Flow Control
Some of the most powerful tools of Java programming are control structures.
Control structures are used to create the basic logic that drives many of the routines used in your applications.
These control structures use Boolean logic to imbue your script with the intelligence necessary to manage the diverse needs of the clients.
All control structures are divided into the control statement (which we will explain below) and the statement block.
The statement block is simply a group of commands that are executed together.
This block is grouped by enclosing the commands within curly braces ({}).
For example, the following is a simple statement block.
{
statement one
statement two
statement three
}
Java will execute each statement in a statement block from beginning to end as a group.
When, how, or if the script will execute the commands however, is determined by the control statement.
if / else if / else
The most common control statement is the "if" test.
The if test checks to see if some expression is true, and if so, executes the routines in the statement block.
Java uses a simple binary comparison as a test of truth.
If the result of some operation is true, the operation returns a one and the statement block is executed.
If the result is false, it returns a zero, and the statement block is not executed.
For example, consider the following code:
if (name.equals("Selena Sol"))
{
System.out.println("Hello Selena.\n");
}
In this example, Java checks to see if the scalar variable name has the value of "Selena Sol".
If the patterns match, the matching operation will return true and the application will execute the print statement within the statement block.
If Java discovers that name is not equal to "Selena Sol" however, the print will not be executed.
The if test also provides for alternatives: the "else" and the "else if" control statements.
The else if alternative adds a second check for truth and the else alternative defines a final course of action for every case of failed if or else if tests.
The following code snippet demonstrates the usage of if, else if, and else.
if (age == 28)
{
System.out.println("You are 28!");
}
else if (age == 29)
{
System.out.println("You are 29!");
}
else
{
System.out.println("You're neither!");
}
while
The while control statement allows you to loop through a control block until some condition is met.
while (x <=100)
{
X++
System.out.println("x is: " x);
}
// In a while loop, the action is
// performed only if the test is true.
// An interesting alternative is
// the do / while loop that performs
// the action "before" testing.
for
The for loop is another excellent control statement tool.
The basic syntax of a for loop follows:
for ([initial condition]; [test]; [incrementation])
{
[action to perform]
}
The "initial condition" defines where the loop should begin.
The "test" defines the logic of the loop by letting the script know the conditions that determine the application's actions.
The "incrementation" defines how the script should perform the loop.
For example, we might produce a visible countdown with the following for loop:
for (int x = 10;X >=0;x--)
{
System.out.println("x is: " x);
}
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NOTE: To decrement, you use variableName--.
To increment, you use variableName++.
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Testing a Condition
We can use the ==, !=, >, >=, <, <= operators to test equality.
Further, we can use the && and || operators to perform multiple tests.
Consider the following:
(8 > 2) && (2 == 3) // Returns false because
// one of the statements is false
(8 >= 2) || (2 == 3) // Returns true because
// at least one is true.
// Notice that the second
// test is short circuited
// by the true result in
// the first.
The Switch Statement
However, Java does not stop there.
Java introduces another control structure called the switch statement.
The switch statement allows you to input a single variable and test for multiple values.
Consider the following example:
switch (input)
{
case 1;
{
System.out.println("You entered a 1");
break;
}
case 2;
{
System.out.println("You entered a 2");
break;
}
case 3;
{
System.out.println("You entered a 3");
break;
}
}
Breaks
As you can see, Java also provides a break statement that allows you to break out of a loop.
Selena Sol contributes to the JavaBoutique's Introduction to Java. Selena curently works for Barclays Capital in London, one of the leading global investment banks in Europe and has worked as a software developer for the National Center for Human Genome research, Microline Software, Neuron Data, and Electric Eye in Singapore. Selena is perhaps best-known for creating the Public Domain Web Script Archive (Extropia) and writing several books on Web Programming (Perl, CGI, Java).
Email: selena@extropia.com
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