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White Space
• Spaces, blank lines, and tabs are
collectively called white space and are used
to separate words and symbols in a program
• Extra white space is ignored
• A valid Java program can be formatted
many different ways
• Programs should be formatted to enhance
readability, using consistent indentation
• Comments in a program are also called
inline documentation
• They should be included to explain the
purpose of the program and describe
processing steps
• Java comments can take two forms:
– // comment runs to the end of the line
– /* comment runs to terminating symbol, even
across line breaks */
• Identifiers are the words a programmer uses
in a program
• Most identifiers have no predefined
meaning except as specified by the
• An identifier can be made up of letters,
digits, the underscore character (_), and the
dollar sign
• They cannot begin with a digit
Reserved Words
• Some identifiers, called reserved words,
have specific meanings in Java and cannot
be used in other ways
• abstract boolean break byte
byvalue case cast catch char
class const continue default do
double else extends false final
finally float for future generic
goto if implements import inner
• A literal is an explicit data value used in a
• Integer literals: 25 69 -4288
• Floating point literals: 3.14159 42.075 0.5
• String literals: "The result is: " "To
thine own self be true."
The Java API
• The Java Application Programmer Interface
(API) is a collection of classes that can be
used as needed
• The println and print methods are part of the
Java API; they are not part of the Java
language itself
• Both methods print information to the
screen; the difference is that println
moves to the next line when done, but print
• A program can have three types of errors
• The compiler will find problems with
syntax and other basic issues (compiletime errors)
• If compile-time errors exist, an executable
version of the program is not created
• A problem can occur during program
execution, such as trying to divide by
zero, which causes a program to terminate
Command Line Arguments
• See
• The main method accepts extra information
on the command line when a program is
• > java Name_Tag John
• Each extra value is called command line
• In Java, command line arguments are
always read as a list of character strings
Object-Oriented Programming
• Java is object-oriented language
• Programs are made from software
components called objects
• An object contains data and methods
• An object is defined by a class
• Multiple objects can be created from the
same class
Object-Oriented Programming
• A class represents a concept and an object
represents the realization of that concept
• Car Class Objects
Object-Oriented Programming
• Objects can also be derived from each other
using a process called inheritance
• Objects, classes, and inheritance will be
discussed in greater detail later
Class Libraries
• The Java API is a class library, a group of
classes that support program
development Classes in a class hierarchy
are often related by inheritance
• The classes in the Java API is separated into
• The System class, for example, is in
package java.lang
• Each package contains a set of classes that
The Java API Packages
• Some packages in the Java API:
• java.lang package: free gift
Importing Packages
• Using a class from the Java API can be
accomplished by
using its fully qualified name:
java.lang.System.out.println ();
Or, the package can be imported using an
import statement, which has two forms:
import java.applet.*;
import java.util.Random;
• The java.lang package is automatically
Primitive Data Types
• A data type is defined by a set of values and
the operators you can perform on them
• Each value stored in memory is associated
with a particular data type
• The Java language has several predefined
types, called primitive data types. The
following reserved words represent eight
different primitive types:
– byte, short, int, long, float, double,
• There are four separate integer primitive
data types
They differ by the amount of memory
used to store them
A boolean value represents a true or false
They can also be used to represent any
two states, such as a
light bulb being on or off
The reserved words true and false are the
only valid values
for a boolean type
• The ASCII character set is still the basis for
many other programming languages
• ASCII is a subset of Unicode, including:
– uppercase letters
– lowercase letters
– punctuation
special symbols
control characters
A, B, C, …
• For each primitive data type there is a
corresponding wrapper class.
• For example:
Wrapper classes are useful in situations
where you need an object instead of a
primitive type
They also contain some useful methods
Numeric Input
• Converting a string that holds an integer into
the integer value can be done with a method in
the Integer wrapper class:
• A value can be read and converted in one line:
Integer.parseInt (stdin.readLine());
• An expression is a combination of
operators and operands
– The arithmetic operators include addition (+),
subtraction (-),multiplication (*), and division
– Operands can be literal values, variables, or
other sources of data
– The programmer determines what is done with
the result of an expression (stored, printed, etc.)
Operator Precedence
• The order in which operands are evaluated
in an expression
is determined by a well-defined
precedence hierarchy
Operators at the same level of
precedence are evaluated
according to their associativity (right to
left or left to right)
The if Statement
The Java if statement has the following
if (condition)
If the boolean condition is true, the
statement is executed; if
Block Statements
• Several statements can be grouped together
into a block statement
– Blocks are delimited by braces
– A block statement can be used wherever a
statement is called for in the Java syntax
The if-else Statement
• An else clause can be added to an if
statement to make it an
if-else statement:
if (condition)
Nested if Statements
• The body of an if statement or else clause
can be another if
These are called nested if statements
Note: an else clause is matched to the
The while Statement
• A while statement has the following syntax:
while (condition)
If the condition is true, the statement is
executed; then the
condition is evaluated again
The while Statement
• If the condition of a while statement is false
initially, the statement is never executed
Therefore, we say that a while statement
executes zero or more times
Infinite Loops
• The body of a while loop must eventually
make the condition
If not, it is an infinite loop, which will
execute until the user
interrupts the program
This is a common type of logical error --
Program Development
• The creation of software involves four basic
establishing the requirements
creating a design
implementing the code
testing the implementation
• The development process is much more
involved that this, but these basic steps are a
• Requirements specify the tasks a program
must accomplish (what to do, not how to do
– They often address the user interface
– An initial set of requirements are often
provided, but usually must be critiqued,
modified, and expanded
– It is often difficult to establish detailed,
unambiguous, complete requirements
– Careful attention to the requirements can save
significant time and money in the overall
• A program follows an algorithm, which is a
step-by-step process for solving a problem
• The design specifies the algorithms and data
• In object-oriented development, it
establishes the classes, objects, and methods
that are required
• The details of a method may be expressed
in pseudo-code, which is code-like, but does
not necessarily follow any specific syntax
• Implementation is the process of translating
a design into source code
• Most novice programmers think that writing
code is the heart of software development,
but it actually should be the least creative
• Almost all important decisions are made
during requirements analysis and design
• Implementation should focus on coding
details, including style guidelines and
• A program should be executed multiple
times with various input in an attempt to
find errors
• Debugging is the process of discovering the
cause of a problem and fixing it
• Programmers often erroneously think that
there is "only one more bug" to fix
• Tests should focus on design details as well
as overall requirements