面试必问之ArrayList
- 2019 年 10 月 3 日
- 笔记
ArrayList??
?1?ArrayList ???????????????????
?2?ArrayList ???????????? ArrayList ???????????????????????????????????????
?3?ArrayList ????????????????? O(1) ?????????????
?4?ArrayList ?????????????????????? ArrayList???????????????
ArrayList?????
?????ArrayList????????????????????DEFAULTCAPACITY_EMPTY_ELEMENTDATA?EMPTY_ELEMENTDATA?????????????????????DEFAULTCAPACITY_EMPTY_ELEMENTDATA????????????
//??????? private static final int DEFAULT_CAPACITY = 10; //??????????????????????????????? private static final Object[] EMPTY_ELEMENTDATA = {}; //????size??????????EMPTY_ELEMENTDATA????? //???????????????????????? private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; //ArrayList????????????????ArrayList?????????? //??????elementData????DEFAULTCAPACITY_EMPTY_ELEMENTDATA???????????? //???????????????????DEFAULT_CAPACITY transient Object[] elementData; // non-private to simplify nested class access //arrayList??? private int size;static???EMPTY_ELEMENTDATA?DEFAULTCAPACITY_EMPTY_ELEMENTDATA
ArrayList????
?1?????????????
- ????0?elementData????initialCapacity?????
- ????0?elementData???????
- ????0?????
//???????? public ArrayList(int initialCapacity) { //???????? if (initialCapacity > 0) { //elementData??????????? this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { //????????0??????????????????(?????) this.elementData = EMPTY_ELEMENTDATA; } else { throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); } }?2?????
- ??????elementData???????DEFAULTCAPACITY_EMPTY_ELEMENTDATA
- ???add??????????????????
- ??????DEFAULT_CAPACITY=10
//??????????size?10?????????????????????????????add????????? public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }?3????Collection??????
//??????Collection??????ArrayList????????????????????????? //??????null???????????c.toArray()?????? public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { //c.toArray()??????????? Object[]???????Arrays.copyOf()?? if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { //????????????????0?????????????????elementData this.elementData = EMPTY_ELEMENTDATA; } }? ??????????????????????????????????????????? elementData(this.elementData=XXX)????????????? elementData ?????????????????????????????????????????? elementData ??????????>= 0????????????????????????????????? ArrayList ??????????????????????
? ??????????????????add????????????????????add?????????
ArrayList?add??
add??????
//????????list??? public boolean add(E e) { //???????????????????????????????????????? ensureCapacityInternal(size + 1); // Increments modCount!!???????fast-fail? elementData[size++] = e; return true; }????add???????????????size???????????ensureCapacityInternal?????
ensureCapacityInternal????
private void ensureCapacityInternal(int minCapacity) { //??????elementData????????? //????????????elementData=DEFAULTCAPACITY_EMPTY_ELEMENTDATA? //????????size+1(?????add???size+1=1)?DEFAULT_CAPACITY? //???????10 if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity); }? ? ? add ??1?????minCapacity?(size+1=0+1=)1??Math.max()??????minCapacity ?10????????ensureExplicitCapacity??modCount????????????
ensureExplicitCapacity????
private void ensureExplicitCapacity(int minCapacity) { modCount++; //????add??????Increments modCount //?? if (minCapacity - elementData.length > 0) grow(minCapacity);//??????????? }? ?????????????grow?
grow????
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; private void grow(int minCapacity) { // oldCapacity??????? int oldCapacity = elementData.length; // newCapacity????????oldCap+oldCap/2:????????1.5?? int newCapacity = oldCapacity + (oldCapacity >> 1); // ?????????????????????????????????????? if (newCapacity - minCapacity < 0) newCapacity = minCapacity; //???????MAX_ARRAY_SIZE???hugeCapacity???? 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); }hugeCapacity??
???????hugeCapacity??
private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); //?minCapacity?MAX_ARRAY_SIZE???? //?minCapacity???Integer.MAX_VALUE???????? //?MAX_ARRAY_SIZE???MAX_ARRAY_SIZE???????? //MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; }add????????
? ???????????????????????????????add?????????
? ???????add??????????capacity?10???
-
?????2?????????ensureCapacityInternal????????size+1=1+1=2?
- ?ensureCapacityInternal????elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA??????????ensureExplicitCapacity??
- ensureExplicitCapacity???minCapacity??????2??????if???2-10=-8???????newCapacity ?? MAX_ARRAY_SIZE???????
grow????????10?add??? return true,size??1? - ?????3?4……10?????????????????grow?????
-
?add?11?????????grow??????newCapacity?15??minCapacity??10+1=11??????if?????????????????size?????hugeCapacity?????????15?add???return true,size??11?
add(int index,E element)??
//????? index ????? public void add(int index, E element) { rangeCheckForAdd(index); //?????index??????? // 1. ???????? ensureCapacityInternal(size + 1); // Increments modCount!! // 2. ? index ??????????????? System.arraycopy(elementData, index, elementData, index + 1, size - index); // 3. ??????? index ? elementData[index] = element; size++; } private void rangeCheckForAdd(int index) { if (index > size || index < 0) //?????index>size???????????index??0 throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }add(int index, E element)??????????????????????????????????
- ????????????(?????????)
- ? index ??????????????
- ??????? index ?.
????????????????????????????????????????????????????????O(N)??????????????????????????????????????????????????????????????????
ArrayList?remove??
ArrayList???????????
1?remove(int index) ??????
public E remove(int index) { rangeCheck(index); //???????????index>size????IndexOutOfBoundsException??? modCount++;//??list??????????? E oldValue = elementData(index); //??????????? int numMoved = size - index - 1;//???????????? //???????0 ???????????(size=index+1) //???0????????????????(?????????) if (numMoved > 0) //??index??????????? ???index?????? System.arraycopy(elementData, index+1, elementData, index, numMoved); //?????????size??null elementData[--size] = null; // clear to let GC do its work //???? return oldValue; } //src:??? //srcPos:?????srcPos??????? //dest:???? //desPos:????srcPos?????????????????????desPos????? //length:???????? public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length);? ?????????
2?remove(Object o) ?????????????????????
public boolean remove(Object o) { //?????null ?????????null if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { //???????null????? ??????????? fastRemove(index); return true; } } else { //????????? ??????????? for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } //????????????????????????????? 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 }ArrayList?????
ensureCapacity??
??? add ??????? ensureCapacity ???????????????
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); } }ArrayList??
?1?ArrayList ????????????????????????????????????10?1.7????????????????add???????????elementData??????10??
?2?ArrayList ???????????? ArrayList ???????????????????????????????????????ArrayList??????????1.5?
?3??? ArrayList ????????????????? O(1) ?????????????
?4?ArrayList ?????????????????????? ArrayList???????????????
?5????????
?6?????????????????????LinkindList?
?7?Integer.MAX_VALUE – 8 ??????????JVM,??JVM?????????,?????????MAX_ARRAY_SIZE,?????????????MAX_ARRAY_SIZE???,?????, ???Integer.MAX_VALUE,???Integer.MAX_VALUE -8? ????jdk1.7?????
fast-fail??
fail-fast????
In systems design, a fail-fast system is one which immediately reports at its interface any condition that is likely to indicate a failure. Fail-fast systems are usually designed to stop normal operation rather than attempt to continue a possibly flawed process. Such designs often check the system’s state at several points in an operation, so any failures can be detected early. The responsibility of a fail-fast module is detecting errors, then letting the next-highest level of the system handle them.
? ????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
? ????????????????????????????????????????????????
//?????????????????????fast_fail_method????????????????????????0????????????????????????????fail-fast???????? public int fast_fail_method(int arg1,int arg2){ if(arg2 == 0){ throw new RuntimeException("can't be zero"); } return arg1/arg2; }? ?Java?????????????????????????????fail-fast????????????????????????Java??fail-fast????????Java??????????????????????????????????????????????????????????ConcurrentModificationException.????????????????foreach???????add/remove???????????????fast-fail???????????
? ??????ConcurrentModificationException?????????????????for???????for???????????iterator?????????add/remove????????????????????iterator??????????????????????????????/?????????????????????????????????Java????????????ConcurrentModificationException?????fail-fast????????????????????????Iterator???fail-fast????????????????????????????????????
