How to search in a List of Java object

Question

I have a List of object and the list is very big  The object is   class Sample       String value1      String value2      String value3      String value4      String value5       Now I have to search for a specific value of an object in the list  Say if value3   three   I have to return those objects  My search is not always based on value3   The list is   List lt Sample gt  list   new ArrayList lt Sample gt       What is the efficient way of doing it   Thanks

User · Answer

As your list is an ArrayList  it can be assumed that it is unsorted  Therefore  there is no way to search for your element that is faster than O n    If you can  you should think about changing your list into a Set  with HashSet as implementation  with a specific Comparator for your sample class   Another possibility would be to use a HashMap  You can add your data as Sample  please start class names with an uppercase letter  and use the string you want to search for as key  Then you could simply use  Sample samp   myMap get myKey     If there can be multiple samples per key  use Map lt String  List lt Sample gt  gt   otherwise use Map lt String  Sample gt   If you use multiple keys  you will have to create multiple maps that hold the same dataset  As they all point to the same objects  space shouldn t be that much of a problem

User · Answer

You can give a try to Apache Commons Collections   There is a class CollectionUtils that allows you to select or filter items by custom Predicate   Your code would be like this   Predicate condition   new Predicate        boolean evaluate Object sample            return   Sample sample  value3 equals  three            List result   CollectionUtils select  list  condition      Update   In java8  using Lambdas and StreamAPI this should be   List lt Sample gt  result   list stream         filter item - gt  item value3 equals  three          collect Collectors toList       much nicer

User · Answer

I modifie this list and add a List to the samples try this  Pseudocode  Sample      List lt String gt  values     List lt String gt  getList        return values       for Sample s   list       if s getString getList contains  three           return s

User · Answer

Using Java 8  With Java 8 you can simply convert your list to a stream allowing you to write   import java util List  import java util stream Collectors   List lt Sample gt  list   new ArrayList lt Sample gt     List lt Sample gt  result   list stream        filter a - gt  Objects equals a value3   three         collect Collectors toList       Note that   a - gt  Objects equals a value3   three   is a lambda expression result is a List with a Sample type It s very fast  no cast at every iteration If your filter logic gets heavier  you can do list parallelStream   instead of list stream    read this      Apache Commons  If you can t use Java 8  you can use Apache Commons library and write   import org apache commons collections CollectionUtils  import org apache commons collections Predicate   Collection result   CollectionUtils select list  new Predicate          public boolean evaluate Object a             return Objects equals   Sample  a  value3   three                    If you need the results as a typed array  Sample   resultTyped    Sample    result toArray new Sample result size        Note that    There is a cast from Object to Sample at each iteration If you need your results to be typed as Sample    you need extra code  as shown in my sample        Bonus  A nice blog article talking about how to find element in list

User · Answer

If you always search based on value3  you could store the objects in a Map   Map lt String  List lt Sample gt  gt  map   new HashMap  lt  gt       You can then populate the map with key   value3 and value   list of Sample objects with that same value3 property   You can then query the map   List lt Sample gt  allSamplesWhereValue3IsDog   map get  Dog      Note  if no 2 Sample instances can have the same value3  you can simply use a Map lt String  Sample gt

[java] How to search in a List of Java object

Examples related to java

Examples related to list

Examples related to search

Examples related to collections