"if" statements
Now that we know about conditions, we can talk about conditional execution. Thus far in our program the execution of the code has been very straightforward. The program starts on the first line of Stakeout's "main" function and proceeds down to the end of the main function (denoted by the "}" character) executing each line of code on the way. Now I will introduce a feature of the Java language which will complicate this somewhat, but which will give us the ability to respond to the user input. It is called the "if" statement. Here's the general structure:
if (someCondition) {
someStatement;
someOtherStatement;
...
}
"someCondition" can be any condition. If it evaluates to true, then the statements enclosed in the following brackets will be executed. If "someCondition" evaluates to false or zero, these statements will be skipped.
Here's an example that shows the usefulness of this feature. Say that our game allowed 2, 3, or 4 players. In our startup sequence we need prompt the user for how many players he wants to play with and we should only accept the value if it is "2", "3", or "4".
public class Stakeout {
public static void main (String args[]) throws java.io.IOException {
System.out.println ("How many players do you want to use? [2, 3, or 4]");
String input = CmdInput.GetInput ();
if (input.contentEquals ("2")) {
System.out.println ("Two players it is!");
}
if (input.contentEquals ("3")) {
System.out.println ("Three players it is!");
}
if (input.contentEquals ("4")) {
System.out.println ("Four players it is!");
}
System.out.println ("Game over!");
}
}
Try to compile this program and then see how it responds to the different input.
This code introduces the
contentEquals function of the String class. Look at the documentation to familiarize yourself with it. Note that it returns a boolean value.
Now the trick comes when we try to respond to an entry other than the 3 valid values that we accept. We will use the System.exit function to immediately quit the program and we will use complex conditionals to detect the situation:
public class Stakeout {
public static void main (String args[]) throws java.io.IOException {
System.out.println ("How many players do you want to use? [2, 3, or 4]");
String input = CmdInput.GetInput ();
if (input.contentEquals ("2")) {
System.out.println ("Two players it is!");
}
if (input.contentEquals ("3")) {
System.out.println ("Three players it is!");
}
if (input.contentEquals ("4")) {
System.out.println ("Four players it is!");
}
if (! input.contentEquals ("2")
&& ! input.contentEquals ("3")
&& ! input.contentEquals ("4")) {
System.out.println ("You entered an invalid number of players!");
System.out.println ("Exiting...");
System.exit (0);
}
System.out.println ("Game over!");
}
}
But this is a little cumbersome, not to mention redundant. There's an easier way...
"else" and "else if" statements
To make this task a easier, the Java language has an "else" statement and an "else if" statement. It can be used to solve our problem above like so:
public class Stakeout {
public static void main (String args[]) throws java.io.IOException {
System.out.println ("How many players do you want to use? [2, 3, or 4]");
String input = CmdInput.GetInput ();
if (input.contentEquals ("2")) {
System.out.println ("Two players it is!");
}
else if
(input.contentEquals ("3")) {
System.out.println ("Three players it is!");
}
else if
(input.contentEquals ("4")) {
System.out.println ("Four players it is!");
}
else
{
System.out.println ("You entered an invalid number of players!");
System.out.println ("Exiting...");
System.exit (0);
}
System.out.println ("Game over!");
}
}
The program will run each line of the main function in order until it reaches the first "if" statement. If the condition evaluates to true, it will execute the statements inside the "if" and then skip the rest of the "else" statements and execute the last println. If the first "if" condition evaluates to false, it will then go on and evaluate the first "else if" statement. If the first "else if" statement evaluates to true it will execute the statements inside and then skip the rest and execute the final prinln. If the first "else if" condition evaluates to false, it will move on to the second "else if" statement. If the second "else if" condition evaluates to true, it will execute the code inside and skip the rest and execute the last println. If the second "else if" condition evaluates to false, it will immediately execute the statements inside the "else" statement (no conditional to evealuate) and then move on and execute the final println. Notice that we no longer need the redundant checks to make sure the input is neither 2, 3, or 4.
A note on indetation: leading whitespace is totally unnecessary. The program above could have been written like this:
public class Stakeout {
public static void main (String args[]) throws java.io.IOException {
System.out.println ("How many players do you want to use? [2, 3, or 4]");
String input = CmdInput.GetInput ();
if (input.contentEquals ("2")) {
System.out.println ("Two players it is!");
}
else if
(input.contentEquals ("3")) {
System.out.println ("Three players it is!");
}
else if
(input.contentEquals ("4")) {
System.out.println ("Four players it is!");
}
else
{
System.out.println ("You entered an invalid number of players!");
System.out.println ("Exiting...");
System.exit (0);
}
System.out.println ("Game over!");
}
}
In fact the whole program could have been written on a single line, but this would hardly have made it readable. It's important to indent the code inside "if" and "else" statements so that it's easier to follow the flow of execution in the program.
