Lesson 1
August 9, 2000
I recommend that you also make use of my online Java tutorial lessons, which are designed from a more conventional textbook viewpoint. Those tutorial lessons are published at Gamelan.com.
For your convenience, I also maintain a consolidated Table of Contents on my personal web site that links to the individual lessons on the Gamelan site.
Insofar as possible, I will make use of Sun Java in these lessons. However, it will not be possible for me to go back and do a full update each time Sun releases a new version, so over the course of time, I expect to use different versions of Sun Java.
Just in case you would like to sneak a peek, the answers to the questions, and the explanations of those answers are located (in reverse order) at the end of this file.
The questions and the answers are connected by hyperlinks to make it
easy for you to navigate from the question to the answer and back.
It is recommended that you make your first pass through the questions in
the order that they appear so as to avoid inadvertently seeing the answer
to a question before you provide your own answer.
2. Separate source File Requirements: True or false? A source file may contain an unlimited number of non-public class definitions.
3. Class file names: True or false? If a source file includes a public class, the class name must match the unextended filename.
4. True or false: Each source file that you compile will produce one file with an extension .class
5. Required programming elements: True or false. At least one of the following three top-level elements must appear in every source file. If they are included in the source file, they must appear in the following order.
A. package declaration
B. import declarations
C. class definitions
6. Import directives: True or false? The compiler does not require you to use import directives.
7. Import directives: What output is produced by the following program?
//File Q001_07.java
import java.awt.*;
public class Q001_07{
public static void main
(String args[]){
Button aButton =
new Button("Button");
Label aLabel =
new Label("Label");
System.out.println("OK");
}//end mail
}//end class Q001_07
|
8. goto and const: True or false? As in C and C++, goto and const are keywords that are actively used in Java.
9. Which of the following characters may be used as the first character in a Java identifier?
A. x (the letter x)
B. 4 (the number 4)
C. $ (the dollar sign character)
D. _ (the underscore character)
Bonus question. The following question is considerably more difficult than the previous nine questions, and is included here to challenge you if the previous nine questions have been too easy.
10. Access Control: What output is produced by compiling and running the following program? Note that the instance variable named x is declared private (as highlighted in boldface in the code).
//File Q001_10.java
class Q001_10{
public static void main(
String args[]){
AClass ref1 = new AClass(5);
AClass ref2 = new AClass(10);
System.out.println(
ref1.add(ref2));
}//end main()
}//end class definition
class AClass{
private int x;
AClass(int x){//constructor
this.x = x;
}// end constructor
int add(AClass ref){
return x + ref.x;
}//end add()
}//end class AClass
|
Richard has participated in numerous consulting projects involving Java, XML, or a combination of the two. He frequently provides onsite Java and/or XML training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Java Programming Tutorials, which has gained a worldwide following among experienced and aspiring Java programmers. He has also published articles on Java Programming in Java Pro magazine.
Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.
10. The answer is C. The output produced by compiling and running the program is 15.
The important point is that access modifiers dictate which classes -- not which instances of a class -- may access features. Thus, a private instance variable can be accessed by any instance of the class in which the private instance variable is defined.
The first character must be a letter.
All subsequent characters must be letters or numerals from any alphabet that Unicode supports.
The underscore character (_) and the dollar sign ($) are considered to be letters and may be used as any character including the first one.
Alternatively, an import directive can use the wildcard character (*) to "import" all of the class files in a specified package. This program uses the following wildcard version of the import directive to import all of the class files in the package named awt.
import java.awt.*;
This includes both Button and Label. Therefore, it is not necessary to provide fully-qualified path and package information when referring to either of these classes in the program code.
A word of caution is in order, however. The purpose of packages is to resolve name clashes among class files having the same names. If you find yourself referring to two or more different class files that have the same name, they must be stored in different packages, and you cannot use import directives for those class files. In that case, you must provide a fully-qualified path and package name each time you refer to one of those classes.
It is possible that you could use the wildcard character to import two different packages that contain one or more duplicate class file names. This could lead to problems.
Therefore, the safest approach is to avoid the use of the wildcard character altogether and to use a separate import directive for each class that you need to refer to in the program. I have worked as a Java consultant for a couple of companies whose internal programming standards prohibit the use of the wildcard character in import directives.
However, you can often save yourself a lot of extra work by using import directives. If you don't use import directives, every reference to a class must provide a fully qualified path and package name for the package that contains the class file.
For example, the following simple program contains references to two classes: Button and Label. An import directive, highlighted in boldface, is used to inform the compiler where it can find the class file for the Button class. No such import directive is provided for the Label class.
As a result, the code required to refer to the Label class (highlighted
in red) must specify the package that contains the class file for the Label
class. Otherwise, the program cannot be compiled. As you can
see, considerably more typing is required to refer to the Label
class than is required to refer to the Button class (highlighted
in blue).
//File Q001_06.java
import java.awt.Button;
public class Q001_6{
public static void main
(String args[]){
Button aButton =
new Button("Button");
java.awt.Label aLabel =
new java.awt.Label("Label");
System.out.println(
aButton.getLabel());
System.out.println(
aLabel.getText());
}//end main
}//end class
|
First, it is easy to demonstrate that none of these programming elements are required to be present in a source file, by using the Sun javac.exe program to compile an empty source file. The fact that javac.exe will happily compile an empty source file (producing no output) causes the overall answer to the question to be false.
However, and this is more important, if any of the three programming elements are included in the source file, they must appear in the following order, so the answer to the second question is true.
A. package declaration
B. import declarations
C. class definitions
The fact that each public class requires its own source file is easy enough to demonstrate. The following simple Java application fails to compile with a compiler error to the effect that "Public class Dummy must be defined in a file called "Dummy.java"."
If the public modifier is removed from the definition of the class named Dummy, leaving only one public class in the source file, the application will compile and execute successfully.
(Note that boldface and color were used here to assist the eye in separating
the different parts of the program.)
public class Q001_2{
public static void main
(String args[]){
System.out.println("OK");
}//end main
}//end class
public class Dummy{
//empty class definition
}//end class definition
class AnotherDummy{
//empty class definition
}//end class definition
|
The output from the editor must not contain any control characters indicating bold, italics, underline, etc.
Just about any text editor can be used for this purpose. Various text editors are available on the web that can make the creation of source code easier. These editors provide features such as causing the various parts of the program to appear in different colors during the editing process , or automatically indenting each line of source code according to generally accepted indentation standards.
Some of the available editors will allow you to compile and execute the Java program from within the editor program, which can be a real time saver during development. Editors of this type are often referred to as Integrated Development Environments (IDE).
Back to the point of the discussion, the file name of all source code must end with the extension .java If your editor doesn't put it there, you will need to rename the file manually to create the correct extension.
When you successfully compile the program, one or more files will be produced by the compiler having the extension .class
Richard has participated in numerous consulting projects involving Java, XML, or a combination of the two. He frequently provides onsite Java and/or XML training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Java Programming Tutorials, which has gained a worldwide following among experienced and aspiring Java programmers. He has also published articles on Java Programming in Java Pro magazine.
Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.
-end-