Difference Betwixt Arraylist Together With Arraylist≪?≫ Inward Coffee - Raw Type Vs Wildcard

One of my readers asked me almost the difference betwixt ArrayList vs ArrayList< inward Java?>, which was truly asked to him on a recent Java evolution interview. The key departure betwixt them is that ArrayList is non using generics spell ArrayList is a generic ArrayList but they looks really similar. If a method accepts ArrayList or ArrayList<?> every bit a parameter hence it tin have got whatsoever type of ArrayList e.g. ArrayList of String, Integer, Date, or Object, but if you lot await closely you lot volition notice that 1 is raw type spell other is using an unbounded wildcard. What departure that could make? Well, that makes a meaning departure because ArrayList alongside raw type is non type safe but ArrayList<?> alongside the unbounded wildcard is type safe.

You tin add together objects of whatsoever type into raw ArrayList but you lot cannot exercise that alongside a generic ArrayList alongside unbounded wildcard i.e. ArrayList, it volition last a compile-time error, every bit we'll run into yesteryear a code event inward this article.

This is 1 of the really interesting Java Interview questions from Generics together with Collection, but unfortunately, I didn't have got that 1 inward my listing of Java Generics Interview questions but volition add together it soon.

I truly dear whenever I got this form of interesting questions which is both thoughts provoking together with truly attempt your noesis of Java fundamentals. So, if you lot have got such questions, delight part alongside us too.

Difference betwixt Raw Type vs Unbounded Wildcard ArrayList

Now, let's run into a Java programme to sympathize the quest I brand inward higher upwards paragraph. I said, that when you lot utilisation a raw type ArrayList or ArrayList alongside unbounded wildcard every bit method arguments you lot tin have got whatsoever type of ArraList i.e. your customer tin top ArrayList of String, ArrayList of Integer, or ArrayList of Date object, but if you lot endeavor to add together novel objects to that listing hence raw type volition allow but generics listing alongside unbounded wildcard volition non allow.

It volition throw a compile-time mistake because it doesn't know which type of ArrayList it has received together with last beingness type-safe it throws an mistake every fourth dimension you lot endeavor to add together a novel element. You cannot fifty-fifty add together java.lang.Object into a listing of unbounded wildcards. That's why Effective Java recommend using bounded wildcard together with PECS pattern spell creating API using generics. They brand your API to a greater extent than flexible.

In this program, I have got 2 methods, printWildcardList and printRawTypeList former accepts an ArrayList spell later on accepts a raw type ArrayList every bit an argument. Both methods, commencement prints all elements of the given ArrayList yesteryear using Java 1.5 enhanced for loop together with hence tries to add together novel elements to list. You tin run into that solely raw type ArrayList allows that because it is non type-safe. The same functioning is prohibited on ArrayList of unbounded wildcard together with you lot volition run into a compile-time mistake there.

I know, Generic sometimes tin last truly confusing, peculiarly if you lot have got a one-half noesis together with that's why Interviewer wants to know during an interview. If you lot tin explicate the fact that ArrayList is a raw type together with non type-safe together with ArrayList is type rubber but non flexible together with how you lot tin acquire inward flexible e.g. yesteryear using bounded wildcards similar ArrayList hence you lot volition score skilful marks.

If you lot are swell to acquire Generics, peculiarly heart together with mortal concepts similar this hence at that topographic point is no improve majority than Java Generics together with Collection yesteryear Maurice Naftalin. I strongly advise every experienced Java developer read that majority non in 1 trial but twice to acquire a skilful noesis of this key feature.

Java Program to present departure betwixt ArrayList together with ArrayList

import java.util.ArrayList; import java.util.HashMap; import java.util.Map;   /**

 * Java Program to demonstrate departure betwixt ArrayList together with ArrayList>  *   * @author WINDOWS 8  */  public class Test {      public static void main(String args[]) {                  ArrayList<String> names = new ArrayList<String>();         names.add("Java");                  // you lot tin top whatsoever type of ArrayList of to both methods         printRawTypeList(names);         printWildcardList(names);     }                   /*      * Java method which takes an ArrayList of raw types      * together with impress elements on console      */     public static void printRawTypeList(ArrayList rawType){         for(Object obj : rawType){             System.out.println(obj);         }                  rawType.add(101); // OK         rawType.add("101");  // OK         rawType.add(121.00);  // OK     }                    /*      * Java method which prints ArrayList of unbounded wildcard      */     public static void printWildcardList(ArrayList<?> unbounded){                  // since ? doesn't specify whatsoever type, you lot tin merely utilisation object         for(Object obj : unbounded){             System.out.println(obj);         }                  unbounded.add(101); // NOT OK         unbounded.add("101");  //  NOT OK         unbounded.add(121.00);  // NOT OK      }      }

When you lot endeavor to run this programme inward your favorite IDE or command-line you lot volition run into a lot of compile fourth dimension mistake on the places where I have got mentioned NOT OK because you lot cannot add together novel elements into an ArrayList of unknown type.

One again, if you lot are swell to improve your noesis almost Generics, I strongly advise you lot reading next items from Effective Java which are related to Generics together with Collections:

That's all on the departure betwixt ArrayList together with ArrayList inward Java. Remember raw type is non type safe but ArrayList alongside the unbounded wildcard is type safe because you lot tin add together whatsoever type (String, Integer, Object into ArrayList of raw type, but you lot cannot add together whatsoever chemical component on ArrayList of unknown type.

Further Learning
Introduction to Collections & Generics inward Java
tutorial)

How to create parameterized classes together with methods inward Java? (example)

What is the departure betwixt bounded together with unbounded wildcards inward Generics? (answer)

How to implement singly linked listing using Generics inward Java? (tutorial)

12 Advanced Java Programming books for Experienced Programmers (books)

10 Tips to choke a improve Java Programmer (tips)

Thanks for reading this article. If you lot similar this interview inquiry hence delight part alongside your friends together with colleagues. If you lot have got whatsoever feedback, suggestion, or whatsoever interview inquiry which you lot similar to part alongside us, delight drib a comment.

P.S. - Remember, the unbounded wildcard inward method declaration is inflexible together with non recommended together with I have got solely used hither for demonstration purpose. You should ever follow suggestions given in Effective Java regarding designing a generic API e.g. using bounded wildcards for method arguments.

Share this post