ArrayList及HashMap的扩容规则讲解
1、ArrayList
默认大小为10
/** *Defaultinitialcapacity. */ privatestaticfinalintDEFAULT_CAPACITY=10;
最大容量为2^30-8
/**
*Themaximumsizeofarraytoallocate.
*SomeVMsreservesomeheaderwordsinanarray.
*Attemptstoallocatelargerarraysmayresultin
*OutOfMemoryError:RequestedarraysizeexceedsVMlimit
*/
privatestaticfinalintMAX_ARRAY_SIZE=Integer.MAX_VALUE-8;
/**
*Aconstantholdingthemaximumvaluean{@codeint}can
*have,231-1.
*/
publicstaticfinalintMAX_VALUE=0x7fffffff;
扩容规则为：oldCapacity*1.5
/**
*Increasesthecapacitytoensurethatitcanholdatleastthe
*numberofelementsspecifiedbytheminimumcapacityargument.
*@paramminCapacitythedesiredminimumcapacity
*/
privatevoidgrow(intminCapacity){
//overflow-consciouscode
intoldCapacity=elementData.length;
intnewCapacity=oldCapacity+(oldCapacity>>1);
if(newCapacity-minCapacity<0)
newCapacity=minCapacity;
if(newCapacity-MAX_ARRAY_SIZE>0)
newCapacity=hugeCapacity(minCapacity);
//minCapacityisusuallyclosetosize,sothisisawin:
elementData=Arrays.copyOf(elementData,newCapacity);
}
2、HashMap
默认大小：16
/** *Thedefaultinitialcapacity-MUSTbeapoweroftwo. */ staticfinalintDEFAULT_INITIAL_CAPACITY=1<<4;//aka16
最大容量为：2^30
/** *Themaximumcapacity,usedifahighervalueisimplicitlyspecified *byeitheroftheconstructorswitharguments. *MUSTbeapoweroftwo<=1<<30. */ staticfinalintMAXIMUM_CAPACITY=1<<30;
扩容规则为：大于oldCapacity的最小的2的n次方整数
/**
*Addsanewentrywiththespecifiedkey,valueandhashcodeto
*thespecifiedbucket.Itistheresponsibilityofthis
*methodtoresizethetableifappropriate.
*Subclassoverridesthistoalterthebehaviorofputmethod.
*/
voidaddEntry(inthash,Kkey,Vvalue,intbucketIndex){
if((size>=threshold)&&(null!=table[bucketIndex])){
resize(2*table.length);
hash=(null!=key)?hash(key):0;
bucketIndex=indexFor(hash,table.length);
}
createEntry(hash,key,value,bucketIndex);
}
总结
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