 0, fromIndex, toIndex); } private class SubList extends AbstractList<E> implements RandomAccess { private final AbstractList<E> parent; private final int parentOffset; private final int offset; int size; SubList(AbstractList<E> parent, int offset, int fromIndex, int toIndex) { this.parent = parent; this.parentOffset = fromIndex; this.offset = offset + fromIndex; this.size = toIndex - fromIndex; this.modCount = ArrayList.this.modCount; } public E set(int index, E e) { rangeCheck(index); checkForComodification(); E oldValue = ArrayList.this.elementData(offset + index); ArrayList.this.elementData[offset + index] = e; return oldValue; } public E get(int index) { rangeCheck(index); checkForComodification(); return ArrayList.this.elementData(offset + index); } public int size() { checkForComodification(); return this.size; } public void add(int index, E e) { rangeCheckForAdd(index); checkForComodification(); parent.add(parentOffset + index, e); this.modCount = parent.modCount; this.size++; } public E remove(int index) { rangeCheck(index); checkForComodification(); E result = parent.remove(parentOffset + index); this.modCount = parent.modCount; this.size--; return result; } protected void removeRange(int fromIndex, int toIndex) { checkForComodification(); parent.removeRange(parentOffset + fromIndex, parentOffset + toIndex); this.modCount = parent.modCount; this.size -= toIndex - fromIndex; } public boolean addAll(Collection<? extends E> c) { return addAll(this.size, c); } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); int cSize = c.size(); if (cSize==0) return false; checkForComodification(); parent.addAll(parentOffset + index, c); this.modCount = parent.modCount; this.size += cSize; return true; } public Iterator<E> iterator() { return listIterator(); } public ListIterator<E> listIterator(final int index) { checkForComodification(); rangeCheckForAdd(index); final int offset = this.offset; return new ListIterator<E>() { int cursor = index; int lastRet = -1; int expectedModCount = ArrayList.this.modCount; public boolean hasNext() { return cursor != SubList.this.size; } @SuppressWarnings("unchecked") public E next() { checkForComodification(); int i = cursor; if (i >= SubList.this.size) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i + 1; return (E) elementData[offset + (lastRet = i)]; } public boolean hasPrevious() { return cursor != 0; } @SuppressWarnings("unchecked") public E previous() { checkForComodification(); int i = cursor - 1; if (i < 0) throw new NoSuchElementException(); Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) throw new ConcurrentModificationException(); cursor = i; return (E) elementData[offset + (lastRet = i)]; } @SuppressWarnings("unchecked") public void forEachRemaining(Consumer<? super E> consumer) { Objects.requireNonNull(consumer); final int size = SubList.this.size; int i = cursor; if (i >= size) { return; } final Object[] elementData = ArrayList.this.elementData; if (offset + i >= elementData.length) { throw new ConcurrentModificationException(); } while (i != size && modCount == expectedModCount) { consumer.accept((E) elementData[offset + (i++)]); } // update once at end of iteration to reduce heap write traffic lastRet = cursor = i; checkForComodification(); } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } public void remove() { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { SubList.this.remove(lastRet); cursor = lastRet; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(offset + lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; SubList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = ArrayList.this.modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } final void checkForComodification() { if (expectedModCount != ArrayList.this.modCount) throw new ConcurrentModificationException(); } }; } public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); return new SubList(this, offset, fromIndex, toIndex); } private void rangeCheck(int index) { if (index < 0 || index >= this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private void rangeCheckForAdd(int index) { if (index < 0 || index > this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+this.size; } private void checkForComodification() { if (ArrayList.this.modCount != this.modCount) throw new ConcurrentModificationException(); } public Spliterator<E> spliterator() { checkForComodification(); return new ArrayListSpliterator<E>(ArrayList.this, offset, offset + this.size, this.modCount); } }
【11】ArrayList和Vector区别
ArrayList:异步
Vector:同步
同步:
定义:多线程对于共享资源同一时间点只允许一个线程占用资源,其他线程等待 (线程安全)
适用:多个线程共享同一个数据,就适用同步,否则会出现例如数据库脏读,不可重复读,幻读等现象
异步:
定义:只有一个线程访问当前数据 (线程不安全)
package com.test; import java.util.ArrayList; import java.util.Iterator; public class Test implements Runnable { private ArrayList<String> aa = new ArrayList<String>(); @Override public void run() { aa.set(0, "Thread"); } public String get() { aa.add("ThreadMain"); System.out.println(aa.get(0).toString()); Iterator it = aa.iterator(); return aa.get(0).toString(); } public static void main(String[] args) throws InterruptedException { Test test = new Test(); new Thread(test).start(); System.out.println(test.get()); /** * 执行结果: ThreadMain Thread */ } }