1

CS2136:Paradigms of Computation

Class 10:Java:

Introduction and/or Review

Copyright 2001, 2002, 2003 Michael J. Ciaraldi and David Finkel

2

Java Programs

Two types of programs: An application runs stand-alone. An applet runs inside a Web browser

Or the utility called “appletviewer.”

Few differences

3

Creating and RunningJava Applications

Create a source file called something.java

Submit it to the compiler javac something.java

This produces an object file called something.class

To run, type [Note: No suffix!] java something

4

A Simple Sample Java Program(Eckel, 1st edition, Chap. 2)

//: Property.javaimport java.util.*;/** The first Thinking in Java example program. Lists system information on current machine. * @author Bruce Eckel * @author http://www.BruceEckel.com * @version 1.0 */public class Property { /** Sole entry point to class & application * @param args array of string arguments * @return No return value * @exception exceptions No exceptions thrown */ public static void main(String[] args) { System.out.println(new Date()); Properties p = System.getProperties(); p.list(System.out); System.out.println("--- Memory Usage:"); Runtime rt = Runtime.getRuntime(); System.out.println("Total Memory = “ + rt.totalMemory() + " Free Memory = “ +

rt.freeMemory()); }}

5

Java Syntax

Semicolon is the statement terminator, not the statement separator.

Comments can be either: Single line, starting with // Block comments, bracketed by /* … */

Within block comments, lines starting with @ are part of javadoc.

Program blocks enclosed by { … }Case is iMpOrTaNt.

6

Structure of the Samplefile Property.java

import java.util.*; We will use the utilitypackage. Could get asingle class instead.

public class Property { Define a class called Property,

visible to all. Must havesame name as file.

…

7

Structure of the Samplefile Property.java II

public static void main(String[] args) {…}For any application, the method

called main is called at start.This method:

returns nothing (void) is visible to all (public) always exists (static)

Command-line arguments in args[].

8

Structure of the Samplefile Property.java III

System.out.println(new Date());Create a new object of type Date, with

default value (current date and time).Within the class System is an object

out, which is the standard output; this has a method called println().

When you pass an argument to println(), it gets printed, then we move to a new line.

9

Structure of the Samplefile Property.java IV

What happens to the newly-created object of type Date? It gets passed to the println(), then it is

not saved anywhere, so it gets garbage collected.

10

Structure of the Samplefile Property.java IV

Properties p = System.getProperties();p.list(System.out);We create a variable called p which

can hold a reference to an object of class Properties.

We invoke the getProperties() method to create an object, and store its reference (handle) in p.

We invoke the list() method on p, which asks it to print itself.

11

Structure of the Samplefile Property.java V

Runtime rt = Runtime.getRuntime();System.out.println(“Total Memory = “

+ rt.totalMemory() + “ Free Memory = “ + rt.freeMemory());

We invoke getRuntime() to get a reference to the current runtime environment (not a copy).

We concatenate with some text to print some of the runtime values.

12

Access Methods

Usually an object has attributes (a.k.a. data fields, a.k.a. instance variables), but you cannot access them directly.

Instead, there are access methods.To see some, check out the API docs at

http://www.cs.wpi.edu/~ciaraldi/jdk1.3/docs/api/

Remember, you don’t know how the data is represented internally.

13

Access Methods Example:The Date Class

long getTime() Returns the number of milliseconds since

January 1, 1970, 00:00:00 GMT represented by this Date object.

String toString() Converts this Date object to a String of

the form: dow mon dd hh:mm:ss zzz yyyy E.g. Tue Mar 21 08:11:13 EST 2000 Note: println() calls toString()

automatically when given a non-string object.

14

Access Methods Example:The Date Class II

void setTime(long time) Sets this Date object to represent a

point in time that is time milliseconds after January 1, 1970 00:00:00 GMT.

15

Allocating an Object

(Almost) every class has a constructor function. We’ll see how to define them later. Constructor has same name as class. Triggered by keyword new.

A constructor might take a variable number of parameters. Including none at all!

16

Constructor Example:The Date ClassDate()

Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.

Date(long date) Allocates a Date object and initializes it

to represent the specified number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT.

17

A Warning

Some features of Java are

Deprecated!

18

The Garbage Collector

Really should be called the Recycler!It runs in the background.When it finds an object which is not

referred to, it destroys the object so the memory can be reused.

Can be invoked explicitly with System.gc() Will try its best.

19

Miscellaneous Java Features

20

Variables

Defined using this syntax: type name ;

Can also define and initialize in same statement: type name = value ;

Can hold either: Primitive Reference (“Handle”) to an object.

Not the object itself. Not the object itself.Not the object itself.

21

Variables

1.234

float x

1234

int y

String s

Date d

November 20, 2000.11:00 am

"Hello"String

Date

22

Java Primitive Types(Not Objects)

Integer (only signed) 8-bit byte 16-bit short 32-bit int 64-bit long

Real 32-bit float 64-bit double

char (16 bit) Uses Unicode

boolean Values:

truefalse

23

Assignment

For primitives, = copies the value.For objects, = copies the reference

(handle) to the object.foo a = new foo();

foo b;b = a;

Now, b and a refer to the same object.A very important difference!

24

Assignment

Foo b;

Foo a = new Foo();

b = a;

Foo b

Foo b

Foo a

some Fooish data

Foo

Foo b

Foo a some Fooish dataFoo

After Line 1

After Line 2

After Line 3

25

Kinds of Variables

Instance Variables Defined as part of a class. Allocated as each object is instantiated. Destroyed with the object.

Local Variables Defined as part of a method (within a

class). Allocated when the method is invoked. Destroyed when the method exits.

26

Math Operators

Same as C or C++

27

Relational OperatorsUnlike C or C++, return boolean, not

integer.Use with boolean:

== !=Use with all other primitive types:

< > <= >= == !=Use with objects:

== != Test for same object, not equal value. Can test value with equals(), if

available.

28

Other Operators

BitwiseShiftsString concatenation

+

29

Flow Control

if-elsewhiledo-whilefor

The test must be booleanswitchreturn

30

Next Time

JavadocClasses

Data Methods Encapsulation