int main()
{
  return 0;
}

public class Ex0
{
  public static void main(String [] args)
  {
    return;
  }
}

The source code for the simplest C++ programs consist of

1. a file named something.cpp
2. that defines a function int main().

The source code for the simplest Java program consists of

1. a file named is something.java
2. that consists solely of the definition of a public class named something   ← i.e. it matches the file name!
3. that defines a member function public static void main(String[] args).   ← Note: member functions are called methods in Java parlance.

Note: It is not the case that public static void main(String[] args) has to be the only member function (or, as Java programmers prefer, method) in the class definition. You can add more function/method definitions in your class just as you would add multiple function definitions in your .cpp file while writing a C++ program.

Step 1. Writing/Compiling/Running Java code

1. Using your favorite text editor (as long as it's emacs or vi) put the following Java code into a file named Lab1a.java:

   public class Lab1a 
   {
     public static void main(String[] args) 
     {
           
     }
   }

   Note: The name of the file must be exactly Lab1a.java. Yes, upper-case vs. lower-case matters!
2. Add the following to the main method:

     int[] x = new int[10];
     int k = 0;
     while(K < 10) {
        x[k] = k*k;
        k++;
     }
     for (int j = 0; j < 10;j++) {
        System.out.println(j + " squared is " + x[j]);
     }

   What do you think it will do when run?
3. Now it is time to compile this program. You do this by typing the command

   javac Lab1a.java

   (javac = "java compiler"). If you typed it in exactly as above, you should have gotten an error. Take a look at it, and notice how helpful javac is being. Fix the error.
4. Recompile the program (so you got no errors). If you type

   ls

   you'll notice a new file, Lab1a.class, which is the bytecode to be run by the java interpreter. Run the interpreter on your program with the command

   java Lab1a

5. Modify the program by replacing the while loop with a for loop. Compile it and run it again.

Step 2. Your first Program

1. Make a new file called Lab1b.java. Write the lines you need to write to make it a Java program.
2. Add the line

   import java.util.*;

   at the very top of the file saying This is like #include <iostream> in C++. It allows us to use the Scanner class.
3. Add the following code to main. Run to see what it does:

   Scanner in = new Scanner(System.in);
   System.out.print("Please input an integer ");
   int n = in.nextInt();
   System.out.print("Please input a second integer ");
   int k = in.nextInt();
   System.out.println("The two ints were " + n + " and " + k);

4. What happens if you type your name instead of an integer? Try it! What you should see is that the JVM prints out the call stack, i.e. the stack of function call records as it appeared when the error occurred. One nice feature of an interpreted language is that when a run-time error occurs, the interpreter can tell you all sorts of things about the state of the program when the error occurred. This is really nice. For programs running on the CPU, the only information you get when a run-time error occurs is a message like "seg-fault".
5. Scanner is a class that is part of Java's utility package (java.util). A package is "roughly" analogous to a C++ library. Scanner is used to facilitate reading information from an input stream. In this case, we are reading from System.in which is that standard input stream. In order to use the Scanner class, you have to import the java.util package. That's the purpose of the 'import' line of the file.
6. Did you know that if you want to play a simple lottery where there are n possible numbers (the number of balls), and you have to pick k of them, then your odds of winning are 1 in (n * (n-1) * (n-2) * ... * (n-k+1))/k!, where ! means "factorial". If not, don't worry. Your discrete math course will teach you these kinds of things. Just accept this formula for purposes of the lab.
7. Anyway, modify your program so that it reads in the n and k, and computes that value. You should be able to do this with code that (not counting I/O) looks just like C++.
8. You may wonder what else the Scanner class can do. The Scanner class, and much of the rest of what is built into Java, is described in your book. Documentation for the complete "Java API", i.e. descriptions of all packages/classes that are part of the language standard, is available online: Java 6 API Documentation. To find Scanner, look under java.util.*, and it is alphabetical from there (alternatively, google for "java scanner"). You'll notice a long list of functions included in the Scanner class. Which one do you think will read in a whole line? What type does it return? Change your code so that before the integers are entered, the user enters his/her name, and this is printed back to him in the final println() command.
9. Add a function which takes an int and returns the factorial of that integer. Place that function immediately before the main function, i.e. still inside the class definition. Functions (for now) should start with the magic incantation public static, otherwise they look just like C++ functions. Note that, unlike C++, there are no function prototypes separate from function definitions. Test your function before continuing on.
10. Add another function which, given n and k from the above program, returns the product n*(n-1)*...*(n-k+1).
11. Modify your main function to use these two new functions to calculate and output the odds from the above section.

Step 3. Your second program

Random rand = new Random(System.currentTimeMillis());

Write a program that generates a random number between 0 and 10 (inclusive!), and then asks the user for a guess until they get it correct. The more violent the berating on each wrong guess, the better. Code this in a file called Lab1c.java.