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

One of my readers asked me well-nigh the difference betwixt ArrayList vs ArrayList< inwards Java?>, which was genuinely 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 rattling similar. If a method accepts ArrayList or ArrayList<?> equally a parameter thence it tin sack convey whatever type of ArrayList e.g. ArrayList of String, Integer, Date, or Object, but if y'all hold off closely y'all volition discovery that i is raw type spell other is using an unbounded wildcard. What departure that could make? Well, that makes a pregnant departure because ArrayList alongside raw type is non type safe but ArrayList<?> alongside the unbounded wildcard is type safe.

You tin sack add together objects of whatever type into raw ArrayList but y'all cannot practise that alongside a generic ArrayList alongside unbounded wildcard i.e. ArrayList, it volition endure a compile-time error, equally we'll run into yesteryear a code event inwards this article.

This is i of the rattling interesting Java Interview questions from Generics in addition to Collection, but unfortunately, I didn't have got that i inwards my listing of Java Generics Interview questions but volition add together it soon.

I genuinely dearest whenever I got this variety of interesting questions which is both thoughts provoking in addition to genuinely testify your cognition of Java fundamentals. So, if y'all have got such questions, delight percentage alongside us too.

Difference betwixt Raw Type vs Unbounded Wildcard ArrayList

Now, let's run into a Java plan to sympathise the indicate I brand inwards to a higher house paragraph. I said, that when y'all usage a raw type ArrayList or ArrayList alongside unbounded wildcard equally method arguments y'all tin sack convey whatever type of ArraList i.e. your customer tin sack transcend ArrayList of String, ArrayList of Integer, or ArrayList of Date object, but if y'all travail to add together novel objects to that listing thence 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 in addition to endure beingness type-safe it throws an mistake every fourth dimension y'all travail 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 in addition to 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 after accepts a raw type ArrayList equally an argument. Both methods, kickoff prints all elements of the given ArrayList yesteryear using Java 1.5 enhanced for loop in addition to thence tries to add together novel elements to list. You tin sack run into that exclusively raw type ArrayList allows that because it is non type-safe. The same performance is prohibited on ArrayList of unbounded wildcard in addition to y'all volition run into a compile-time mistake there.

I know, Generic sometimes tin sack endure genuinely confusing, peculiarly if y'all have got a one-half cognition in addition to that's why Interviewer wants to know during an interview. If y'all tin sack explicate the fact that ArrayList is a raw type in addition to non type-safe in addition to ArrayList is type condom but non flexible in addition to how y'all tin sack acquire inwards flexible e.g. yesteryear using bounded wildcards similar ArrayList thence y'all volition score adept marks.

If y'all are neat to acquire Generics, peculiarly nub concepts similar this thence in that location is no amend majority than Java Generics in addition to Collection yesteryear Maurice Naftalin. I strongly propose every experienced Java developer read that majority non in i lawsuit but twice to acquire a adept cognition of this key feature.

Java Program to demo departure betwixt ArrayList in addition to ArrayList

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

 * Java Program to demonstrate departure betwixt ArrayList in addition to ArrayList>  *   * @author WINDOWS 8  */  public class Test {      public static void main(String args[]) {                  ArrayList<String> names = new ArrayList<String>();         names.add("Java");                  // y'all tin sack transcend whatever type of ArrayList of to both methods         printRawTypeList(names);         printWildcardList(names);     }                   /*      * Java method which takes an ArrayList of raw types      * in addition to 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 whatever type, y'all tin sack merely usage 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 y'all travail to run this plan inwards your favorite IDE or command-line y'all volition run into a lot of compile fourth dimension mistake on the places where I have got mentioned NOT OK because y'all cannot add together novel elements into an ArrayList of unknown type.

One again, if y'all are neat to improve your cognition well-nigh Generics, I strongly propose y'all reading next items from Effective Java which are related to Generics in addition to Collections:

That's all on the departure betwixt ArrayList in addition to ArrayList inwards Java. Remember raw type is non type safe but ArrayList alongside the unbounded wildcard is type safe because y'all tin sack add together whatever type (String, Integer, Object into ArrayList of raw type, but y'all cannot add together whatever chemical factor on ArrayList of unknown type.

Further Learning
Introduction to Collections & Generics inwards Java
tutorial)

How to create parameterized classes in addition to methods inwards Java? (example)

What is the departure betwixt bounded in addition to unbounded wildcards inwards Generics? (answer)

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

12 Advanced Java Programming books for Experienced Programmers (books)

10 Tips to acquire a amend Java Programmer (tips)

Thanks for reading this article. If y'all similar this interview inquiry thence delight percentage alongside your friends in addition to colleagues. If y'all have got whatever feedback, suggestion, or whatever interview inquiry which y'all similar to percentage alongside us, delight drib a comment.

P.S. - Remember, the unbounded wildcard inwards method declaration is inflexible in addition to non recommended in addition to I have got exclusively 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.

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