Adding values to Arraylist

Question

Code 1   ArrayList arr   new ArrayList    arr add 3   arr add  ss      Code 2   ArrayList lt Object gt  arr   new ArrayList lt Object gt     arr add 3   arr add  ss      Code 3   ArrayList lt Object gt  arr   new ArrayList lt Object gt     arr add new Integer 3    arr add new String  ss       all the above three codes are working fine   can some one tell me the which is prefered and why   and why the eclipse compiler always gives warning when type of arguments are not mentioned to the Arraylist     thanks in advance

User · Accepted Answer

First simple rule  never use the String String  constructor  it is absolutely useless       So arr add  ss   is just fine   With 3 it s slightly different  3 is an int literal  which is not an object  Only objects can be put into a List  So the int will need to be converted into an Integer object  In most cases that will be done automagically for you  that process is called autoboxing   It effectively does the same thing as Integer valueOf 3  which can  and will  avoid creating a new Integer instance in some cases   So actually writing arr add 3  is usually a better idea than using arr add new Integer 3    because it can avoid creating a new Integer object and instead reuse and existing one   Disclaimer  I am focusing on the difference between the second and third code blocks here and pretty much ignoring the generics part  For more information on the generics  please check out the other answers       there are some obscure corner cases where it is useful  but once you approach those you ll know never to take absolute statements as absolutes  -

User · Answer

There s a faster and easy way in Java 9 without involving much of code  Using Collection Factory methods   List lt String gt  list   List of  first    second    third

User · Answer

The second form is preferred   ArrayList lt Object gt  arr   new ArrayList lt Object gt     arr add 3   arr add  ss      Always specify generic arguments when using generic types  such as ArrayList lt T gt    This rules out the first form   As to the last form  it is more verbose and does extra work for no benefit

User · Answer

You are getting the warning because ArrayList is part of java generics   Essentially  it s a way to catch your type errors at compile time   For example  if you declare your array list with types Integer  ArrrayList lt Integer gt   and then try to add Strings to it  you ll get an error at compile time - avoiding nasty crashes at runtime   The first syntax is there for backward compatibility and should be avoided whenever possible  note that generics were not there in older versions of java    Second and third examples are pretty much equivalent   As you need to pass an object and not a primitive type to add method  your 3 is internally converted to Integer 3    By writing a string in double-quotes you effectively are creating a String object   When calling String  ss   you are creating a new String object with value being the same as the parameter   ss     Unless you really do need to store different types in your List  I would suggest actually using a proper type declaration  e g  ArrayList lt Integer gt    new ArrayList lt Integer gt    - it ll save you a lot of headache in the long run   If you do need multiple datatypes in the list  then the second example is better

User · Answer

in the first you don t define the type that will be held and linked within your arraylist construct  this is the preferred method to do so  you define the type of list and the ide will handle the rest  in the third one you will better just define List for shorter code

User · Answer

Actually  a third is preferred   ArrayList lt Object gt  array   new ArrayList lt Object gt     array add Integer valueOf 3    array add  ss      This avoids autoboxing  Integer valueOf 3  versus 3  and doesn t create an unnecessary String object   Eclipse complains when you don t use type arguments with a generic type like ArrayList  because you are using something called a raw type  which is discouraged  If a class is generic  that is  it has type parameters   then you should always use type arguments with that class   Autoboxing  on the other hand  is a personal preference  Some people are okay with it  and some not  I don t like it  and I turn on the warning for autoboxing autounboxing

User · Answer

Two last variants are the same  int is wrapped to Integer automatically where you need an Object  If you not write any class in  lt   it will be Object by default  So there is no difference  but it will be better to understanding if you write Object

User · Answer

The second one would be preferred    it avoids unnecessary inefficient constructor calls it makes you specify the element type for the list  if that is missing  you get a warning    However  having two different types of object in the same list has a bit of a bad design smell  We need more context to speak on that

User · Answer

Well by doing the above you open yourself to run time errors  unless you are happy to accept that your arraylists can contains both strings and integers and elephants   Eclipse returns an error because it does not want you to be unaware of the fact that by specifying no type for the generic parameter you are opening yourself up for run time errors   At least with the other two examples you know that you can have objects in your Arraylist and since Inetegers and Strings are both objects Eclipse doesn t warn you   Either code 2 or 3 are ok   But if you know you will have either only ints or only strings in your arraylist then I would do  ArrayList lt Integer gt  arr   new ArrayList lt Integer gt       or  ArrayList lt String gt  arr   new ArrayList lt String gt       respectively

[java] Adding values to Arraylist

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