Java Enterprise Java Java Advanced Placement Study Guide: Introduction to the Lessons, Primitive Types

Java Advanced Placement Study Guide: Introduction to the Lessons, Primitive Types

Welcome


This is the first lesson in a miniseries of tutorial lessons designed to
help you learn the essential features of Java object-oriented programming
as identified by The College
Board
.

Purpose

The purpose of this miniseries is to help you study for, and successfully
complete, the Advanced Placement Examinations designed by the College Board.

Once you understand everything in this miniseries, plus the material
in the lessons that I published earlier on Java
Data Structures
, you should understand the Java programming features
that the College Board considers essential for the first two semesters
of object-oriented programming education at the university level.

Hopefully, that will help you to take and successfully complete the
Advanced Placement Examinations.

Background

In October 2000, the AP Computer Science Development Committee made
a formal request to The
College Board
that the AP Computer Science curriculum be revised to
include object orientation and to use Java as the delivery language for
the AP Computer Science Examinations. The request was approved by the College Board in November 2000.

As of December 2001:

You will have to complete a free registration to gain access
to the subset
material.  Once you register for that material, you
will also have
access to a lot more information as well.

Disclaimer

I have no inside knowledge of the questions that
will appear on the AP exams, either in terms of format or content. 
I have no relationship with the College Board, and did not participate
in any way in the deliberations leading up to the use of Java for the AP
exams.

I have constructed this study guide solely on
the basis of the material in the subset
document and my experience as a college professor teaching Java Programming
to approximately 500 college students and industry programmers each year
since 1997.

Approach

This series of tutorial lessons provides questions, answers, and explanations
designed to help you understand the essential subset of features identified
by The College Board.

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.

Supplementary material

In addition to the material in these lessons, I recommend that you also
study the other lessons in my extensive collection of online Java tutorials,
which are designed from a more conventional textbook approach.  You
will find those lessons published at
Gamelan.com
However, as of the date of this writing, Gamelan doesn’t maintain a consolidated
index of my Java tutorial lessons, and sometimes they are difficult to
locate there.  You will find a consolidated index at
Baldwin’s
Java Programming Tutorials
.

Insofar as possible, I will make use of Sun
Java
in these lessons.  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.

Invoking a Java Program

The subset document mentioned above contains the following statement,
“The AP CS exam does not prescribe any particular approach for program invocation.”

There are several different kinds of Java programs, including applications,
applets, and servlets.  Each different kind of program is invoked
in a different way, and the AP exam doesn’t require the student to know
about all of the different ways that a program can be invoked.  Realistically,
however, in order to write and execute a Java program, you must know how
to invoke that kind of program.

The simplest kind of Java program to invoke is a Java application. 
The questions in these lessons that show program code will almost always
show code that is contained in an application.  The Java virtual machine
always invokes the main method of the controlling class for an application
in order to invoke the program.

In order to illustrate the object-oriented nature of Java, the main
method in these questions will usually create an instance (object) of
some other class and invoke one or more methods on that object.  Thus,
the main method will usually have no purpose other than to cause
the program to start running.  The details of data processing will
rarely be included in the main method.  When all of the methods
invoked by the main method return, the program will terminate and
control will return to the system prompt.

File Management

The controlling class used in the program code in these lessons will
normally be declared public.

Java requires that any class definition that is declared public
be placed in a separate source code file.  The file management required
to maintain large numbers of files can be burdensome for simple programs. 
These are very simple programs and there is no need to place each class
definition associated with a single program in its own separate source
code file.

To avoid the logistical effort required to place the definitions for
classes other than the controlling class in separate source code files,
the other classes defined in a program will not normally be declared public. 
Those classes will not have an access modifier.  (Note that this
is often referred to as package or default access or visibility, and is
not a requirement of the AP CS subset
This approach is being used here strictly for the convenience of the author.)
 
This will make the class definitions accessible to the code in the main
method, without requiring that they be placed in separate source code files.

What is Included?

Click here for a preview of the Java
programming features covered by this lesson.



1.  What output is produced by the following
program?

  • A.  Compiler error
  • B.  Runtime error
  • C.  Hello World
  • D.  Goodbye Cruel World
public class Ap001{
  public static void main(
                        String args[]){
    new Worker().hello();
  }//end main()
}//end class definition

class Worker{
  public void hello(){
    System.out.println("Hello World");
  }//end hello()
}//end class definition

Answer and Explanation

2.  What is the largest (algebraic) value
of type
int?

  • A.  32767
  • B.  2147483647
  • C.  -2147483647
  • D.  -32768

Answer and Explanation

3.  What is the smallest (algebraic) value
of type
int?

  • A.  -2147483648
  • B.  -2147483647
  • C.  32767
  • D.  -32768

Answer and Explanation

4.  What two values are displayed by the following
program?

  • A.  -2147483648
  • B.  1.7976931348623157E308
  • C.  -2147483647
  • D.  4.9E-324
public class Ap003{
  public static void main(
                        String args[]){
    new Worker().printDouble();
  }//end main()
}//end class definition

class Worker{
  public void printDouble(){
    System.out.println(
                     Double.MAX_VALUE);
    System.out.println(
                     Double.MIN_VALUE);
  }//end printDouble()
}//end class definition

Answer and Explanation

5.  What output is produced by the following program?

  • A.  true
  • B.  false
  • C.  1
  • D.  0
public class Ap004{
  public static void main(
                        String args[]){
    new Worker().printBoolean();
  }//end main()
}//end class definition

class Worker{
  private boolean myVar;
  public void printBoolean(){
    System.out.println(myVar);
  }//end printBoolean()
}//end class definition

Answer and Explanation

6.  What output is produced by the following program?

  • A.  Compiler Error
  • B.  Runtime Error
  • C.  true
  • D.  false
public class Ap005{
  public static void main(
                        String args[]){
    new Worker().printBoolean();
  }//end main()
}//end class definition

class Worker{
  public void printBoolean(){
    boolean myVar;
    System.out.println(myVar);
  }//end printBoolean()
}//end class definition

Answer and Explanation

7.  What output is produced by the following program?

  • A.  Compiler Error
  • B.  Runtime Error
  • C.  true
  • D.  false
public class Ap006{
  public static void main(
                        String args[]){
    new Worker().printBoolean();
  }//end main()
}//end class definition

class Worker{
  public void printBoolean(){
    boolean myVar = true;
    myVar = false;
    System.out.println(myVar);
  }//end printBoolean()
}//end class definition

Answer and Explanation

8.  The plus (+) character can be used to perform
numeric addition in Java.  What output is produced by the following
program?  Pay particular attention to the boldface expression.

  • A.  Compiler Error
  • B.  Runtime Error
  • C.  true
  • D.  2
  • E.  1
public class Ap007{
  public static void main(
                        String args[]){
    new Worker().printBoolean();
  }//end main()
}//end class definition

class Worker{
  public void printBoolean(){
    boolean myVar = true;
    System.out.println(1 + myVar);
  }//end printBoolean()
}//end class definition

Answer and Explanation

9.  The plus (+) character can be used to perform
numeric addition in Java.  What output is produced by the following
program?

  • A.  Compiler Error
  • B.  Runtime Error
  • C.  6
  • D.  6.0
public class Ap008{
  public static void main(
                        String args[]){
    new Worker().printMixed();
  }//end main()
}//end class definition

class Worker{
  public void printMixed(){
    double x = 3;
    int y = 3;
    System.out.println(x+y);
  }//end printMixed()
}//end class definition

Answer and Explanation

10.  The slash (/) character can be used to perform
numeric division in Java. What output is produced by the following program?

  • A.  Compiler Error
  • B.  Runtime Error
  • C.  0.33333334
  • D.  0.3333333333333333
public class Ap009{
  public static void main(
                        String args[]){
    new Worker().printMixed();
  }//end main()
}//end class definition

class Worker{
  public void printMixed(){
    System.out.println(1.0/3);
  }//end printMixed()
}//end class definition

Answer and Explanation



Copyright 2001, Richard G. Baldwin.  Reproduction in whole or
in part in any form or medium without express written permission from Richard
Baldwin is prohibited.

About the author

Richard Baldwin
is a college professor (at Austin Community College in Austin, TX) and
private consultant whose primary focus is a combination of Java and XML.
In addition to the many platform-independent benefits of Java applications,
he believes that a combination of Java and XML will become the primary
driving force in the delivery of structured information on the Web.

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.

[email protected]


What is Included?


According to the subset
document, “Primitive types: int, double, boolean are part of the AP Java
subset.”


Answers and Explanations

Answer 10


D.  0.3333333333333333

Back to Question 10

Explanation 10


Divide floating type by integer type

This program divides the literal floating value
of 1.0 by the literal integer value of 3 (no decimal point is specified
in the integer literal value).

Automatic conversion from narrow to wider type

To begin with, whenever division is performed
between a floating type and an integer type, the integer type is automatically
converted (sometimes called promoted) to a floating type and floating
arithmetic is performed.

What is the actual floating type, float or
double?

The real question here is, what is the type of
the literal shown by 1.0 (with a decimal point separating the 1 and
the 0). 
Is it a double or a float?

Type double is the default

By default, a literal floating value is treated
as a double.

The result is type double

Consequently, this program divides a double
type by an integer type, producing a result of type double
This is somewhat evident in the output, which shows about 17 digits plus
a decimal point in the result (recall that the maximum value for a float
shown earlier had only about eight digits plus the decimal point and the
exponent)
.

How can you create literals of type float?

What if you don’t want your literal floating value
to be treated as a double, but would prefer that it be treated as
a float instead.

You can usually force this to be the case by adding
a suffix of either F or f to the end of the literal value
(as
in 1.0F)
.  If you were to modify this program to cause it to divide
1.0F by 3, the output would be
0.33333334 with only nine
digits in the result.

Answer 9


D.  6.0

Back to Question 9

Explanation 9


Declare and initialize two local variables

This program declares and initializes two local
variables, one of type double and the other of type int
Each variable is initialized with the integer value 3.

Automatic conversion to floating type double

However, before the value of 3 is stored in the
double
variable, it is automatically converted to type double.

Automatic conversion in mixed-type arithmetic

Numeric addition is performed on the two variables. 
Whenever addition is performed between a floating type and an integer type,
the integer type is automatically converted to a floating type and floating
arithmetic is performed.

A floating result

This produces a floating result.  When this
floating result is passed to the println() method for display, a
decimal point and a zero are displayed to indicate a floating type, even
though in this case, the fractional part of the result is zero.

Answer 8


A.  Compiler Error

Back to Question 8

Explanation 8


Initialize boolean variable to true

This program declares and initializes a boolean
variable with the value true.  Then it attempts to add the
literal value 1 to the value stored in the boolean variable named
myVar.

Arithmetic with booleans is not allowed

As mentioned earlier, unlike C++, boolean
types in Java cannot participate in arithmetic expressions.

Therefore, this program will not compile. 
The compiler error produced by this program under JDK 1.3 reads partially
as follows:

Ap007.java:13: operator + cannot be applied
to int,boolean


    System.out.println(1
+ myVar);

Answer 7


D.  false

Back to Question 7

Explanation 7


Format for variable initialization

This program declares a local boolean variable and initializes
it to the value true.  All variables, local or otherwise,
can be initialized in this manner provided that the expression on the right
of the equal sign evaluates to a value that is assignment compatible
with the type of the variable (I will have more to say about assignment
compatibility in a subsequent lesson).

Value is changed before display

However, before invoking the println() method to display the
initial value of the variable, the program uses the assignment operator
(=) to assign the value false to the variable.  Thus, when
it is displayed, the value is false.

Answer 6


A.  Compiler Error

Back to Question 6

Explanation 6


A local boolean variable

In this program, the primitive variable named
myVar
is a local variable belonging to the method named printBoolean().

Local variables are not automatically initialized

Unlike instance variables, if you fail to initialize
a local variable, the variable is not automatically initialized.

Cannot access value from uninitialized local
variable

If you attempt to access and use the value from
an uninitialized local variable before you assign a value to it, you will
get a compiler error.  The compiler error produced by this program
under JDK 1.3 reads partially as follows:

Ap005.java:13: variable myVar might not
have been initialized


    System.out.println(myVar);

Must initialize or assign value to all local
variables

Thus, the programmer is responsible for either
initializing all local variables, or assigning a value to them before attempting
to access their value with code later in the program.

Answer 5


B.  false

Back to Question 5

Explanation 5


The boolean type

In this program, the primitive variable named myVar is an instance
variable of the type boolean.

What is an instance variable?

 (An instance variable is a variable that is declared inside
a class, outside of all methods of the class, and is not declared static. 
Every object instantiated from the class has one.  That is why it
is called an instance variable.)

Cannot use uninitialized variables in Java

One of the great things about Java is that it is not possible to make
the mistake of using variables that have not been initialized.

Can initialize when declared

All Java variables can be initialized when they are declared.

Member variables are automatically initialized

If the programmer doesn’t initialize the variables declared inside the
class but outside of a method (often referred to as member variables
as opposed to local variables),
they are automatically initialized
to a default value.  The default value for a boolean variable
is false.

Did you know the boolean default value?

I wouldn’t be overly concerned if you had selected the answer A. 
true
, because I wouldn’t necessarily expect you to memorize the default
initialization value.

Great cause for concern

However, I would be very concerned if you selected either C. 1
or D. 0.

Java has a true boolean type

Unlike C++, Java does not represent true and false by the numeric values
of 1 and 0 (at least the numeric values that represent true and false
are not readily accessible by the programmer)
.

Thus, you cannot include boolean types in arithmetic expressions,
as is the case in C++.

Answer 4


B.  1.7976931348623157E308

D.  4.9E-324

Back to Question 4

Explanation 4


Floating type versus integer type

If you missed this one, shame on you!

I didn’t expect you to memorize the maximum and
minimum values represented by the floating type double, but I did
expect you to be able to distinguish between the display of a floating
value and the display of an integer value.

Both values are positive

Note that both of the values given above are positive
values.

Unlike the integer types discussed earlier, the
constants named MAX_VALUE and MIN_VALUE don’t represent the ends of a signed
number range for type double.  Rather, they represent the largest
and smallest (non-zero) values that can be expressed by the type.

An indication of granularity

MIN_VALUE is an indication of the degree of granularity
of values expressed as type double.  Any double value
can be treated as either positive or negative.

Two floating types are available

Java provides two floating types:  float
and double.  The double type provides the greater range,
or to use another popular terminology, it is the wider of the two.

What is the value range for a float?

In case you are interested, using the same syntax
as above, the value range for type float is from 1.4E-45
to 3.4028235E38

Double is often the default type

There is another thing that is significant about
type double.  In many cases where a value is automatically
converted to a floating type, it is converted to type double rather
than to type float.  This will come up in future lessons.

Answer 3


A.  -2147483648

Back to Question 3

Explanation 3


Could easily have guessed

As a practical matter, you had one chance in two
of guessing the answer to this question, already having been given the
value of the largest algebraic value for type int.

And the winner is …

Did you answer B.  -2147483647?

If so, you may be wondering why the most negative
value isn’t equal to the negative version of the most positive value?

A twos-complement characteristic

Without going into the details of why, it is a
well-known characteristic of binary twos-complement notation that the value
range extends one unit further in the negative direction than in the positive
direction.

What about the other two values?

Do the values of  -32768 and 32767 in the
set of multiple-choice answers to this question represent anything in particular?

Yes, they represent the extreme ends of the value
range for a 16-bit binary number in twos-complement notation.

Does Java have a 16-bit integer type?

Just in case you are interested, the short
type in Java is represented in 16-bit binary twos-complement signed notation,
so this is the value range for type short.

What about type byte?

Similarly, a value of type byte is represented
in 8-bit binary twos-complement signed notation, with a value range extending
from -128 to 127.

Answer 2


B.  2147483647

Back to Question 2

Explanation 2


First question on types

This is the first question on Java types in this series of tutorial
lessons.

What does the College Board have to say?

To set the stage for the discussion of types, the version of the Java
subset
document that is available on 10/30/01 contains the following statement:

“Primitive types: int, double, boolean
are part of the AP CS subset. The other primitive types short,
long,
byte,
char
and float are not in the subset.”

Concentrate on int, double, and boolean

Therefore, this series of lessons will concentrate on int, double,
and boolean.

All eight primitive types are important

However, this should not be taken to mean that the other five types
are unimportant.  If you expect to work as a Java programmer, you
must be fully capable of using all of the eight primitive types, not just
the three covered here.

32-bit signed twos-complement integers

Now back to the question at hand.  In Java, values of type int
are stored as 32-bit signed integers in twos-complement notation.

Can you calculate the values?

There are no unsigned integer types in Java, as there are in
C++.  If you are handy with binary notation, you could calculate the
largest positive value that can be stored in 32 bits in twos-complement
notation.

See documentation for the Integer class

Otherwise, you can visit the documentation for the Integer class,
which provides a symbolic constant (public static final variable)
named MAX_VALUE.  The description of MAX_VALUE reads
as follows:

“The largest value of type int. The constant value of this
field is 2147483647.”

Answer 1


C.  Hello World

Back to Question 1

Explanation 1


Pretty easy huh?

The answer to this first question is intended to be easy.  The
purpose of the first question is to introduce you to the syntax that will
frequently be used for program code in this series of lessons.

The controlling class and the main method

In this example, the class named Ap001 is the controlling class. 
It contains a method named main(), whose signature matches the required
signature for the main() method.  When the user executes this
program, the Java virtual machine automatically invokes the method named
main()
in the controlling class.

Create an instance of Worker

The main() method uses the new operator along with the
default
constructor for the class named Worker to create a new instance
of the class named Worker (an object of the Worker class). 
This is often referred to as instantiating an object.

A reference to an anonymous object

The combination of the new operator and the default constructor
for the Worker class returns a reference to the new object. 
In this case, the object is instantiated as an anonymous object,
meaning that the reference to the object is not saved in a named reference
variable.  (Instantiation of a non-anonymous object will be illustrated
later.)

Invoke hello method on Worker object

The main() method contains a single executable statement that
is highlighted in boldface.  (Note:  Although the statement
is highlighted in boldface for presentation purposes, actual Java source
code never contains boldface, Italics, underlines, or any other form of
decoration.)

As soon as the reference to the new object is returned, the single statement
in the main() method invokes the hello() method on that reference.

Output to standard output device

This causes the hello() method belonging to the new object (of
the class named Worker)
to execute.  The code in the hello()
method invokes the println() method on the static variable
of the System class named out.

Lots of OOP embodied in the hello method

I often tell my students that I can tell a lot about whether a student
really understands object-oriented programming in Java by asking them to
explain everything that they know about the following statement:

System.out.println("Hello World");

I would expect the answer to consume about ten to fifteen minutes if
the student really understands Java OOP.

The one-minute version

When the virtual machine starts a Java application running, it automatically
instantiates an I/O stream object linked to the standard output device
(normally the screen) and stores a reference to that object in the static
variable named
out belonging to the class named System.

Invoke the println instance method on out

Invoking the
println() method on that reference, and passing
a literal string (“Hello World”) to that method causes the contents
of the literal String object to be displayed on the standard output
device.

Display Hello World on the screen

In this case, this causes the words Hello World to be displayed
on the standard output device.  This is the answer to the original
question.

Time for main method to terminate

When the hello() method returns, the main() method has
nothing further to do, so it terminates.  When the main() method
terminates in a Java application, the application terminates and returns
control to the operating system.  This causes the system prompt to
reappear.

A less-cryptic form

A less cryptic form of this program is shown below.

 

public class Ap002{
  public static void main(
                        String args[]){
    Worker refVar = new Worker();
    refVar.hello();
  }//end main()
}//end class definition

class Worker{

  public void hello(){
    System.out.println("Hello World");
  }//end hello()
}//end class definition

Decompose single statement into two statements

In this version, the single statement in the earlier version of the
main()
method is replaced by two statements, which are highlighted in boldface.

A non-anonymous object

In the class named Ap002, the reference to the object of the
class named Worker is not instantiated anonymously.  Rather,
a new object of the Worker class is instantiated and a reference
to the object is stored in (assigned to) the reference variable
named refVar.

Invoke hello method on named reference

Then the hello() method is invoked on that reference in a separate
statement.

Produces the same result as before

The final result is exactly the same as before.  The only difference
is that a little more typing is required to create the source code for
the second version.

Will often use anonymous objects

In order to minimize the amount of typing required, I will probably
use the anonymous form of instantiation whenever appropriate in these tutorial
lessons.

Now that you understand the framework …

Now that you understand the framework for the program code, I can present
more specific questions based on the material that is expected to be included
in the AP exam.

About the author

Richard Baldwin
is a college professor (at Austin Community College in Austin, TX) and
private consultant whose primary focus is a combination of Java and XML.
In addition to the many platform-independent benefits of Java applications,
he believes that a combination of Java and XML will become the primary
driving force in the delivery of structured information on the Web.

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.

[email protected]

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