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

One of my readers asked me most the difference betwixt ArrayList vs ArrayList< inwards Java?>, which was genuinely asked to him on a recent Java evolution interview. The key divergence 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 therefore it tin lead maintain whatsoever type of ArrayList e.g. ArrayList of String, Integer, Date, or Object, but if yous await closely yous volition notice that ane is raw type spell other is using an unbounded wildcard. What divergence that could make? Well, that makes a pregnant divergence 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 yous cannot practise that alongside a generic ArrayList alongside unbounded wildcard i.e. ArrayList, it volition locomote a compile-time error, equally we'll run across yesteryear a code instance inwards this article.

This is ane of the rattling interesting Java Interview questions from Generics too Collection, but unfortunately, I didn't lead maintain that ane 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 too genuinely bear witness your noesis of Java fundamentals. So, if yous lead maintain such questions, delight part alongside us too.

Difference betwixt Raw Type vs Unbounded Wildcard ArrayList

Now, let's run across a Java plan to empathize the indicate I brand inwards to a higher house paragraph. I said, that when yous utilization a raw type ArrayList or ArrayList alongside unbounded wildcard equally method arguments yous tin lead maintain whatsoever type of ArraList i.e. your customer tin transcend ArrayList of String, ArrayList of Integer, or ArrayList of Date object, but if yous endeavour to add together novel objects to that listing therefore raw type volition allow but generics listing alongside unbounded wildcard volition non allow.

It volition throw a compile-time fault because it doesn't know which type of ArrayList it has received too locomote existence type-safe it throws an fault every fourth dimension yous endeavour 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 too PECS pattern spell creating API using generics. They brand your API to a greater extent than flexible.

In this program, I lead maintain 2 methods, printWildcardList and printRawTypeList former accepts an ArrayList spell afterwards 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 too therefore tries to add together novel elements to list. You tin run across that solely raw type ArrayList allows that because it is non type-safe. The same functioning is prohibited on ArrayList of unbounded wildcard too yous volition run across a compile-time fault there.

I know, Generic sometimes tin locomote genuinely confusing, particularly if yous lead maintain a one-half noesis too that's why Interviewer wants to know during an interview. If yous tin explicate the fact that ArrayList is a raw type too non type-safe too ArrayList is type condom but non flexible too how yous tin acquire inwards flexible e.g. yesteryear using bounded wildcards similar ArrayList therefore yous volition score skillful marks.

If yous are swell to larn Generics, particularly nub concepts similar this therefore in that location is no amend mass than Java Generics too Collection yesteryear Maurice Naftalin. I strongly propose every experienced Java developer read that mass non ane time but twice to acquire a skillful noesis of this key feature.

Java Program to exhibit divergence betwixt ArrayList too ArrayList

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

 * Java Program to demonstrate divergence betwixt ArrayList too ArrayList>  *   * @author WINDOWS 8  */  public class Test {      public static void main(String args[]) {                  ArrayList<String> names = new ArrayList<String>();         names.add("Java");                  // yous tin transcend whatsoever type of ArrayList of to both methods         printRawTypeList(names);         printWildcardList(names);     }                   /*      * Java method which takes an ArrayList of raw types      * too 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, yous tin but utilization 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 yous endeavour to run this plan inwards your favorite IDE or command-line yous volition run across a lot of compile fourth dimension fault on the places where I lead maintain mentioned NOT OK because yous cannot add together novel elements into an ArrayList of unknown type.

One again, if yous are swell to improve your noesis most Generics, I strongly propose yous reading next items from Effective Java which are related to Generics too Collections:

That's all on the divergence betwixt ArrayList too ArrayList inwards Java. Remember raw type is non type safe but ArrayList alongside the unbounded wildcard is type safe because yous tin add together whatsoever type (String, Integer, Object into ArrayList of raw type, but yous cannot add together whatsoever chemical portion on ArrayList of unknown type.

Further Learning
Introduction to Collections & Generics inwards Java
tutorial)

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

What is the divergence betwixt bounded too 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 yous similar this interview inquiry therefore delight part alongside your friends too colleagues. If yous lead maintain whatsoever feedback, suggestion, or whatsoever interview inquiry which yous similar to part alongside us, delight drib a comment.

P.S. - Remember, the unbounded wildcard inwards method declaration is inflexible too non recommended too I lead maintain solely used hither for demonstration purpose. You should e'er follow suggestions given in Effective Java regarding designing a generic API e.g. using bounded wildcards for method arguments.