savitchAbsJavaPPT Java Programming Part 1

Slides prepared by Rose Williams, Binghamton University
Chapter 1
Getting Started

© 2006 Pearson Addison-Wesley. All rights reserved 1-2
Introduction To Java
• Most people are familiar with Java as a
language for Web applications
• We will study Java as a general
purpose programming language
– The syntax of expressions and
assignments will be similar to that of other
high-level languages
– Details concerning the handling of strings
and console output will probably be new

History of the Java Language
• Created by Sun Microsystems team led by
James Gosling (1991)
• Originally designed for programming home
appliances
– Difficult task because appliances are controlled
by a wide variety of computer processors
– To simplify the task, team developed a two-step
translation process to avoid having a separate
compiler for each class of appliances (which is
very costly)

Origins of the Java Language
• Two-step translation process
1) Developed an intermediate language
that is the same for all types of
processors : Java byte-code
2) Needed an easy to write program called
the interpreter to translate byte-code into
the machine code for each processor

Objects and Methods
• Java is an object-oriented
programming (OOP) language
– Programming methodology that views a
program as consisting of objects that
interact with one another by means of
actions (called methods)
– Objects of the same kind are said to have
the same type or be in the same class

Java Application Programs
• There are two types of Java programs:
applications and applets
• A Java application program or "regular"
Java program is a class with a method
named main
– When a Java application program is run, the run-
time system automatically invokes the method
named main
– All Java application programs start with the main
method

Applets
• A Java applet (little Java application) is a
Java program that is meant to be run from a
Web browser
• In contrast, application programs may have
a Graphical User Interface or Console
Interface (input read from the console and
output written to the console)

A Sample Java Application Program

System.out.println
• Java programs work by having things
called objects perform actions
– System.out: an object used for sending
output to the screen
• The actions performed by an object
are called methods
– println: the method or action that the
System.out object performs

System.out.println
• Invoking or calling a method: When an
object performs an action using a method
– Also called sending a message to the object
– Method invocation syntax (in order): an object, a
dot (period), the method name, and a pair of
parentheses
– Arguments: Zero or more pieces of information
needed by the method that are placed inside the
parentheses
System.out.println("This is an argument");

Variable declarations
• A variable is a value that can change,
depending on conditions or on
information passed to the program
• Variable declarations in Java are similar
to those in other programming languages
– Simply give the type of the variable followed
by its name and a semicolon
int answer;

Using = and +
• In Java, the equal sign (=) is used as the
assignment operator
– The variable on the left side of the assignment
operator is assigned the value of the expression
on the right side of the assignment operator
answer = 2 + 2;
• In Java, the plus sign (+) can be used to
denote addition (as above) or concatenation
– Using +, two strings can be connected together
System.out.println("2 plus 2 is " + answer);

Program terminology
• Source code (or source program): A
program written in a high-level language
such as Java
– The input to the compiler program
• Object code: The translated low-level
program
– The output from the compiler program, e.g., Java
byte-code
– In the case of Java byte-code, the input to the
Java byte-code interpreter

Syntax and Semantics
• Syntax: The arrangement of words
and punctuations that are legal in a
language, the grammar rules of a
language
• Semantics: The meaning of things
written while following the syntax rules
of a language

Tip: Error Messages
• Bug: A mistake in a program
– The process of eliminating bugs is called
debugging
• Syntax error: A grammatical mistake
in a program
– The compiler can detect these errors, and
will output an error message saying what
it thinks the error is, and where it thinks
the error is

Tip: Error Messages
• Run-time error: An error that is not detected
until a program is run
– The compiler cannot detect these errors: an error
message is not generated after compilation, but after
execution
• Logic error: A mistake in the underlying
program which makes it operate
incorrectly
– The compiler cannot detect these errors, and no error
message is generated after compilation or execution,
but the program does not do what it is supposed to
do

Identifiers
• Identifier: The name of a variable or other
item (class, method, object, etc.) defined in
a program
– A Java identifier must not start with a digit, and
all the characters must be letters, digits, or the
underscore symbol
– Java identifiers can theoretically be of any length
– Java is a case-sensitive language: Rate, rate,
and RATE are the names of three different
variables

Identifiers
• Reserved words: Identifiers that have a
predefined meaning in Java
– Do not use them to name anything else

Naming Conventions
• Start the names of variables, methods, and
objects with a lowercase letter, indicate
"word" boundaries with an uppercase letter,
and restrict the remaining characters to
digits and lowercase letters:
topSpeed bankRate1 timeOfArrival
• For classes, use an uppercase letter for
each word:
Employee FirstProgram MyClass

Variable Declarations
• Every variable in a Java program must be declared
before it is used
– A variable declaration tells the compiler what kind of data
(type) will be stored in the variable
– The type of the variable is followed by one or more
variable names separated by commas, and terminated
with a semicolon
– Variables are typically declared just before they are used
or at the start of a block (indicated by an opening brace { )
– Basic types in Java are called primitive types
int numberOfBeans;
double oneWeight, totalWeight;

Primitive Types

Assignment Statements With Primitive
Types
• In Java, the assignment statement is used
to change the value of a variable
– The equal sign (=) is used as the assignment
operator
– An assignment statement consists of a variable
on the left side of the operator, and an
expression on the right side of the operator
Variable = Expression;
– An expression consists of a variable, number, or
mix of variables, numbers, operators, and/or
method invocations
temperature = 98.6;
count = numberOfBeans;

Assignment Statements With Primitive
Types
– When an assignment statement is executed, the
expression is first evaluated, and then the
variable on the left-hand side of the equal sign is
set equal to the value of the expression
distance = rate * time;
– Note that a variable can occur on both sides of
the assignment operator
count = count + 2;
– The assignment operator is automatically
executed from right-to-left, so assignment
statements can be chained
number2 = number1 = 3;

Tip: Initialize Variables
• A variable that has been declared but that
has not yet been given a value by some
means is said to be uninitialized
• In certain cases an uninitialized variable is
given a default value
– It is best not to rely on this
– Explicitly initialized variables have the added
benefit of improving program clarity

Tip: Initialize Variables
• The declaration of a variable can be
combined with its initialization via an
assignment statement
int count = 0;
double distance = 55 * .5;
char grade = 'A';
– Note that some variables can be initialized and
others can remain uninitialized in the same
declaration
int initialCount = 50, finalCount;

Shorthand Assignment Statements
• Shorthand assignment notation combines the
assignment operator (=) and an arithmetic operator
• It is used to change the value of a variable by
adding, subtracting, multiplying, or dividing by a
specified value
• The general form is
Variable Op = Expression
which is equivalent to
Variable = Variable Op (Expression)
– The Expression can be another variable, a constant, or
a more complicated expression
– Some examples of what Op can be are +, -, *, /, or %

Shorthand Assignment Statements
Example: Equivalent To:
count += 2; count = count + 2;
sum -= discount; sum = sum – discount;
bonus *= 2; bonus = bonus * 2;
time /=
rushFactor;
time =
time / rushFactor;
change %= 100; change = change % 100;
amount *=
(count1 + count2);
amount = amount *
(count1 + count2);

Type Casting
• A type cast takes a value of one type and produces
a value of another type with an "equivalent" value
– If n and m are integers to be divided, and the fractional
portion of the result must be preserved, at least one of the
two must be type cast to a floating-point type before the
division operation is performed
double ans = n / (double)m;
– Note that the desired type is placed inside parentheses
immediately in front of the variable to be cast
– Note also that the type and value of the variable to be cast
does not change

More Details About Type Casting
• When type casting from a floating-point to an
integer type, the number is truncated, not rounded
– (int)2.9 evaluates to 2, not 3
• It is illegal to place a double value into an int
variable without an explicit type cast
int i = 5.5; // Illegal
int i = (int)5.5 // Correct i is 5

Assignment Compatibility
• In general, the value of one type cannot be
stored in a variable of another type
int intVariable = 2.99; //Illegal
– The above example results in a type mismatch
because a double value cannot be stored in an
int variable
• However, there are exceptions to this
double doubleVariable = 2;
– For example, an int value can be stored in a
double type

Assignment Compatibility
• More generally, a value of any type in the following list can
be assigned to a variable of any type that appears to the
right of it
byteshortintlongfloatdouble
char
• An explicit type cast is required to assign a value of one
type to a variable whose type appears to the left of it on
the above list (e.g., double to int)
int x;
double y = 35.5;
x = (int)y;
x will be set to 35
• In the above example, note that assigning x to y, explicit
cast is not required as an implicit cast will be applied
automatically:
y = x; // no explicit cast needed: y = (double) x;

Constants
• Constant (or literal): An item in Java which
has one specific value that cannot change
– Constants of an integer type may not be written
with a decimal point (e.g., 10)
– Constants of a floating-point type can be written
in ordinary decimal fraction form (e.g., 3.67)
– Constant of a floating-point type can also be
written in floating-point or scientific notation
(e.g., 5.89e4)
• Note that the number before the e may contain a
decimal point, but the number after the e may not

Constants
• Constants of type char are expressed by
placing a single character in single quotes
(e.g., 'Z')
• Constants for strings of characters are
enclosed by double quotes (e.g., "Welcome
to Java")
• There are only two boolean type
constants, true and false
– Note that they must be spelled with all lowercase
letters

Arithmetic Operators and Expressions
• As in most languages, expressions
can be formed in Java using variables,
constants, and arithmetic operators
– These operators are + (addition), -
(subtraction), * (multiplication), /
(division), and % (modulo, remainder)
– Example expression:
z = w + 2;

Arithmetic Operators and Expressions
• If an arithmetic operator is combined with int operands,
then the resulting type is int
• Similarly, if an arithmetic operator is combined with at
double operands, then the resulting type is double
• If different types are combined in an expression, then the
resulting type is the right-most type on the following list
that is found within the expression
byteshortintlongfloatdouble
char
– Exception: If the type produced should be byte or short
(according to the rules above), then the type produced will
actually be an int
• Question: What will be the resulting type of the following
expression?
float a=11.5f;
double b =12.2;
a + b

Parentheses and Precedence Rules
• An expression can be fully parenthesized in
order to specify exactly what
subexpressions are combined with each
operator
• If some or all of the parentheses in an
expression are omitted, Java will follow
precedence rules to determine, in effect,
where to place them
– However, it's best (and sometimes necessary) to
include them

Precedence Rules

Precedence and Associativity Rules
• When the order of two adjacent operations
must be determined, the operation of higher
precedence (and its apparent arguments) is
grouped before the operation of lower
precedence
base + rate * hours is evaluated as
base + (rate * hours)
• When two operations have equal precedence,
the order of operations is evaluated left to
right:
int x;
x = 6 * 4 / 2;
is evaluated as
x = (6 * 4) /2

Integer and Floating-Point Division
• When one or both operands are a floating-point
type, division results in a floating-point type
15.0/2 evaluates to 7.5
• When both operands are integer types, division
results in an integer type
– Any fractional part is discarded
– The number is not rounded
15/2 evaluates to 7
• Be careful to make at least one of the operands a
floating-point type using explicit type cast if the
fractional portion is needed:
(double)15/2 evaluates to 7.5

Unary +,- vs Binary +, -
• When +,- are used as unary operators, they
operate on one operand:
int x=4, y;
y = +x; // y becomes 4, note + is redundant
y = -x; // y becomes -4
• When +,- are used as binary operators, they
operate on two operands:
int x=4, y=2, z;
z = x + y; // z becomes 6
z = x – y; // z becomes 2

The ! Operator
• Called the logical NOT operator
• It operates on a boolean expression and
reverses it (if true it becomes false and if false, it
becomes true
double balance=0;
if (!(balance > 0))
System.out.println("No money left!");
• The if statement will be covered in Chapter 3.

The % Operator
• The % operator is used with operands of
type int
• It divides left-hand operand by right-hand
operand and returns remainder.
15 % 2 evaluates to 1

Increment and Decrement Operators
• The increment operator (++) adds one
to the value of a variable
– If n is equal to 2, then n++ or ++n will
change the value of n to 3
• The decrement operator (--) subtracts
one from the value of a variable
– If n is equal to 4, then n-- or --n will
change the value of n to 3

Increment and Decrement Operators
• When either operator precedes its variable,
and is part of an expression, then the
expression is evaluated using the changed
value of the variable
– If n is equal to 2, then 2*(++n) evaluates to 6
• When either operator follows its variable,
and is part of an expression, then the
expression is evaluated using the original
value of the variable, and only then is the
variable value changed
– If n is equal to 2, then 2*(n++) evaluates to 4
– The value of n will then become 3

The Class String
• The class String is a predefined class in Java that
is used to store and process strings
• Objects of type String are made up of strings of
characters that are written within double quotes
– Any quoted string is a constant of type String
"Live long and prosper."
• A variable of type String can be given the value of
a String object
String blessing = "Live long and prosper.";

Concatenation of Strings
• Concatenation: Using the + operator on two strings
in order to connect them to form one longer string
– If greeting is equal to "Hello ", and javaClass is
equal to "class", then greeting + javaClass is
equal to "Hello class"
• Any number of strings can be concatenated
together
• When a string is combined with almost any other
type of item, the result is a string
– "The answer is " + 42 evaluates to
"The answer is 42"

String Methods
• The String class contains many useful methods
for string-processing applications
– A String method is called by writing a String object, a
dot, the name of the method, and a pair of parentheses to
enclose any arguments
– If a String method returns a value, then it can be placed
anywhere that a value of its type can be used
String greeting = "Hello";
int count = greeting.length();
System.out.println("Length is " + count);
– Always count from zero when referring to the position or
index of a character in a string

String Indexes

Some Methods in the Class String
(Part 1 of 8)

(Part 2 of 8)

(Part 3 of 8)

(Part 4 of 8)

(Part 5 of 8)

(Part 6 of 8)

(Part 7 of 8)

Escape Sequences
• A backslash () immediately preceding
a character (i.e., without any space)
denotes an escape sequence or an
escape character
– The character following the backslash
does not have its usual meaning
– Although it is formed using two symbols,
it is regarded as a single character

Escape Sequences

Character Sets
• ASCII: A character set used by many programming
languages that contains all the characters normally
used on an English-language keyboard, plus a few
special characters
– Each character is represented by a particular number
• Unicode: A character set used by the Java
language that includes all the ASCII characters plus
many of the characters used in languages with a
different alphabet from English

Naming Constants
• Instead of using "anonymous" numbers in a
program, always declare them as named constants,
and use their name instead
public static final int INCHES_PER_FOOT = 12;
public static final double RATE = 0.14;
– This prevents a value from being changed inadvertently
– It has the added advantage that when a value must be
modified, it need only be changed in one place
– Note the naming convention for constants: Use all
uppercase letters, and designate word boundaries with an
underscore character

Comments
• A line comment begins with the symbols //,
and causes the compiler to ignore the
remainder of the line
– This type of comment is used for the code writer
or for a programmer who modifies the code
• A block comment begins with the symbol
pair /*, and ends with the symbol pair */
– The compiler ignores anything in between
– This type of comment can span several lines
– This type of comment provides documentation
for the users of the program

savitchAbsJavaPPT Java Programming Part 1

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