Part 1 The Empty Program

Transcription

1 California State University, Sacramento College of Engineering and Computer Science Computer Science 10: Introduction to Programming Logic Activity L From Flowgorithm To Java Throughout this semester, you have been creating programs using flowcharts. Flowcharts are not just used in this class. They are used everywhere by scientists, business, government, and many more. Flowgorithm allowed you to take your flowcharts and execute them like programs. But, this is not the only way. In fact, most computer scientists don't use a nice graphical tool to create their programs. Instead, they create their programs by writing plain text. We have done that too using the pseudocode found in the book. Real-world computer scientists write programs using one of the major programming languages such as C#, Java, Lua, Python, etc Java, in particular, is currently one the most popular programming languages in the World. Part 1 The Empty Program For this activity, you are going to use Flowgorithm to create some actual Java programs. Even though programming with flowcharts is quite easy it's not how the "real" world does it. Fortunately, Flowgorithm has features designed to help students transition from flowcharts to a "real" programming language. In particular, it has a feature called the "Source Code Viewer" that can convert your flowchart to actual code! Woo hoo! Yes, it can't get easier than that! 1. Open Flowgorithm. Right now your flowchart should be completely empty 2. Open the Source Code Viewer. Either click on the toolbar icon or select it from the Tools Menu. The icon to the right is the source code viewer icon. 3. Make sure that Java is selected in the Language dropdown menu. Oh, let's make it easy to compare your flowchart and Java code side-by-side. 4. Go to the Tools Menu on Flowgorithm and select "Layout Windows" (you can also press Control+L). 5. Select "code" from the window Okay, now we can see your flowchart and the Java code at the same time. Notice that even though your flowchart doesn't do anything yet the Java program has a lot of code! import java.util.*; import java.lang.math; class MyProgram { public static void main(string[] args) { } } All of this means something important in Object Oriented Programming. But, it's not easy to look at. In fact, it's quite scary. But, let's explain what all of this means.

2 2 The lines that start with import tell Java that you need some built-in intrinsic functions. In Flowgorithm, all your intrinsic functions (e.g. the cos() function) are always available. However, in Java, you must tell the compiler which ones you plan to use. The line that starts with class defines a new class of objects called "MyProgram". Everything in a Java program is considered an object and those are defined using classes. The next line defines the Main Function. This is exactly the same as your Flowgorithm programs! Flowgorithm and Java always start with a function called main. At the start of any function definition, Java uses a number of special words called "modifiers" which tells it how the function is going to be used. The word public means that other objects can call this function (otherwise it is private and hidden). Next, Java uses the word static. This modifier means that this function "sticks around" and is shared by all the objects of this class. Now that we are done with the modifiers, we finally get to the function definition. The word void is Java's way of saying the function doesn't return any value. So, it acts just like a module in the textbook. Finally, there is the name of the function "main" followed by some truly odd looking code. That basically says that the function has one parameter an array of strings called "args". Whew! Yes, that's a lot to take in. But it's not so bad once you get used to it. Part 2 Hello World Now that you have seen that a Java Program is pretty simple (though rather strange looking), it is time to try some experiments. 1. In your flowchart, add an Output Shape 2. Double-click on it and make it print "Hello". 3. Observe the generated code. Ah, it appears that Java outputs text by using a function called "System.out.println()". Also notice that the line is colored green the same color as the flowchart's shape. Flowgorithm likes to do this: color the generated code based on the shape that it represents. EXPLORE: Flowgorithm Color Coding When Flowgorithm generates source code, it converts each of your flowchart's shapes into the equivalent Java code. So you can visually match the code to the shape, it displays both using the same color. Try highlighting shapes in the flowchart (like you are going to copy them) and see what happens. Let's do some more work with the Output Shape. 4. Double click on the shape and change the expression to "Hello " & "World" 5. Observe the results It appears that Java uses a plus sign + to concatenate two strings. Wait! Isn't that used to add numbers together? Yes, that's true too. Java uses the same operator for both!

3 Part 3 Input 3 Now that we have gotten an idea on how to output information, let's try to input some data as well. 1. Add variable declaration for a string called age. 2. Add an input shape below the declaration and input age. 3. Change the Output Shape to output: "Your age is" & age. 4. Observe the results. Wow! Look at all the Java code! Flowgorithm is using something called "input". What is that? Look further up the program right before where the main function is defined. Ah! There it is! It is something called a "scanner". NOTE: Java Scanner Class The Scanner Class is commonly used to read, and interpret, raw data coming from a source (called a stream). So, to read data from the keyboard, the class is created using System.in. To print text to the screen, data is sent to System.out.

4 4 Part 4 From Java to Flowcharts and Back to Java Below is a pretty simple Java program. It reads the user's age from the keyboard and them tells them if they are old enough to vote. Even though you haven't done any real Java programming yet, you probably understand what you are looking at. This is pretty much like what you created earlier, but contains a bit more logic. Hmmm... What is that construct that starts with the word "if"? 1. Study the program above. 2. Now, create this program in Flowgorithm. 3. Check the Source Code Viewer window. Does the generated code match? If not, go back and double-check your program. Now that your program is complete and working, it is finally time to make the leap into real Java! 4. Open the Java compiler that your instructor told you about. 5. In the Source Code Viewer, click on the copy icon. The source will be copied to the clipboard. 6. Paste it into your Java compiler. 7. Execute your Java program! Ta da! You did it. You created a working Java program. Hopefully, at this point, you can see that the logic you learned during the semester is the same logic that is used in popular languages like Java. 8. Execute your flowchart. 9. Try the same data you entered into your Java program. Flowgorithm and Java might look a tad different when they execute, but they behave the same way. Both programs implement the same algorithm. They are the same program!

5 Part 5 Adding More Features 5 Let's build upon your program. Below is a slightly more sophisticated version of the flowchart you created before. Now, if the user is not yet 18, it calculates and displays how many years until they are eligible. 1. Modify your flowchart, again, to make it identical to this program. 2. Check the code in the Source Code Viewer. Did it match? 3. Copy and paste the generated code into the Java compiler. 4. Execute it & observe the results. Here is a really interesting observation. a fact that might impress you. And it's about you! You understood that Java program before you executed it. You saw the logic even before you converted it into a flowchart. While you might not be a seasoned Java programmer yet, you can read a Java program! and that is pretty impressive! Part 6 Into the Breach You have created Java code by using Flowgorithm's Source Code Viewer. It's a nice feature, but, this time, let's not use it. 1. Save your Flowgorithm flowchart. 2. Close Flowgorithm you won t need it at this point. The program that you created before lacked any input validation. In other words, the user could enter an invalid age! So, let's add some input validation to your program, but do it completely in Java! This might be a little scary, so it might be a good idea to save your current solution before you continue. 3. Look at the flowchart below (well, part of a flowchart).

6 6 This contains the input validation loop for age. Basically, it will loop until a valid age is entered each time prompting the user to enter an age between 1 and 100. The syntax of the Java While loop is shown below. Also note: the Logical Or operator is: (two pipe symbols, next to the backslash on most keyboards). (In Java, Logical And is && ) while(expression) { } 4. Add the input validation loop to your Java program 5. Execute and check your results If your program executed correctly, then you have written Java completely on your own! Congratulations!