The generic collections should be used whenever possible instead of classes such as ArrayList in the System. Collections namespace. You can create your own generic interfaces, classes, methods, events, and delegates. Generic classes may be constrained to enable access to methods on particular data types. Information on the types that are used in a generic data type may be obtained at run-time by using reflection.
C language specification For more information, see the C Language Specification. See also System. Generic Generics in. NET Is this page helpful? Yes No. Any additional feedback? Skip Submit. Submit and view feedback for This product This page. View all page feedback. Is this page helpful? I don't like restating things multiple times with slight differences. I can build one reusable class I'm now 3x more efficient and I only have to maintain one copy.
Not needing to typecast is one of the biggest advantages of Java generics , as it will perform type checking at compile-time. This will reduce the possibility of ClassCastException s which can be thrown at runtime, and can lead to more robust code.
Every time I look at Generics it gives me a headache. I find the best part of Java to be it's simplicity and minimal syntax and generics are not simple and add a significant amount of new syntax. At first, I didn't see the benefit of generics either. I started learning Java from the 1. Most modern, decent IDEs are smart enough to assist with writing code with generics, especially with code completion.
The code I would have to type in is:. And indeed, that's a lot to type just to make a new HashMap. However, in reality, I only had to type this much before Eclipse knew what I needed:. True, I did need to select HashMap from a list of candidates, but basically the IDE knew what to add, including the generic types. With the right tools, using generics isn't too bad.
In addition, since the types are known, when retrieving elements from the generic collection, the IDE will act as if that object is already an object of its declared type -- there is no need to casting for the IDE to know what the object's type is. A key advantage of generics comes from the way it plays well with new Java 5 features. Here's an example of tossing integers in to a Set and calculating its total:. The integer 10 is autoboxed into an Integer with the value of And same for Then that Integer is tossed into the Set which is known to hold Integer s.
Trying to throw in a String would cause a compile error. First, the Set containing Integer s are used in a for-each loop. Each element is declared to be an int and that is allowed as the Integer is unboxed back to the primitive int.
And the fact that this unboxing occurs is known because generics was used to specify that there were Integer s held in the Set.
Generics can be the glue that brings together the new features introduced in Java 5, and it just makes coding simpler and safer. And most of the time IDEs are smart enough to help you with good suggestions, so generally, it won't a whole lot more typing. And frankly, as can be seen from the Set example, I feel that utilizing Java 5 features can make the code more concise and robust. The following is an illustration of the above Set example without the use of generics. It is possible, but isn't exactly pleasant:.
When using non-generics collections, the types that are entered into the collection is objects of type Object. Therefore, in this example, a Object is what is being add ed into the set. In the above lines, autoboxing is in play -- the primitive int value 10 and 42 are being autoboxed into Integer objects, which are being added to the Set. However, keep in mind, the Integer objects are being handled as Object s, as there are no type information to help the compiler know what type the Set should expect.
This is the part that is crucial. The reason the for-each loop works is because the Set implements the Iterable interface, which returns an Iterator with type information, if present.
However, since there is no type information, the Set will return an Iterator which will return the values in the Set as Object s, and that is why the element being retrieved in the for-each loop must be of type Object. Now that the Object is retrieved from the Set , it needs to be cast to an Integer manually to perform the addition:.
Here, a typecast is performed from an Object to an Integer. In this case, we know this will always work, but manual typecasting always makes me feel it is fragile code that could be damaged if a minor change is made else where.
I feel that every typecast is a ClassCastException waiting to happen, but I digress The Integer is now unboxed into an int and allowed to perform the addition into the int variable total. I hope I could illustrate that the new features of Java 5 is possible to use with non-generic code, but it just isn't as clean and straight-forward as writing code with generics.
And, in my opinion, to take full advantage of the new features in Java 5, one should be looking into generics, if at the very least, allows for compile-time checks to prevent invalid typecasts to throw exceptions at runtime. If you were to search the Java bug database just before 1. So it doesn't seem that it is a great feature to find bugs, or at least bugs that persist after a little smoke testing.
For me the huge advantage of generics is that they document in code important type information. If I didn't want that type information documented in code, then I'd use a dynamically typed language, or at least a language with more implicit type inference. Keeping an object's collections to itself isn't a bad style but then the common style is to effectively ignore encapsulation.
It rather depends upon what you are doing. Passing collections to "algorithms" is slightly easier to check at or before compile-time with generics. Generics in Java facilitate parametric polymorphism.
By means of type parameters, you can pass arguments to types. So List is a actually a type constructor. It takes a type as an argument and constructs another type as a result.
Here are a couple of examples of generic types I use every day. First, a very useful generic interface:. This interface says that for some two types, A and B , there's a function called f that takes an A and returns a B. When you implement this interface, A and B can be any types you want, as long as you provide a function f that takes the former and returns the latter.
Here's an example implementation of the interface:. Before generics, polymorphism was achieved by subclassing using the extends keyword. With generics, we can actually do away with subclassing and use parametric polymorphism instead.
For example, consider a parameterised generic class used to calculate hash codes for any type. Instead of overriding Object. This is much more flexible than using inheritance, because we can stay with the theme of using composition and parametric polymorphism without locking down brittle hierarchies. Java's generics are not perfect though. You can abstract over types, but you can't abstract over type constructors, for example. That is, you can say "for any type T", but you can't say "for any type T that takes a type parameter A".
I wrote an article about these limits of Java generics, here. One huge win with generics is that they let you avoid subclassing. Subclassing tends to result in brittle class hierarchies that are awkward to extend, and classes that are difficult to understand individually without looking at the entire hierarchy. Generics avoid the performance hit of boxing and unboxing.
The best benefit to Generics is code reuse. Lets say that you have a lot of business objects, and you are going to write VERY similar code for each entity to perform the same actions. E Linq to SQL operations. With generics, you can create a class that will be able to operate given any of the types that inherit from a given base class or implement a given interface like so:. Notice the reuse of the same service given the different Types in the DoSomething method above.
Truly elegant! I just like them because they give you a quick way to define a custom type as I use them anyway. So for example instead of defining a structure consisting of a string and an integer, and then having to implement a whole set of objects and methods on how to access an array of those structures and so forth, you can just make a Dictionary.
A Dictionary in particular lets you use the first type as a key no repeated values. Typed collections - even if you don't want to use them you're likely to have to deal with them from other libraries , other sources.
Where you're essentially checking for a condition that should only exist because the interface is expressed in terms of objects. My initial reaction to generics was similar to yours - "too messy, too complicated". My experience is that after using them for a bit you get used to them, and code without them feels less clearly specified, and just less comfortable.
Aside from that, the rest of the java world uses them so you're going to have to get with the program eventually, right? Example 1 Now you want to have a collection of Foo objects. You have two options, LIst or ArrayList, both of which work in a similar manner. Now you have your two array lists and you want to call the Bar function on each. Since hte ArrayList is filled with Objects, you have to cast them before you can call bar.
I want a List of Strings, not a List of things I hope are strings! Don't forget that generics aren't just used by classes, they can also be used by methods. For example, take the following snippet:. It is simple, but can be used very elegantly. The nice thing is that the method returns whatever it was that it was given. This helps out when you are handling exceptions that need to be re-thrown back to the caller:. The point is that you don't lose the type by passing it through a method.
You can throw the correct type of exception instead of just a Throwable , which would be all you could do without generics. This is just a simple example of one use for generic methods. There are quite a few other neat things you can do with generic methods. The coolest, in my opinion, is type inferring with generics. This doesn't do a lot, but it does cut down on some clutter when the generic types are long or nested; i.
The primary advantage, as Mitchel points out, is strong-typing without needing to define multiple classes. Java generics are just syntactic sugar. Java collections use generics since Java 1. So, a good place to use them is when you are creating your own collection-like object. An example I see almost everywhere is a Pair class, which holds two objects, but needs to deal with those objects in a generic way.
Whenever you use this Pair class you can specify which kind of objects you want it to deal with and any type cast problems will show up at compile time, rather than runtime.
Generics can also have their bounds defined with the keywords 'super' and 'extends'. For example, if you want to deal with a generic type but you want to make sure it extends a class called Foo which has a setTitle method :. While not very interesting on its own, it's useful to know that whenever you deal with a FooManager, you know that it will handle MyClass types, and that MyClass extends Foo. Once the compiler knows the element type of the collection, the compiler can check that you have used the collection consistently and can insert the correct casts on values being taken out of the collection The code using generics is clearer and safer Because the program compiles without warnings, we can state with certainty that it will not throw a ClassCastException at run time.
The net effect of using generics, especially in large programs, is improved readability and robustness. If we already know that our list only holds string data then we need not typecast it every time. Generics promotes code reusability. Implementing generic algorithms: By using generics, we can implement algorithms that work on different types of objects and at the same, they are type safe too. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute geeksforgeeks.
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