Converting List Integer to List String

Question

I have a list of integers  List lt Integer gt  and I d like to convert all the integer objects into Strings  thus finishing up with a new List lt String gt    Naturally  I could create a new List lt String gt  and loop through the list calling String valueOf   for each integer  but I was wondering if there was a better  read  more automatic  way of doing it

User · Answer

Lambdaj allows to do that in a very simple and readable way. For example, supposing you have a list of Integer and you want to convert them in the corresponding String representation you could write something like that;

List<Integer> ints = asList(1, 2, 3, 4);
Iterator<String> stringIterator = convertIterator(ints, new Converter<Integer, String> {
    public String convert(Integer i) { return Integer.toString(i); }
}

Lambdaj applies the conversion function only while you're iterating on the result.

User · Answer

I think using Object toString   for any purpose other than debugging is probably a really bad idea  even though in this case the two are functionally equivalent  assuming the list has no nulls    Developers are free to change the behavior of any toString   method without any warning  including the toString   methods of any classes in the standard library   Don t even worry about the performance problems caused by the boxing unboxing process   If performance is critical  just use an array   If it s really critical  don t use Java   Trying to outsmart the JVM will only lead to heartache

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What you re doing is fine  but if you feel the need to  Java-it-up  you could use a Transformer and the collect method from Apache Commons  e g    public class IntegerToStringTransformer implements Transformer lt Integer  String gt       public String transform final Integer i          return  i    null   null   i toString                and then    CollectionUtils collect     collectionOfIntegers      new IntegerToStringTransformer        newCollectionOfStrings

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As far as I know  iterate and instantiate is the only way to do this   Something like  for others potential help  since I m sure you know how to do this    List lt Integer gt  oldList          Specify the size of the list up front to prevent resizing     List lt String gt  newList   new ArrayList lt  gt  oldList size     for  Integer myInt   oldList       newList add String valueOf myInt

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Using Streams   If let say result is list of integers  List lt Integer gt  result  then    List lt String gt  ids    List lt String gt   result stream   map intNumber - gt  Integer toString intNumber   collect Collectors toList       One of the ways to solve it  Hope this helps

User · Answer

I just wanted to chime in with an object oriented solution to the problem   If you model domain objects  then the solution is in the domain objects   The domain here is a List of integers for which we want string values   The easiest way would be to not convert the list at all   That being said  in order to convert without converting  change the original list of Integer to List of Value  where Value looks something like this     class Value       Integer value      public Integer getInt                return value            public String getString                return String valueOf value             This will be faster and take up less memory than copying the List   Good Luck

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Another Solution using Guava and Java 8  List lt Integer gt  numbers   Arrays asList 1  2  3  4  5   List lt String gt  strings   Lists transform numbers  number - gt  String valueOf number

User · Answer

Jonathan  I could be mistaken  but I believe that String valueOf   in this case will call the String valueOf Object  function rather than getting boxed to String valueOf int    String valueOf Object  just returns  null  if it is null or calls Object toString   if non-null  which shouldn t involve boxing  although obviously instantiating new string objects is involved

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The source for String valueOf shows this   public static String valueOf Object obj        return  obj    null     null    obj toString        Not that it matters much  but I would use toString

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An answer for experts only       List lt Integer gt  ints            String all   new ArrayList lt Integer gt  ints  toString        String   split   all substring 1  all length  -1  split            List lt String gt  strs   Arrays asList split

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List lt String gt  stringList   integerList stream   map  Object s - gt String valueOf s   collect Collectors toList

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Just for fun  a solution using the jsr166y fork-join framework that should in JDK7   import java util concurrent forkjoin     private final ForkJoinExecutor executor   new ForkJoinPool        List lt Integer gt  ints        List lt String gt  strs       ParallelArray create ints size    Integer class  executor       withMapping new Ops Op lt Integer String gt      public String op Integer i            return String valueOf i                all        asList       Disclaimer  Not compiled  Spec is not finalised  Etc    Unlikely to be in JDK7 is a bit of type inference and syntactical sugar to make that withMapping call less verbose        withMapping   Integer i  String valueOf i

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A slightly more concise solution using the forEach method on the original list       List lt Integer gt  oldList   Arrays asList 1  2  3  4  5       List lt String gt  newList   new ArrayList lt  gt  oldList size         oldList forEach e - gt  newList add String valueOf e

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Solution for Java 8  A bit longer than the Guava one  but at least you don t have to install a library   import java util Arrays  import java util List  import java util stream Collectors          List lt Integer gt  integers   Arrays asList 1  2  3  4   List lt String gt  strings   integers stream   map Object  toString                                           collect Collectors toList

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Here s a one-liner solution without cheating with a non-JDK library   List lt String gt  strings   Arrays asList list toString   replaceAll                 1   split

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Using Google Collections from Guava-Project  you could use the transform method in the Lists class  import com google common collect Lists  import com google common base Functions  List lt Integer gt  integers   Arrays asList 1  2  3  4    List lt String gt  strings   Lists transform integers  Functions toStringFunction       The List returned by transform is a view on the backing list - the transformation will be applied on each access to the transformed list   Be aware that Functions toStringFunction   will throw a NullPointerException when applied to null  so only use it if you are sure your list will not contain null

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I didn t see any solution which is following the principal of space   complexity  If list of integers has large number of elements then it s   big problem    It will be really good to remove the integer from the List lt Integer gt  and free the space  once it s added to List lt String gt     We can use iterator to achieve the same        List lt Integer gt  oldList   new ArrayList lt  gt         oldList add 12       oldList add 14                                List lt String gt  newList   new ArrayList lt String gt  oldList size         Iterator lt Integer gt  itr   oldList iterator        while itr hasNext             newList add itr next   toString             itr remove

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To the people concerned about  boxing  in jsight s answer  there is none  String valueOf Object  is used here  and no unboxing to int is ever performed   Whether you use Integer toString   or String valueOf Object  depends on how you want to handle possible nulls  Do you want to throw an exception  probably   or have  null  Strings in your list  maybe   If the former  do you want to throw a NullPointerException or some other type   Also  one small flaw in jsight s response  List is an interface  you can t use the new operator on it  I would probably use a java util ArrayList in this case  especially since we know up front how long the list is likely to be

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You can t avoid the  boxing overhead   Java s faux generic containers can only store Objects  so your ints must be boxed into Integers   In principle it could avoid the downcast from Object to Integer  since it s pointless  because Object is good enough for both String valueOf and Object toString  but I don t know if the compiler is smart enough to do that   The conversion from String to Object should be more or less a no-op  so I would be disinclined to worry about that one

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Instead of using String valueOf I d use  toString    it avoids some of the auto boxing described by  johnathan holland  The javadoc says that valueOf returns the same thing as Integer toString     List lt Integer gt  oldList       List lt String gt  newList   new ArrayList lt String gt  oldList size      for  Integer myInt   oldList       newList add myInt toString

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This is such a basic thing to do I wouldn t use an external library  it will cause a dependency in your project that you probably don t need    We have a class of static methods specifically crafted to do these sort of jobs  Because the code for this is so simple we let Hotspot do the optimization for us  This seems to be a theme in my code recently  write very simple  straightforward  code and let Hotspot do its magic  We rarely have performance issues around code like this - when a new VM version comes along you get all the extra speed benefits etc   As much as I love Jakarta collections  they don t support Generics and use 1 4 as the LCD  I am wary of Google Collections because they are listed as Alpha support level

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Not core Java  and not generic-ified  but the popular Jakarta commons collections library has some useful abstractions for this sort of task   Specifically  have a look at the collect methods on  CollectionUtils  Something to consider if you are already using commons collections in your project

[java] Converting List<Integer> to List<String>

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