The only people who never make mistakes are
the people who never do anything at all. But careful planning can help you
avoid some of the most common Java goofs, such as the following.
Putting capital
letters where they belong
Java is a case-sensitive language, so you
really have to mind your Ps
and Qs — along with every
other letter of the alphabet. Here are some things to keep in mind as you
create Java programs:
·
Java's keywords are all completely lowercase. For instance, in a
Java if statement, the word ifcan't
be If or IF.
·
When you use names from the Java API (Application Programming
Interface), the case of the names has to match what appears in the API.
·
You also need to make sure that the names you make up yourself
are capitalized the same way throughout your entire program. If you declare a
myAccount variable, you can't refer to it as MyAccount, myaccount, or
Myaccount. If you capitalize the variable name two different ways, Java thinks
you're referring to two completely different variables.
Breaking out of
a switch statement
If you don't break out of a switch statement,
you get fall-through. For instance, if the value of verse is 3, the following
code prints all three lines — Last refrain, He's a pain, and Has no brain.
switch (verse) {
case 3:
out.print("Last refrain, ");
out.println("last refrain,");
case 2:
out.print("He's a pain, ");
out.println("he's a pain,");
case 1:
out.print("Has no brain, ");
out.println("has no brain,");
}
switch (verse) {
case 3:
out.print("Last refrain, ");
out.println("last refrain,");
case 2:
out.print("He's a pain, ");
out.println("he's a pain,");
case 1:
out.print("Has no brain, ");
out.println("has no brain,");
}
Comparing
values with a double equal sign
When you compare two values with one another,
you use a double equal sign. The line
if (inputNumber == randomNumber)
if (inputNumber == randomNumber)
is correct, but the line
if (inputNumber = randomNumber)
if (inputNumber = randomNumber)
is not correct.
Adding
components to a GUI
Here's a constructor for a Java frame:
public SimpleFrame() {
JButton button = new JButton("Thank you...");
setTitle("...Katie Feltman and Heidi Unger");
setLayout(new FlowLayout());
add(button);
button.addActionListener(this);
setSize(300, 100);
setVisible(true);
}
public SimpleFrame() {
JButton button = new JButton("Thank you...");
setTitle("...Katie Feltman and Heidi Unger");
setLayout(new FlowLayout());
add(button);
button.addActionListener(this);
setSize(300, 100);
setVisible(true);
}
Whatever you do, don't forget the call
to the add method. Without this call, you go to all the work of creating a
button, but the button doesn't show up on your frame.
Adding
listeners to handle events
Look again at the previous section's code to
construct a SimpleFrame. If you forget the call to addActionListener, nothing
happens when you click the button. Clicking the button harder a second time
doesn't help.
Defining the
required constructors
When you define a constructor with parameters,
as in
public Temperature(double number)
public Temperature(double number)
then the computer no longer creates a default
parameterless constructor for you. In other words, you can no longer call
Temperature roomTemp = new Temperature();
Temperature roomTemp = new Temperature();
unless you explicitly define your own
parameterless Temperature constructor.
Fixing
non-static references
If you try to compile the following code, you
get an error message:
class WillNotWork {
String greeting = "Hello";
public static void main(String args[]) {
System.out.println(greeting);
}
}
class WillNotWork {
String greeting = "Hello";
public static void main(String args[]) {
System.out.println(greeting);
}
}
You get an error message because main is
static, but greeting isn't static.
Staying within
bounds in an array
When you declare an array with ten components,
the components have indices 0 through 9. In other words, if you declare
int guests[] = new int[10];
int guests[] = new int[10];
then you can refer to the guests array's
components by writing guests[0], guests[1], and so on, all the way up to
guests[9]. You can't write guests[10], because the guests array has no
component with index 10.
Anticipating
null pointers
In real-life Java programming, you see that
exception all the time. A NullPointerException comes about when you call a
method that's supposed to return an object, but instead the method returns
nothing. Here's a cheap example:
import static java.lang.System.out;
import java.io.File;
class ListMyFiles {
public static void main(String args[]) {
File myFile = new File("\\windows");
String dir[] = myFile.list();
for (String fileName : dir) {
out.println(fileName);
}
}
}
import static java.lang.System.out;
import java.io.File;
class ListMyFiles {
public static void main(String args[]) {
File myFile = new File("\\windows");
String dir[] = myFile.list();
for (String fileName : dir) {
out.println(fileName);
}
}
}
This program displays a list of all the files
in the windows directory.
But what happens if you change \\windows to
something else — something that doesn't represent the name of a directory?
File myFile = new File("#*%$!!");
File myFile = new File("#*%$!!");
Then the new File call returns null (a special
Java word meaning nothing),
so the variable myFile has nothing in it. Later in the code, the variable dir
refers to nothing, and the attempt to loop through all the dir values fails
miserably. You get a big NullPointerException, and the program comes crashing
down around you.
To avoid this kind of calamity, check Java's
API documentation. If you're calling a method that can return null, add
exception-handling code to your program.
Helping Java
find its files
You're compiling Java code, minding your own
business, when the computer gives you a NoClassDefFoundError. All kinds of
things can be going wrong, but chances are that the computer can't find a
particular Java file. To fix this, you must align all the planets correctly.
·
Your project directory has to contain all the Java files whose
names are used in your code.
·
If you use named packages, your project directory has to have
appropriately named subdirectories.
·
Your CLASSPATH must be set properly.
