If (expression) Code Segments


Introduction.  In almost all instances, when writing computer code, developers are going to want the code's execution to depend on other things such as user selections or other internal computations.  The following list contains a very few of the sorts of situations where this might arise.
  • base execution on which command button a user selects
  • base computations on user inputs
  • base computations on values in a database

Obviously, this list can go on and on.  In order for these sorts of decisions to be made within computer code, computer languages have built-in constructs for control of the program flow.  That is, the programming languages have built-in control structures to allow determination of what code will execute based on other inputs.  While there are all kinds of control structures, in this webpage we will focus on one of the most fundamental, if (expression) code segments.

In general the syntax for this sort of control structure looks like the following.

if (expression)


program statements;


The code between the braces will execute only if the (expression) evaluates as true.  The expression must be contained within parentheses.

If there is only a single code statement after the if (expression) there doesn't need to be any braces.

if (expression)

single program statement;

This isn't really any different than the If - Then statements used in Visual Basic except in VB you don't need to enclose the condition in parentheses and multiple program statements within the decision block are terminated by an End If.

Many programmers, and I strongly encourage you to do so, use some sort of indentation to help them when debugging.  Code can get complicated and determining what belongs in what blocks of code can become very difficult!

Some Verbal Examples.  We will start with some relatively common language examples and then progress to computer code.

if (userAge >= 18)


user can serve in military
user can vote
user can't drink


Another example might be

if (availableCredit > itemPrice)

buyer can buy

Well, this can go on and on.  Obviously, some of these depend on having other sources for inputs.

A Simple Code Example.  Since we have been somewhat shortchanging your experience with applets we will develop another applet.  This applet will do very little other than ask the user to enter a number within a particular range.  If they do it they will get a message that they have done so.  You should call the applet NumberRangeApplet.java.


import javax.swing.JOptionPane;
import javax.swing.JApplet;

public class NumberRangeApplet extends JApplet

public void init()

String userEntry;
float floatUserEntry = 0;

// Prompt user for input
userEntry = JOptionPane.showInputDialog("Please enter a number between 10 and 20:");

// convert these string values to floats

floatUserEntry = Float.parseFloat(userEntry);


catch (NumberFormatException nfeFloat)

JOptionPane.showMessageDialog( null, "The entry needs to be a number",
"Not a Number", JOptionPane.INFORMATION_MESSAGE);


if ((floatUserEntry > 10) && (floatUserEntry < 20))

// dislplaying the textarea within an MessageDialog
JOptionPane.showMessageDialog( null, "You entered " + floatUserEntry +
"\n which is between 10 and 20", "Valid Entry", JOptionPane.INFORMATION_MESSAGE);

} // end method init

} // end class NumberRangeApplet


The html file should be NumberRangeApplet.html.


<APPLET CODE="NumberRangeApplet.class" WIDTH=200 HEIGHT=100> </APPLET>


Since we will use this construct so much throughout the course I will not present another example in this webpage.

The next webpage will improve on this example somewhat by doing a better job of dealing with the situation where the user doesn't input a number in the appropriate range.  This will involve using else clauses in our If (expression) code segments.