1. Variables in Java A variable holds either
Primitive value
int num;
Or an object
Scanner scan;
Once declared => un-initialized -
num -
scan

2. Object declaration Variable referring to an object
must be declared with the class name as the object type
String name;
=> name is an object reference variable
No object is created with this declaration
doesn’t hold an object, but a reference to the object
can be set to null => does not refer to an object
name = null;

3. Creating an object Generally, we use the new operator
To create an object
Creating an object is called instantiation
Returns a reference to the newly created object
The object is said to be an instance of a particular class
name = new String (“James Bond");
This calls the String constructor, which is
a special method that sets up the object

4. Reference variables Object reference variable
name
“James Bond”
Object reference variable
Pointer to the location of the created object in memory
uses the dot operator to access the objects methods
int count ;
count = name.length( );
A method invocation can be thought of
As asking an object to perform a service
Appended
after the object
reference.
Method
being invoked

5. Shortcuts Initializing the variable in the declaration
String title = new String(“JAVA software solution”);
Whenever a string literal appears
A String object is created automatically
=> String city = “London”; // statement is VALID
For String, the explicit use of new can be eliminated

6. Assignment revisited The act of assignment Before: After:
Takes a copy of a value and stores it in a variable
For primitive types:
num1 38
num2 96
Before:
num2 = num1;
num1 38
num2
After:

7. Reference assignment For object references Before: After:
Assignment copies the address
name1
name2
Before:
"Steve Jobs"
"Steve Wozniak"
name2 = name1;
name1
name2
After:
"Steve Jobs"

8. Aliases Effect of assignment on primitive values
Effect of assignment on object references 5 12
num1
num2
num1
num1 = num2; 12
num2
name1
“string1”
aliases
name1
“string1”
name2 = name1;
“string2”
name2
name2

9. Aliases (cont’d) 2 or more references referring to same object
Are called aliases of each other
=> multiple reference variable can be used
To access the object
Changing an object thru one reference
Changes it for all of its aliases
Because there is really only one object

10. Garbage collection When an object In other languages (like C++)
no longer has any valid references to it
It becomes useless, and therefore is called garbage
Java performs automatic garbage collection periodically
Returning an object’s memory to the system for future use
In other languages (like C++)
The programmer is responsible for
Performing garbage collection

11. Background information
Keep in mind
Object reference variable stores an address
interaction with an object occurs via reference variables
You can use a variable, ONLY if you have reference to it
When last reference to object is lost
Object can no longer contribute to the program
At this point, the object is called garbage
Occasionally, JAVA collects all garbage and free memory

12. Outline Creating Objects The String Class Packages Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images

13. Class library JAVA class library
Provides support when developing JAVA programs
includes classes containing valuable methods
String class is part of the JAVA standard class library
was created by employees at Sun (who created JAVA)
is made up of clusters of related classes
Called JAVA APIs, or application programming interfaces

14. Packages Classes of the JAVA standard library
are grouped into packages
Each class is part of a particular package
String is part of the java.lang package
Scanner is part of the java.util package
Different packages exist in JAVA
Package
java.lang
java.applet
java.awt
javax.swing
java.net
Purpose
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities
Network communication

15. import declaration To use classes from any package, either
Fully qualify the reference
including the package name,
Every time it is referenced
java.util.Scanner scan = new java.util.Scanner(System.in)
This however becomes tiring!!

16. import declaration Or, use an import declaration
To simplify these references
Import the class, and then use just the class name
import java.util.Scanner
Refer to Import_example.java
to import all classes in a particular package
You can use the * wildcard character
import java.util.*;

17. Different forms of import declaration
Import uses
an asterisk to indicate
Any class inside the package might be used
import java.util.*;
the name of the particular class
If only one class of a certain package is needed
import java.util.Scanner;

18. Java.lang package import java.lang.*;
All classes of the java.lang package
are imported automatically into all programs
It is as if all programs contain the following line
That is why we didn’t have to import String classes
The Scanner class
Is part of java.util package
And therefore must be imported
import java.lang.*;

19. Java.lang java.lang package Automatically imported
Fundamental, thought of as basic extension of language
Therefore
Any class in java.lang package can be used
Such as System, String, etc…
Without an explicit import declaration
Any program contains: import java.lang.*

20. JAVA 10 commandments You must declare every identifier
that is not a JAVA reserved word
Not doing so results in an error message
JAVA is a case-sensitive language
so two identifiers that are capitalized differently
are treated differently
Check for mismatched quotes in char literals
Each char literal starts and ends with an apostrophe
In an assignment statement,
make sure that the identifier to the left of =
is a variable and not a named constant

21. JAVA 10 commandments (cont’d)
Make sure your statements end with semicolons
The file name
that holds the program
must be the same as the name of the public class
but with the extension .java
If the public class is named Driver => file: Driver.java
Be careful when using the /* */ to delimit comments
If you forget the */,
then everything that follows will be treated as a comment
Confirm that every open brace { in the program is matched by a close brace }

22. JAVA 10 commandments (cont’d)
To print a double quote
within a literal string, use the symbol \”
Make sure
that the application class and main are public

23. Outline Character Strings Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes

24. Interactive programs Programs generally It is often useful That way,
Need input on which to operate
It is often useful
To design a program that reads data interactively
During execution
That way,
New results are computed each time the program is run
Depending on the data that is entered

25. The Scanner class How to design programs Scanner class
reading data from the user interactively?
Scanner class
provides convenient methods
for reading input values of various types
Can be setup to read input from the keyboard
Keyboard input is represented by the System.in object

26. Reading input First, you create a Scanner object
The following line creates the Scanner object
That reads from the keyboard
The new operator creates the Scanner object
Use the Scanner object to invoke methods
answer = scan.nextLine();
Reads the input line and returns it as one String
scan.nextInt(), scan.nextDouble
Used to read data of particular types
Scanner scan = new Scanner (System.in);

27. Example See Echo.java The Scanner class
is part of the java.util library
that must be imported into a program to use Scanner
Methods such as nextInt()and nextDouble()
Read data of particular types
See GasMileage.java