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Ryan Harrison My blog, portfolio and technology related ramblings

Compile from the Command Line

It’s sometimes frustrating when you want to compile and test a short snippet of code, yet find yourself modifying an existing project or even creating a whole new one just to compile the code. Most developers write and compile their code straight from their IDE of choice and never bother compiling from the command prompt. For most situations this is sensible, however in some specific scenarios the command line can save you time when it’s not feasible to modify or create a new project. In this short tutorial I will show you how to compile your Java, C# and C++ code straight from the command line, including all the necessary tweaks of your Environment Variables.


The first language is Java. I will assume that you have the JDK (Java Development Kit) installed, as this is the package that includes the Java compiler.

The first step is to navigate to the installation directory of your JDK installation and copy the path to the bin folder which resides inside. The path should look a bit like this (depending the version of the JDK you have installed) -

JDK Path

Once you have the path in the clipboard, you have to add it to your system’s Environment Variables, which will essentially allow you to run the Java compiler from anywhere on your computer.

In Windows 7, click the Start orb and right click on the Computer shortcut and click on Properties in the context menu. This should take you to a window that looks something like this -

System Properties

Then, click on Advanced System Settings on the right hand side. Under the Advanced tab of the new window, click on Environment Variables….

Next, you want to locate the variable named Path in your System Variables. If you don’t have one, click on the new button to create a new variable (make sure you name it Path). With it selected, click on edit. A new window pops up showing the value of the Path variable. Navigate to the end of the value string and add a semi-colon to signify a new entry.

Finally, simply paste in the path to the JDK’s bin directory and click on OK. The value field should look a little like this -

JDK Environment Variable

We have now configured your system to allow you to compile Java programs from any directory on you hard drive.

To use the compiler from the command line, open up a new command prompt window by typing cmd in the Run box (or search box in the Windows 7/Vista start menu).

Type in javac (stands for Java Compiler) and hit enter. You should see a lot writing being printed to the window. It should look a little like this -

Java Compiler

This signifies that you can actually use the compiler and that you have successfully followed the previous steps.

In this example I will compile and run a simple ‘Hello World!’ program from the desktop.

public class javacomp  
  public static void main(String[] args)  
    System.out.println("Hello World!");

Save this Java source file to any directory on your computer, (in this case my Desktop) and again open up a command prompt window.

Next, navigate to the directory of your source file using the cd command.

e.g - cd C:\Users\YourUserName\Desktop

When this is done, type javac again, yet this time followed by the name of the source file you just created, (make sure you add the .java extension) and hit enter. If there are no syntax errors you should see a new .class file in the same directory. This is where the compiled bytecode is stored. Finally, to run the program type java and the name of the .class file (this time no need to add the extension). The program will now run the window and you should see something like this -

Compiling and Running the Java Program

More information about the Java compiler, including the many command line arguments.


Next for C#. This is pretty much the same as for Java apart from a different path to the compiler.

In this case you need to copy the path to the .NET Framework folder in the root Windows folder. The path should be a bit like this depending on the version of the .NET framework you have installed -

The Path to the .Net Framework installation

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C# - Auto Clicker

I recently got a link to a Facebook app which challenges you to how many times you can click a button in 30 seconds. After a couple of tries I got to around 180, yet I thought that I must be able to do better by making the computer click for me.

After a bit of research I came across two functions from MSDN named SendInput and mouse_event, both of which are provided in the Win32 API, and allow you to programatically simulate both mouse clicks and keyboard input. Thus I thought it was a great opportunity to try out C#’s p/invoke feature which allows the programmer to ‘import’ unmanaged functions from DLL libraries and use them in a C# application.

In this example I will only be using the mouse_event function, however an implementation of SendInput is included in the sources files, which are available for download at the bottom of this post.

The MSDN page of the mouse_event function gives us some good information on how to implement the function, including a function header and a rundown on each of the parameters. They give the mouse_event function the following header -

VOID WINAPI mouse_event(  
  __in DWORD dwFlags,  
  __in DWORD dx,  
  __in DWORD dy,  
  __in DWORD dwData,  
  __in ULONG_PTR dwExtraInfo  

Which can then be imported for use in our C# application using using the DLLImport attribute and the extern keyword (which simply indicates that the method is implemented externally) -

public static extern void mouse_event(int dwFlags, int dx, int dy, int dwData, int dwExtraInfo);  

We can now call this unmanaged function as we would any other method already in our project. However to make good use of them, we need to find out a bit more information on each of the parameters.

They can be summed up as follows -

  • dwFlags - In our case this parameter specifies which mouse button we would like to press depending on the integer value we pass
  • dx - The relative mouse position along the x-axis
  • dy - The relative mouse position along the y-axis
  • dwData - Contains how much we would like to move the mouse wheel and in which direction
  • dwExtraInfo - Contains extra information on the function call (not needed in our case)

In the case of this example we only have a need for the dwFlags parameter as we can easily set the cursor position using C# -

Cursor.Position.X = ...
Cursor.Position.Y = ...

Before we finally call this method, it is a good idea to declare some constants, each describing which mouse button we would like to press. Details can again be found on the relevant MSDN page -

public const int MOUSEEVENTF_LEFTDOWN = 0x0002;  
public const int MOUSEEVENTF_LEFTUP = 0x0004;  
public const int MOUSEEVENTF_RIGHTDOWN = 0x0008;  
public const int MOUSEEVENTF_RIGHTUP = 0x0010;  
public const int MOUSEEVENTF_MIDDLEDOWN = 0x0020;  
public const int MOUSEEVENTF_MIDDLEUP = 0x0040;  

Finally we can now call the function, which will be wrapped into two methods -

private void ClickLeftMouseButtonMouseEvent()  
  //Send a left click down followed by a left click up to simulate a  
  //full left click  
  mouse_event(MOUSEEVENTF_LEFTDOWN, 0, 0, 0, 0);  
  mouse_event(MOUSEEVENTF_LEFTUP, 0, 0, 0, 0);  

private void ClickRightMouseButtonMouseEvent()  
  //Send a left click down followed by a right click up to simulate a  
  //full right click  
  mouse_event(MOUSEEVENTF_RIGHTDOWN, 0, 0, 0, 0);  
  mouse_event(MOUSEEVENTF_RIGHTUP, 0, 0, 0, 0);  

As you can see we actually need to call the function twice, once for the mouse down action, and again for the mouse up action, to simulate a full mouse click. In this example we only use the dwFlags parameter and pass zero for the rest as we have no need for them.

So we now have two methods that we can use to simulate a mouse click, yet no way of letting the user determine where to click, how many times to click, and what mouse button to click - the perfect chance to create a C# Windows Forms Application (a.ka. GUI) to make our program a little more presentable.

In my program, I have created a simple User Interface containing a list view, a few buttons, and a couple of textboxes, which will allow the user to specify a queue of points to click in sequence, as well as the ability to insert additional information for each click (button to press, time in between, etc).

Screenshot of the final Auto Clicker program

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Hello world!

Welcome to my blog! Stay tuned for posts on software development, animation and current technology…

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