How to print out all the elements of a List in Java

Question

I am trying to print out all the elements of a List  however it is printing the pointer of the Object rather than the value   This is my printing code     for int i 0 i lt list size   i         System out println list get i         Could anyone please help me why it isn t printing the value of the elements

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For a list of array of String  list forEach s - gt  System out println Arrays toString  String    s

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Consider a List lt String gt  stringList which can be printed in many ways using Java 8 constructs   stringList forEach System out  println                                 1  Iterable forEach stringList stream   forEach System out  println                        2  Stream forEach  order maintained generally but doc does not guarantee  stringList stream   forEachOrdered System out  println                 3  Stream forEachOrdered  order maintained always  stringList parallelStream   forEach System out  println                4  Parallel version of Stream forEach  order not maintained  stringList parallelStream   forEachOrdered System out  println         5  Parallel version ofStream forEachOrdered  order maintained always    How are these approaches different from each other   First Approach  Iterable forEach - The iterator of the collection is generally used and that is designed to be fail-fast which means it will throw ConcurrentModificationException if the underlying collection is structurally modified during the iteration  As mentioned in the doc for ArrayList      A structural modification is any operation that adds or deletes one or   more elements  or explicitly resizes the backing array  merely setting   the value of an element is not a structural modification    So it means for ArrayList forEach setting the value is allowed without any issue  And in case of concurrent collection e g  ConcurrentLinkedQueue the iterator would be weakly-consistent which means the actions passed in forEach are allowed to make even structural changes without ConcurrentModificationExceptionexception being thrown  But here the modifications might or might not be visible in that iteration   Second Approach  Stream forEach - The order is undefined  Though it may not occur for sequential streams but the specification does not guarantee it  Also the action is required to be non-interfering in nature  As mentioned in doc      The behavior of this operation is explicitly nondeterministic  For   parallel stream pipelines  this operation does not guarantee to   respect the encounter order of the stream  as doing so would sacrifice   the benefit of parallelism    Third Approach  Stream forEachOrdered - The action would be performed in the encounter order of the stream  So whenever order matters use forEachOrdered without a second thought  As mentioned in the doc      Performs an action for each element of this stream  in the encounter   order of the stream if the stream has a defined encounter order    While iterating over a synchronized collection the first approach would take the collection s lock once and would hold it across all the calls to action method  but in case of streams they use collection s spliterator  which does not lock and relies on the already established rules of non-interference  In case collection backing the stream is modified during iteration a ConcurrentModificationException would be thrown or inconsistent result may occur     Fourth Approach  Parallel Stream forEach - As already mentioned no guarantee to respect the encounter order as expected in case of parallel streams  It is possible that action is performed in different thread for different elements which can never be the case with forEachOrdered   Fifth Approach  Parallel Stream forEachOrdered - The forEachOrdered will process the elements in the order specified by the source irrespective of the fact whether stream is sequential or parallel  So it makes no sense to use this with parallel streams

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The following is compact and avoids the loop in your example code  and gives you nice commas    System out println Arrays toString list toArray        However  as others have pointed out  if you don t have sensible toString   methods implemented for the objects inside the list  you will get the object pointers  hash codes  in fact  you re observing  This is true whether they re in a list or not

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System out println list   works for me   Here is a full example   import java util List      import java util ArrayList   public class HelloWorld        public static void main String   args            final List lt String gt  list   new ArrayList lt  gt             list add  Hello            list add  World            System out println list              It will print  Hello  World

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List lt String gt  textList   messageList stream                                map Message  getText                               collect Collectors toList              textList stream   forEach System out  println           public class Message             String name          String text           public Message String name  String text                this name   name              this text   text                     public String getName                 return name                   public String getText             return text

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Use String join   for example  System out print String join  quot  n quot   list

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Or you could simply use the Apache Commons utilities   https   commons apache org proper commons-lang apidocs org apache commons lang3 ArrayUtils html toString-java lang Object-  List lt MyObject gt  myObjects       System out println ArrayUtils toString myObjects

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You haven t specified what kind of elements the list contains  if it is a primitive data type then you can print out the elements  But if the elements are objects then as Kshitij Mehta mentioned you need to implement  override  the method  toString  within that object - if it is not already implemented - and let it return something meaning full from within the object  example   class Person       private String firstName      private String lastName        Override     public String toString             return this firstName         this lastName

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I happen to be working on this now     List lt Integer gt  a   Arrays asList 1  2  3   List lt Integer gt  b   Arrays asList 3  4   List lt int   gt  pairs   a stream      flatMap x - gt  b stream   map y - gt  new int   x  y       collect Collectors toList      Consumer lt int   gt  pretty   xs - gt  System out printf   n  d  d    xs 0   xs 1    pairs forEach pretty

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public static void main String   args            answer 10 60              public static void answer int m int k           AtomicInteger n   new AtomicInteger m           Stream lt Integer gt  stream   Stream generate    - gt  n incrementAndGet    limit k           System out println Arrays toString stream toArray

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Here is some example about getting print out the list component   public class ListExample        public static void main String   args            List lt Model gt  models   new ArrayList lt  gt                 TODO  First create your model and add to models ArrayList  to prevent NullPointerException for trying this example             Print the name from the list             for Model model   models                System out println model getName                           Or like this            for int i   0  i  lt  models size    i                  System out println models get i  getName                        class Model        private String name       public String getName             return name             public void setName String name            this name   name

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try to override toString   method as you want that the element will be printend   so the method to print can be this   for int i 0 i lt list size   i         System out println list get i  toString

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I wrote a dump function  which basicly prints out the public members of an object if it has not overriden toString    One could easily expand it to call getters  Javadoc      Dumps an given Object to System out  using the following rules         If the Object is Iterable  all of its components are dumped     If the Object or one of its superclasses overrides toString    the  toString  is dumped     Else the method is called recursively for all public members of the Object              Dumps an given Object to System out  using the following rules  lt br gt      lt ul gt      lt li gt  If the Object is   link Iterable   all of its components are dumped  lt  li gt      lt li gt  If the Object or one of its superclasses overrides   link  toString     the  toString  is dumped lt  li gt      lt li gt  Else the method is called recursively for all public members of the Object  lt  li gt      lt  ul gt      param input     throws Exception     public static void dump Object input  throws Exception      dump input  0      private static void dump Object input  int depth  throws Exception      if input  null           System out print  null n  indent depth            return             Class lt   extends Object gt  clazz   input getClass        System out print clazz getSimpleName             if input instanceof Iterable lt   gt            for Object o    Iterable lt   gt  input                System out print   n  indent depth 1                dump o  depth 1                  else if clazz getMethod  toString   getDeclaringClass   equals Object class            Field   fields   clazz getFields            if fields length    0               System out print input   n  indent depth                      System out print   n  indent depth 1            for Field field  fields               Object o   field get input               String s     -   field getName                     System out print s               dump o  depth 1                  else           System out print input   n  indent depth             private static String indent int depth        StringBuilder sb   new StringBuilder        for int i 0  i lt depth  i            sb append            return sb toString

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For loop to print the content of a list     List lt String gt  myList   new ArrayList lt String gt     myList add  AA    myList add  BB     for   String elem   myList       System out println  Element     elem       Result     Element   AA Element   BB   If you want to print in a single line  just for information      String strList   String join       myList   System out println  Elements     strList     Result     Elements   AA  BB

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Since Java 8  List inherits a default  forEach  method which you can combine with the method reference  System out  println  like this   list forEach System out  println

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The Java 8 Streams approach     list stream   forEach System out  println

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The objects in the list must have toString implemented for them to print something meaningful to screen   Here s a quick test to see the differences   public class Test        public class T1           public Integer x             public class T2           public Integer x            Override         public String toString                 return x toString                         public void run             T1 t1   new T1            t1 x   5          System out println t1            T2 t2   new T2            t2 x   5          System out println t2               public static void main String   args                    new Test   run              And when this executes  the results printed to screen are   t1   Test T1 19821f t2   5   Since T1 does not override the toString method  its instance t1 prints out as something that isn t very useful  On the other hand  T2 overrides toString  so we control what it prints when it is used in I O  and we see something a little better on screen

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I have faced similar problems  My code   List lt Integer gt  leaveDatesList   new ArrayList lt  gt           inserted value in list          Way 1  printing a list in a for loop  for int i 0 i lt leaveDatesList size   i         System out println leaveDatesList get i        Way 2  printing the list in a forEach  for loop  for Integer leave   leaveDatesList       System out println leave       Way 3  printing the list in java 8  leaveDatesList forEach System out  println

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list stream   map x - gt  x getName    forEach System out  println

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System out println list    toString   is easy and good enough for debugging    toString   of AbstractCollection  will be clean and easy enough to do that  AbstractList is a subclass of AbstractCollection  so no need to for loop and no toArray   needed      Returns a string representation of this collection  The string representation consists of a list of the collection s elements in the   order they are returned by its iterator  enclosed in square brackets           Adjacent elements are separated by the characters       comma   and space   Elements are converted to strings as by   String valueOf Object     If you are using any custom object in your list  say Student   you need to override its toString   method it is always good to override this method  to have a meaningful output  See the below example   public class TestPrintElements        public static void main String   args               Element is String  Integer or other primitive type         List lt String gt  sList   new ArrayList lt String gt             sList add  string1            sList add  string2            System out println sList              Element is custom type         Student st1 new Student 15  Tom            Student st2 new Student 16  Kate            List lt Student gt  stList new ArrayList lt Student gt             stList add st1           stList add st2           System out println stList            public  class Student      private int age      private String name       public Student int age  String name           this age age          this name name              Override     public String toString            return  student   name    age    age            output    string1  string2   student Tom age 15  student Kate age 16

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It depends on what type of objects stored in the List  and whether it has implementation for toString   method  System out println list  should print all the standard java object types  String  Long  Integer etc   In case  if we are using custom object types  then we need to override toString   method of our custom object   Example   class Employee    private String name   private long id     Override  public String toString        return  name      this name                    id      this id             Test   class TestPrintList      public static void main String   args         Employee employee1  new Employee  test1   123        Employee employee2  new Employee  test2   453        List lt Employee gt  employees   new ArrayList 2        employee add employee1        employee add employee2        System out println employees

[java] How to print out all the elements of a List in Java?

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