Download ppt - Dr. Wissam Fawaz

Variables in Java

A variable holds either

Primitive value


Or an object


int num;
Scanner scan;
Once declared => un-initialized
-
num
-
scan
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
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No object is created with this declaration
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doesn’t hold an object, but a reference to the object
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can be set to null => does not refer to an object
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String name;
name = null;
Creating an object

Generally, we use the new operator

To create an object
name = new String (“James Bond");
This calls the String constructor, which is
a special method that sets up the 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
Reference variables
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( );
Appended
after the object
reference.

Method
being invoked
A method invocation can be thought of

As asking an object to perform a service
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
Assignment revisited

The act of assignment

Takes a copy of a value and stores it in a variable

For primitive types:
Before:
num1
38
num2
96
num2 = num1;
After:
num1
38
num2
38
Reference assignment

For object references

Assignment copies the address
Before:
name1
"Steve Jobs"
name2
"Steve Wozniak"
name2 = name1;
name1
After:
name2
"Steve Jobs"
Aliases

num1
Effect of assignment on primitive values
5
num1
12
num2
12
num1 = num2;
num2

name1
12
Effect of assignment on object references
“string1”
name1
name2 = name1;
name2
“string2”
name2
aliases
“string1”
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
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Because there is really only one object
Garbage collection
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When an object
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no longer has any valid references to it
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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
Background information

Keep in mind

Object reference variable stores an address

interaction with an object occurs via reference variables
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You can use a variable, ONLY if you have reference to it

When last reference to object is lost
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Object can no longer contribute to the program

At this point, the object is called garbage

Occasionally, JAVA collects all garbage and free memory
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
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)
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is made up of clusters of related classes
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Called JAVA APIs, or application programming interfaces
Packages

Classes of the JAVA standard library
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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
Purpose
java.lang
java.applet
java.awt
javax.swing
java.net
General support
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities
Network communication
import declaration

To use classes from any package, either
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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!!
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
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Refer to Import_example.java
to import all classes in a particular package

You can use the * wildcard character
import java.util.*;
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
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import java.util.Scanner;
Java.lang package

All classes of the java.lang package
 are imported automatically into all programs

It is as if all programs contain the following line
import java.lang.*;


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
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.*
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
JAVA 10 commandments
(cont’d)


Make sure your statements end with semicolons
The file name
 that holds the program



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 */,


must be the same as the name of the public class
then everything that follows will be treated as a comment
Confirm that every open brace { in the program is matched by
a close brace }
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
Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
Interactive programs

Programs generally


Need input on which to operate
It is often useful

To design a program that reads data interactively


During execution
That way,
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New results are computed each time the program is run

Depending on the data that is entered
The Scanner class

How to design programs


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
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Keyboard input is represented by the System.in object
Reading input
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First, you create a Scanner object

The following line creates the Scanner object

That reads from the keyboard
Scanner scan = new Scanner (System.in);


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
Example

See Echo.java

The Scanner class

is part of the java.util library


Methods such as nextInt()and nextDouble()


that must be imported into a program to use Scanner
Read data of particular types
See GasMileage.java