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Generic Functions in C# and Java

without comments

>>> Attached: ( Main.java — in Java | Main.cs — in C# ) <<<

Updated: (1) Made code more readable. (2) Removed unnecessary package (Java) and namespace (C#) and added a function that returns a generic type as well. (3) Attached compilable demo source code in separate files.

The most fun and productive concept in object oriented programming is generics — for me anyway. In C, one could deploy generics hazardously with code that casts the contents of memory addresses with a putative struct. The first field gives away what that chunk of memory is supposed to be at run time (usually, it’s a typdef int or an enum). I still do that when it’s called for, but it’s quite delicate and often leads to insidious bugs that don’t crash immediately. At least one would know what code to suspect when crashes do happen.

In C# and Java, two languages that derive from C — we find full safe support of generics. Generic classes (the things that collections are made of) are interesting, and I’m sure most who have used either of these languages have already played with them and have found them useful. One of the things that don’t receive a healthy dose of spotlight is Generic Functions (“Generic Methods” if you like).

I’ll compare two segments of code, one in C# and one in Java that do exactly the same thing — demonstrate two trivial functions printArrayList() and getElement(). The function printArrayList() prints out the contents of an ArrayList (Java) or a List (C#). The function getElement() retrieves an element from a list. This shows how single generic functions can operate on collections, each with a different defined type without the need for unsafe casting. The only assumption the code makes is that each object in a list implements the toString() method (needed for the printing function).

Note naming convention: In Java, methods are just members of an object, so they are named in lowercase. In C#, methods are capitalized. We will refer to methods by the Java convention here to keep things consistent.

Setting Up in Main…

Let’s declare and fill a few lists for this demonstration. Three generic list objects, cow, dog and elephant are constructed in a for loop. Each gets ten elements. Each list contains objects of a particular type; cow contains integers, dog contains doubles and elephant contains strings.

Java Code C# Code
ArrayList<Integer> cow = new ArrayList();
ArrayList<Double> dog = new ArrayList();
ArrayList<String> elephant = new ArrayList();
List<int> cow = new List();
List<double> dog = new List();
List<string> elephant = new List();

Notice that Java does not autobox the type in the angel brackets so you can’t give it the primitives int and double. In C#, this is allowed plus string is also a primitive. Remember: In both Java and C#, primitives are emulated — they are first class objects that are only different from other objects in that they are pass-by-value rather than pass-by-reference.

Appending ten items to each list. Shown below is the Java version — in C#, change “add()” to “Add()”.

for(int i = 0; i < 10; i ++) {
    cow.add(3 * i);
    dog.add(0.25 * i);
    if(i % 2 == 0)

The below is the code we want to make work — We’ll call printArrayList() to print out all of the elements in each list, then we’ll call getElement() to return a specific element from each list. Notice that this is the Java version below — in C#, we capitalize method names and use Console.Writeline() instead of System.out.println().

System.out.println("== Generic List Printer ==");



System.out.println("== Generic Element Accessing ==");

int    cow_at_7      = getElement(cow, 7);
double dog_at_2      = getElement(dog, 2);
String elephant_at_4 = getElement(elephant, 4);

System.out.println("Cow at 7      = " + cow_at_7);
System.out.println("Dog at 2      = " + dog_at_2);
System.out.println("Elephant at 4 = " + elephant_at_4);

Note that in C#, we may use the keyword “var” instead of typing out the types for cow_at_7, dog_at_2, and elephant_at_4 — the compiler infers the type for us. This is different from unsafely casting with “Object”, as the compiler infers the narrowest possible type and substitutes in that correct type.

Onto the methods …

Below is the Java version of printArrayList().

static <A> void printArrayList(ArrayList<A> animalList) {
    for(A a : animalList)
        System.out.print(a + "\t");

Below is the C# version of PrintArrayList().

static void PrintArrayList<A>(List<A> animalList) {
    foreach(A a in animalList)
        Console.Write(a + "\t");

Notice that printArrayList() is a method that specifies a generic type <A>, but only in its argument list. In Java, <A> appears before the function’s type and in C#, this appears after the function name. In this case, it’s obvious what we would do if we have functions that return specific types — we just substitute the type where the keyword “void” is. So what happens when we want to return the generic type? That’s what getElement() will demonstrate.

Below is the Java version of getElement().

static <A> A getElement(ArrayList<A> what, int which) {
    return what.get(which);

Below is the C# version of GetElement().

static A GetElement<A>(List<A> what, int which) {
    return what[which];

Yes, these are both trivial functions, as you could have easily called ArrayList.get() in Java and List[] in C# respectively — but it does the job in this demonstration. In the Java version, the generic type <A> is placed before the type of the function, A. Don’t let that confuse you, just recall how we specified the return type when it wasn’t the generic type. In C#, we place the generic type <A> after the function name just as before.

Below is the output you should expect if you run the main function.

C# Output

== Generic List Printer ==
0	3	6	9	12	15	18	21	24	27
0	0.25	0.5	0.75	1	1.25	1.5	1.75	2	2.25
Even	Odd	Even	Odd	Even	Odd	Even	Odd	Even	Odd	

== Generic Element Accessing ==
Cow at 7      = 21
Dog at 2      = 0.5
Elephant at 4 = Even

The Java output is the same, except the values that are doubles are always printed with a trailing “.0” even if it is numerically equal to an integer.

Eddie Ma

January 29th, 2010 at 11:41 am