ArrayList implementation of Java

ArrayList is a generic class in Java, using the form of arrays to implement the collection list interface to achieve the role of dynamic arrays, in the case of uncertain data volume, the use of ArrayList is a good choice.

In this demo, only the simple implementation of ArrayList, the purpose is to understand the ArrayList work principle, deepen the understanding of ArrayList.

The secret of ArrayList's ability to implement dynamic arrays is to judge every time an element is added, check if the array is full, and if so, create a new array, twice times the original, and copy the old array into the new array, discarding the old array. This is the key to ArrayList, but it is also the flaw, because this expansion of the operation is to compare the impact of performance, especially when adding many elements at once, the array is extended multiple times. For example, to add 10,000 elements at a time, while the initial size of ArrayList is 10, then 10*2^10=10240, that is, the array has 10 expansion operations. But the developers who developed Java were aware of this, so they provided a construction method that could customize the initial size to avoid multiple expansions of the array.

It is also important to note that ArrayList is thread insecure, and if vector () should be used in multi-threading, it is essentially the same as ArrayList, but with more synchronized keywords.

1  Packagemy;2 3 4  Public classMyarraylist<anytype>5 {6     Private Static Final intDefault_capacity = 10;7     8     Private intthesize;9     Privateanytype[] Theiteme;Ten      One      Publicmyarraylist () A {Clear ();} -      -      Public voidClear () the     {  -thesize = 0; - ensurecapacity (default_capacity); -     } +      -      Public intsize () +{returnthesize;} A      at      Public BooleanIsEmpty () -{returnThesize = = 0; } -      -      Public voidtrimtosize () - {ensurecapacity (thesize);} -      in      PublicAnyType Get (intindex) -     {  to         if(Index < 0 | | index >size ()) +             Throw Newarrayindexoutofboundsexception (); -          the         returnTheiteme[index]; *     } $     Panax Notoginseng      PublicAnyType Set (AnyType newvalue,intindex) -     { the         if(Index < 0 | | index >size ()) +             Throw Newarrayindexoutofboundsexception (); A          theAnyType old =Theiteme[index]; +Theiteme[index] =newvalue; -          $         returnOld ; $     } -      -      Public voidAdd (AnyType x,intindex) the     { -         if(Theiteme.length = =size ())Wuyi         { theEnsurecapacity (Size () + 1); -         } Wu          -          for(inti = size (); i > Index; i--) About         { $Theiteme[i] = theiteme[i-1]; -         } -Theiteme[index] =x; -thesize++; A     } +      the      Public BooleanAdd (AnyType x) -     { $ Add (x, size ()); the         return true; the     } the      the      PublicAnyType Remove (intindex) -     { in         if(Index < 0 | | Index <size ()) the             Throw Newarrayindexoutofboundsexception (); theAnyType value =Theiteme[index]; About          for(inti = index; I < Size ()-1; i++) the         { theTheiteme[i] = theiteme[i + 1]; the         } +         returnvalue; -     } the     Bayi      PublicJava.util.iterator<anytype>iterator () the{return Newarraylistiterator ();} the      -      -     Private voidEnsurecapacity (intnewcapacity) the     { the         if(Newcapacity <thesize) the             return; the          -anytype[] Old =Theiteme; the          theTheiteme = (anytype[])Newobject[newcapacity]; the          for(inti = 0; I < size (); i++ )94         { theTheiteme[i] =Old[i]; the         } the     }98      About     Private classArraylistiteratorImplementsJava.util.iterator<anytype> -     {101         Private intCurrent = 0;102         103          Public BooleanHasnext ()104{returnCurrent <size ();} the         106          PublicAnyType Next ()107         {108             if(!Hasnext ())109                 Throw Newjava.util.NoSuchElementException (); the             returntheiteme[current++];111         } the         113          Public voidRemove () the{myarraylist. This. Remove (-Current );} the     } the}

ArrayList implementation of Java