Even though you might not be aware of it, one of the main concepts you’ve been learning about so far is something called program control.

Program control includes how a program executes its code, makes decisions and organizes its functions. It is not just related to how the code is executed, but also which lines of code get executed as well.

Imagine that the following program is going to be executed.

Can you tell which line of code will be executed first? Second? Last?

Can you tell if there are lines of code that will not be executed?

 

Sequential Program Control

One of the first major concepts involved in program control is the idea that lines of programming code are executed sequentially, from first to last, top to bottom. That’s pretty simple, right?

Eventually you will write programs that bounce around a little bit (with subroutines, etc), but for now let’s just assume that the program executes line 1 first, then line 2, then line 3, etc.

Selection and Program Control

Selection involves having the computer make choices, and having programming code execute or not execute certain lines of code based on data or input. You have been implementing selection in your programs through the use of if-then-else statements.

The program below calculates the area of a rectangle and then outputs one of three statements:

• If the area is larger than 250: "That's a big rectangle!"
• If the area is larger than 150: "That's a medium-sized rectangle!"
• Otherwise: "That's a small rectangle!"

 

You’ve had a brief introduction to selection through the use of if-then-else statements, comparison operators (<, >, <=, >=, ==, etc) and Boolean operators (&&, ||, etc).

It is now time to dig a little deeper to see the alternatives to if-then-else statements, and how if-then-else statements can be nested inside of each other to create really cool decision making structures and game play logic.

Switch Statements

Switch statements in Java are an alternative to the if-then-else statements.

They are useful when the decision being made is based on a single character, word or value. If the decision involves ranges, then a switch statement might not be ideal. For example, you would not use a switch statement for the following, because the condition involves a range of possible numbers:

if (mark >= 50)
  {
     System.out.println (“You passed!”);
  }

else
  {
     System.out.println (“You passed!”);
  }

Take a look at the following two programs.

They accomplish the same task, but the first one is written using the if-then-else statement, while the second one is written using a switch statement:

 

The switch statement above doesn’t change the program in terms of how it works; it only changes the program in terms of how the source code looks.

An advantage to using the switch statement is that some programmers feel it helps keep their programming code look a little more organized.

Case Fall Through

If you choose to use switch statements in your program, just make sure that you properly place the break statement at the end of each case.

The break ensures that only one of the cases is executed. When the program reaches the “break;” in line 28, it then skips to the end of the switch statement (line 39). Without the break in each case, the program will execute the following case as well. This is called case fall-through.

Nested if-then-else Statements

A nested if-then-else statement is when an if-then-else statement is programmed inside of another if-then-else statement. Sounds a bit crazy, doesn’t it? It’s actually pretty cool, and allows the programmer to develop some really interesting programs.

As you continue to work with selection in your programs, you may want to think of if-then-else statements as leading to branches in your programs. With nested if-then-else statements, the branching gets more and more complex.

An example

The flowchart below shows how a programmer might design a program that determines the type of computer a person is looking for. There are many more types of computers and sub-categories, but the following have been selected just to keep things simple:

Notice how the decision of whether or not the computer has a keyboard comes within the decision of whether or not the computer is portable.

To create the above program, the programmer has to insert if-then-else statements inside other if-then-else statements.

The code would resemble the following:

A Second Example

The diagram below shows how a person might determine what type of animal another person is thinking about. Notice that we haven’t included every single possible animal, since that would make this quite complicated and confusing.

In the diagram above, the decision based on whether or not the animal is red or blue falls under the decision based on whether or not they have feathers. If you programmed this, it would mean that the if-then-else statement for bird colour would be nested inside the if-then-else statement for feathers.

In the diagram above, the decision based on whether or not the animal has scales or fur falls under the decision based on whether or not the animal has feathers. Then there is a decision on whether or not the animal has legs, and whether it is large or small.

This will mean you will have if-then-else statements nested inside other nested if-then-else statements. Wow!

The Importance of Indenting and Braces

Once you start to work with nested if-then-else statements, and you start creating larger programs, it is very, very important that you keep your programs organized.

Indenting your code allows you to see which blocks belong to each other. A good example of this in practice is the if-then-else statements above.

Each statement that goes inside another if-then-else statement gets indented further to the right. Therefore, after every open curly bracket you have to indent the next line of code further to the right. Then after closing a curly bracket, the next line of code is no longer indented. This makes it easy to see the statements that are associated with each one:

Braces (or Curly Brackets)

By now, you have probably figured out that the braces (also known as curly brackets) in Java are used to indicate where certain components or blocks of code start and end.

As an example, these braces (or curly brackets) are used to show the start and end of an if-then-else statement.

An opening brace “{“ always needs an associated closing brace “}”.

Programming Style and Organization … You are Not Alone!

The Netbeans IDE includes many features that help keep your programming code clear and organized. You may have already noticed that automatic indenting occurs, and often extra whitespace is added to your code as well.

Also, when you are using Netbeans, the closing brace is often added for you automatically after you type the opening brace.

You can also click on an opening or closing brace and it’s associated brace will automatically highlight for you. This is a very useful trick to stay organized and to figure out where components like if-then-else statements start and end.

Required Reading

Style Guides

A style guide is a set of rules or conventions that outline how a person’s programming code should work. Sometimes workplaces have their own style guide for their programmers. Other times, a programming language itself will have certain rules and conventions connected to its use.

You can read about some of the Java conventions in these style guides:

• Google Java Style Guide
• Oracle Code of Conventions for Java
• Java Code Conventions

 

Choose Your Own Adventure Game

"Choose Your Own Adventure" games and books have always been a popular source of entertainment for kids and adults. The idea behind them is that the user or reader gets to make decisions as he or she progresses through a story or game.

Each decision leads to new ones, and if you play the game or read the book twice, the outcomes can be entirely different based on the decisions made.

Dealing with various outcomes and alternatives in programs, flow charts and diagrams can be quite useful. The use of these tools help in the logic of the program (they will ensure that the program and choices make sense) and they speed up the actual coding of the program because the design is already established.

If you perform a web search for Choose Your Own Adventure Diagram, you might be surprised at the complexity of some of the stories being created. There are a number of alternative paths and outcomes.