Add ArrayList to another ArrayList in java

Question

I am having the following java code  in which I am trying to copy the ArrayList to another ArrayList     ArrayList lt String gt  nodes   new ArrayList lt String gt      ArrayList NodeList new ArrayList     ArrayList list new ArrayList             for int i 0 i lt PropertyNode getLength  -1 i                 Node childNode    PropertyNode item i                   NodeList Children   childNode getChildNodes                     if Children  null                       nodes clear                        nodes add  PropertyStart                        nodes add Children item 3  getTextContent                         nodes add Children item 7  getTextContent                         nodes add Children item 9  getTextContent                         nodes add Children item 11  getTextContent                         nodes add Children item 13  getTextContent                         nodes add  PropertyEnd                                          NodeList addAll nodes                   list add NodeList               I want the  list  array to be in this format      PropertyStart a b c PropertyEnd   PropertyStart d e f PropertyEnd   PropertyStart            But from the above code  the  list  array output is seen like this    PropertyStart a b c PropertyEnd PropertyStart d e f PropertyEnd PropertyStart     PropertyEnd    I think you might have noticed the difference  I am not able to achieve the result in expected format   Please suggest me any solution    Thanks in advance

User · Accepted Answer

Then you need a ArrayList of ArrayLists   ArrayList lt ArrayList lt String gt  gt  nodes   new ArrayList lt ArrayList lt String gt  gt     ArrayList lt String gt  nodeList   new ArrayList lt String gt     nodes add nodeList     Note that NodeList has been changed to nodeList  In Java Naming Conventions variables start with a lower case  Classes start with an upper case

User · Answer

Your Problem  Mainly  you ve got 2 major problems   You are using adding a List of Strings  You want a List containing Lists of Strings   Note as well that when you invoke this   NodeList addAll nodes         all you say is to add all elements of nodes  which is a list of Strings  to the  badly named  NodeList  which is using Objects and thus adds only the strings inside  Which leads me to the next point   You seem to be confused between your nodes and NodeList  Your NodeList keeps growing over time  and that s what you add to your list   So  even if doing things right  if we were to look at the end of each iteration at your nodes  nodeList and list  we d see    i   0  nodes   PropertyStart a b c PropertyEnd  nodeList   PropertyStart a b c PropertyEnd  list    PropertyStart a b c PropertyEnd    i   1  nodes   PropertyStart d e f PropertyEnd  nodeList   PropertyStart a b c PropertyEnd  PropertyStart d e f PropertyEnd  list    PropertyStart a b c PropertyEnd   PropertyStart a b c PropertyEnd  PropertyStart d e f PropertyEnd    i   2  nodes   PropertyStart g h i PropertyEnd  nodeList   PropertyStart a b c PropertyEnd PropertyStart d e f PropertyEnd PropertyStart g h i PropertyEnd  list    PropertyStart a b c PropertyEnd   PropertyStart a b c PropertyEnd  PropertyStart d e f PropertyEnd   PropertyStart a b c PropertyEnd PropertyStart d e f PropertyEnd PropertyStart g h i PropertyEnd    and so on      Some Other Corrections  Follow the Java Naming Conventions  Don t use variable names starting with uppercase letters  So here  replace NodeList with nodeList    Learn a Bit More About Types  You say  I want the  list  array         This is confusing for whoever you will be communicating with  It s not an array  It s an implementation of List backed by an array   There s a difference between a type  an interface  and an implementation   Use Generics for Stronger Typing in Collections  Use generic types  because static typing really helps with these errors  Also  use interfaces where possible  except if you have a good reason to use the concrete type   So your code becomes   List lt String gt  nodes   new ArrayList lt String gt     List lt String gt  nodeList   new ArrayList lt String gt     List lt List lt String gt  gt  list   new ArrayList lt List lt String gt  gt       Remove Unnecessary Code  You could do away with the nodeList entirely  and write the following once you ve fixed your types   list add nodes     Use the Right Scope  Except if you have a very strong reason to do so  prefer to use the inner-most scope to declare variables and limit both their lifespan for their references and facilitate the separation of concerns in your code   Here you could then move List lt String gt  nodes to be declared within the loop  and then forget the nodes clear   invocation    A reason not to do this could be performance  as you might want to avoid recreating an ArrayList on each iteration of the loop  but it s very unlikely that s a concern to you  and clean  readable and maintainable code has priority over pre-optimized code    SSCCE  Last but not least  if you want help give us the exact reproducible case with a short  self-Contained  correct example   Here you give us your program s outputs  but don t mention how you got them  so we re left to assume you did a System out println list   And you confused a lot of people  as I think the output you give us is not what you actually got

User · Answer

Wouldn t it just be a case of   ArrayList lt ArrayList lt String gt  gt  outer   new ArrayList lt ArrayList lt String gt  gt     ArrayList lt String gt  nodeList   new ArrayList lt String gt         Fill in nodeList here     outer add nodeList     Repeat as necesary   This should return you a list in the format you specified

User · Answer

Initiate the NodeList inside the for loop and you will get the desired output   ArrayList lt String gt  nodes   new ArrayList lt String gt      ArrayList list new ArrayList             for int i 0 i lt PropertyNode getLength  -1 i                 ArrayList NodeList new ArrayList                Node childNode    PropertyNode item i                   NodeList Children   childNode getChildNodes                     if Children  null                       nodes clear                        nodes add  PropertyStart                        nodes add Children item 3  getTextContent                         nodes add Children item 7  getTextContent                         nodes add Children item 9  getTextContent                         nodes add Children item 11  getTextContent                         nodes add Children item 13  getTextContent                         nodes add  PropertyEnd                                          NodeList addAll nodes                   list add NodeList               Explanation  NodeList is an object which remains same throughout the loop so adding same variable to list in a loop will actually add it only once  The loop is only adding its variables in single NodeList array hence you must be seeing     list           NodeList             and NodeList contains  prostart  a b c proend prostart d e f proend

User · Answer

Very first will declare outer Arraylist which will contain another inner Arraylist inside it  ArrayList  CompletesystemStatusArrayList  ArrayList systemStatusArrayList  CompletesystemStatusArrayList new ArrayList  systemStatusArrayList new ArrayList         systemStatusArrayList add  1        systemStatusArrayList add  2        systemStatusArrayList add  3        systemStatusArrayList add  4        systemStatusArrayList add  5        systemStatusArrayList add  6        systemStatusArrayList add  7        systemStatusArrayList add  8         CompletesystemStatusArrayList add systemStatusArrayList

User · Answer

The problem you have is caused that you use the same ArrayList NodeList over all iterations in main for loop  Each iterations NodeList is enlarged by new elements    After first loop  NodeList has 5 elements  PropertyStart a b c PropertyEnd  and list has 1 element  NodeList   PropertyStart a b c PropertyEnd   After second loop NodeList has 10 elements  PropertyStart a b c PropertyEnd PropertyStart d e f PropertyEnd  and list has 2 elements  NodeList  with 10 elements   NodeList  with 10 elements     To get you expectations you must replace   NodeList addAll nodes   list add NodeList    by  List childrenList   new ArrayList nodes   list add childrenList     PS  Your code is not readable  keep Java code conventions to have readble code  For example is hard to recognize if NodeList is a class or object

[java] Add ArrayList to another ArrayList in java

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