CS计算机代考程序代写 javascript compiler Java interpreter SWEN20003 Object Oriented Software Development 0.5 cm A Quick Tour of Java

SWEN20003 Object Oriented Software Development 0.5 cm A Quick Tour of Java

SWEN20003
Object Oriented Software Development

A Quick Tour of Java

Shanika Karunasekera
.au

University of Melbourne
c© University of Melbourne 2020

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 1 / 67

The Road So Far

Lectures

Subject Introduction

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 2 / 67

Learning Outcomes

Upon completion of this topic you will be able to:

Identify some of the key Java features

Understand the following in context of Java:
I Identifiers, Data Types, Variables and Constants
I Operators and Expressions
I Flow of control

Write simple Java programs

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 3 / 67

A Brief History of Java

A programming language developed by Sun Microsystems

The project started in 1991 by a team led by James Gosling

James Gosling

The goal of the project at the time was to develop a language that was
suitable for the next wave in computing which was expected to be the
digitally controlled consumer devices (embedded systems)

The language was then named Oak

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 4 / 67

https://en.wikipedia.org/wiki/James_Gosling

A Brief History of Java cont..

In 1993, there was a demise in digital consumer devices and the direction was
re-focussed

The team focussed on the “next-wave” – the Internet

They decided to work on an embedded language for the web browser named
it an applet (a small application)

JavaScript, although the name has similarities has nothing to do with Java

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 5 / 67

Java Features

Following are some the key features of Java which we will introduce in this topic:

Compiled and Interpreted

Platform-Independent and Portable

Object Oriented

We will introduce more features as we learn new topics during the lectures.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 6 / 67

Compiled Languages (example C)

IDE Integrated Development Environment

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 7 / 67

Java is Compiled and Interpreted

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 8 / 67

Java is Compiled and Interpreted

Java Compiler converts java source code (file with extension .java) to bytecode
(file with extension .class).

Bytecode is an intermediate form, closer to machine representation.

An Interpreter (virtual machine) on any target platform interprets the
bytecode.

Porting a java system to any new platform involves writing an interpreter.

The interpreter will figure out the equivalent machine dependent code to run.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 9 / 67

Java Features

Compiled and Interpreted

Platform-Independent and Portable

Object Oriented

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 10 / 67

Platform Independent

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 11 / 67

Java Features

Compiled and Interpreted

Platform-Independent and Portable

Object Oriented

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 12 / 67

Object Oriented

Java is an Object Oriented Programming (OOP) language.

Common programming constructs are: Classes, Objects, Methods etc.

We will learn more in the coming lectures!

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 13 / 67

Applications vs Applets

There are two types of Java programs:

Application
I Is a stand-alone program.
I Has a main method.
I Can be invoked from command line using the Java interpreter.

Applet
I A program embedded in a web page.
I Has no main method.
I Can be be run by a Java enabled web browser.

In the context of this subject we will only study Java Applications; Java Applets is
a bit of an outdated technology – hardly used today.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 14 / 67

Building your first Java Program

Let us try it out!

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 15 / 67

Hello World in Java

1 // HelloWorld.java: Display “Hello World!” on the screen

3 import java.lang.*;

5 public class HelloWorld {

7 public static void main(String args[]) {

9 System.out.println(“Hello World!”);

11 }

13 }

Program Output:

Hello World!

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 16 / 67

Hello World in Java

1 // HelloWorld.java: Display “Hello World!” on the screen

3 import java.lang.*;

5 public class HelloWorld {

7 public static void main(String args[]) {

9 System.out.println(“Hello World!”);

11 }

13 }

Program Output:

Hello World!

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 16 / 67

Hello World in Java and compared with C

1 // HelloWorld.java: Display “Hello World!” on the screen

2 import java.lang.*;

3 public class HelloWorld {

4 public static void main(String args[]) {

5 System.out.println(“Hello World!”);

6 return;

7 }

8 }

1 /* helloworld.c: Display “Hello World!” on the screen */

2 #include

4 int main(int argc, char *argv[]) {

5 printf(“Hello World! \n”);

6 return 0;

7 }

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 17 / 67

Hello World – Line by Line

Line 1:

This is a comment in Java, similar to line 1 of the C program

Java supports 3 types of comments:
I /* */ – Usually used from multi-line comments, similar to C.
I // – Used for single line comments.
I /** */ – Documentation comments, will learn more later.

Line 2: import java.lang.*;

Serves the same purpose as the #include statement used in C.

Is used to import additional classes (similar to libraries used in C).

In Java, classes are grouped into packages.

Packages may be defined by different people and may even have same class
and method names, but they differ by package name (will learn more later):
e.g. ibm.mathlib.*, microsoft.mathlib.*

By default Java imports java.lang.* package, therefore, this statement is
optional.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 18 / 67

Hello World – Line by Line

Line 3: public class HelloWorld {
Class definition – in Java everything is defined in a class (we will learn more
about classes, and the keyword public later).

The name of the class must be the same as the Java file name (HelloWorld
class must be saved in a file HelloWorld.java)

Line 4: public static void main(String args[])

Definition of the main method, very similar to C.

A standalone Java program must have a main method.

A class can have only one main() method.

We will learn about keywords public and static later.

String args[]: Defines command line arguments, similar to C.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 19 / 67

Hello World – Line by Line

Line 5: System.out.println(“Hello World!”);

Serves the same purpose as the printf function in C, except println is
called a method , as opposed to a function in C.

System.out: out is an object in the class System; this class is defined in
the java.lang package – you will learn more about classes, objects, methods
and packages in the coming lectures.

Line 6: return

This is optional, and usually not included, just included here for comparison
with C.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 20 / 67

Compiling and Running HelloWorld.java

1 Write the Java program using a text editor (e.g. notepad, vim), and save in a
file HellowWorld.java – you will learn to use an Integrated Development
Program (IDE) later

2 Ensure that the java build and runtime environment is installed on the
machine.

I Open a command window and type the commands:

javac -version, java -version

3 Compile the program using the following command:

javac HelloWorld.java

The command if successful will generate a file HelloWorld.class

4 Run the program using the following command:

java HelloWorld

You should see the following output:

Hello World!

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 21 / 67

Command Line Arguments

If you run java HelloWorld with command line arguments as follows:

java HelloWorld Asutralia England France

args[] (defined in public static void main(String args[])) will
contain the command line argements:

args[0] -> Australia

args[1] -> England

args[2] -> France

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 22 / 67

Command Line Arguments – Example

1 // CommandLineTest.java – Program with command line arguments

2 public class CommandLineTest {

3 public static void main(String args[]) {

4 int count, i=0;

5 count = args.length;

6 System.out.println(“Number of arguments = ” + count);

7 while(i < count) { 8 System.out.println("arg[" + i + "]: " + args[i]); 9 i = i + 1; 10 } 11 } 12 } If you run: java CommandLineTest Australia England France Program Output: Number of arguments = 3 arg[0]: Australia arg[1]: England arg[2]: France Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 23 / 67 Differences between Java and C Java is an Object Oriented language : C is a Procedural Language. Java has no goto, sizeof and typedef statements. Java has no structures and unions. Java has no explicit pointer type. Java has no Preprocessor: no #define, #include, #indef Java is safe and well defined: e.g. memory is managed by the virtual machine not by the programmer. Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 24 / 67 Identifiers, Data Types, Variables and Constants Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 25 / 67 Identifiers Keyword Identifier: A name that uniquely identifies a program element such as a class, object, variable, method. Java identifier rules: must not start with a digit all the characters must be letters, digits, or the underscore symbol can theoretically be of any length are case-sensitive: Rate, rate, and RATE are different variables Java identifier conventions: variables, methods, and objects: start with a lower case letter, indicate ”word” boundaries with an uppercase letter, and restrict the remaining characters to digits and lowercase letters (e.g. topSpeed, bankRate, timeOfArrival) classes: start with an upper case letter and, otherwise, adhere to the rules above (e.g. PrintDemo, HelloWorld) Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 26 / 67 Identifiers Identifiers that have a predefined meaning in Java, such as keywords and reserved words, must not be used as as identifiers in your programs: e.g. public, class, void, static Identifiers that are defined in libraries required by the Java language standard packages are predefined identifiers; although they can be redefined, this could be confusing and dangerous because doing so would change their standard meaning. e.g System, String, println Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 27 / 67 Java Data Types From: https://www.scientecheasy.com/2018/06/data-types-in-java-primitive-example.html Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 28 / 67 https://www.scientecheasy.com/2018/06/data-types-in-java-primitive-example.html Java Data Types Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 29 / 67 Java Data Types Floating point numbers: float type has single-precision. double type has double-precision. Generally, floating point numbers are treated as double-precision numbers. To force them to be in single precision we must append f or F to the number: e.g. float a = 2.3F; double b = 6.7; Boolean numbers: Java boolean type variables can hold a true or false: e.g. boolean x = true; Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 30 / 67 Variables Keyword Variable: Refers to information that is stored in program memory that can be changed; a variable has a memory location and an identifier. Variables must be declared and initialized before use. Syntax: = ;

Examples:

int count = 1;

float length = 2.3F;

double height = 6.7;

boolean status = true;

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 31 / 67

Variables

Assignment operator (=) is used to change the value of a variable.
Syntax:

= OR OR ;

Examples:

int countX = 1, countY = 2;

countX = countY;

countX = countY + countX;

countX += 3; // Shorthand operator countX = countX + 3;

In general, the value of one type cannot be stored in a variable of another
type, but there are exceptions:
Examples:

int intVariable = 2.99; // Not a valid assignment

double doubleVariable = 2; // Is a valid assignment

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 32 / 67

Variables

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 -> int

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).
Examples:

int x = 2.99; // Not a valid assignment

int y = (int)2.99; // Is a valid assignment; y will be 2 not 3

Variables of type int cannot be assigned to a variable of type boolean, nor
can a variable of type boolean be assigned to a variable of type int.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 33 / 67

Variables

Java variables are classified into three categories.
I instance variables
I static (or class) variables
I local variables

Keyword

Local Variables: Variables defined inside a Java method.

We will introduce instance and static variables in the coming lectures.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 34 / 67

Constants

Keyword

Constant: A value that does not change during the execution of the program; also
called READ only values.

Constants are declared with the Java key word final.
Examples:

final int MAX_LENGTH = 420;

final double PI = 3.1428;

final char CHAR_CONSTANT = ‘Z’;

final boolean BOOL_CONSTANT = true;

final String STRING_CONSTANT = “Welcome to Java”;

By convention upper case letters are used for defining constants.

Data type must be explicitly specified when defining constants; this is not
required in C.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 35 / 67

Variables and Constants – Example Program

// ConstantsVariablesExample.java

// Demonstrates the use of constants and variables

public class ConstantsVariablesExample {

public static void main(String args[]) {

final double PI = 3.1428;

// All the variables below are local variables

float radius = 5.5F;

double area;

boolean x = true;

area = PI * radius * radius;

System.out.println(“Circle Radius = ”

+ radius + ” Area = ” + area);

System.out.println(“Boolean Value = ” + x);

}

Program Output:

Circle Radius = 5.5 Area = 95.0697

Boolean Value = true

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 36 / 67

Operators and Expressions

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 37 / 67

Operators

Java Operators can be classified into the following related categories.

Arithmetic

Relational

Logical

Assignment

Increment and decrement

Conditional

Bitwise

Special

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 38 / 67

Arithmetic Operators

Operator Meaning

+ Addition and unary plus
− Subtraction or unary minus
∗ Multiplication
/ Division
% Modulo division

Note: When one of the operands is real and the other is an integer, the expression
is called a mixed-mode arithmetic expression. If either operand is of real type,
then the other operand is also converted to real and real arithmetic is performed.

Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 39 / 67

Arithmetic Operators – Example

// ArithmeticExample.java – Using arithmetic operators

public class ArithmeticExample {

public static void main(String args[]) {

float a = 20.5F, b = 6.4F;

int c = 11, d = 5;

System.out.println(“a + b = ” + (a+b));

System.out.println(“a – b = ” + (a-b));

System.out.println(“a*b = ” + (a*b) );

System.out.println(“a/b = ” + (a/b));

System.out.println(“c%d = ” + (c%d));

}

Program Output:

a + b = 26.9

a – b = 14.1

a*b = 131.2

a/b = 3.203125

c%d = 1

Relational Operators

Operator Meaning

< Is less than <= Is less than or equal to > Is greater than
>= Is greater than or equal to
== Is equal to
! = Is not equal to

Note: The result of a relational operator is type boolean.

Relational Operators – Example

// RelationalExample.java – Using relational operators

public class RelationalExample {

public static void main(String args[]) {

int a = 3, b = 5;

System.out.println(“a < b = " + (a b = ” + (a>b));

System.out.println(“a <= b = " + (a<=b)); System.out.println("a >= b = ” + (a>=b));

System.out.println(“a == b = ” + (a==b));

System.out.println(“a != b = ” + (a!=b));

}

Program Output:

a < b = true a > b = false

a <= b = true a >= b = false

a == b = false

a != b = true

Logical Operators

Operator Meaning

&& Logical AND
|| Logical OR
! Logical NOT

op1 op2 op1 && op2 op1 || op2 !op1
true true true true false

true false false true false

false true false true true

false false false false true

Bitwise Operators

Operator Meaning

& Bitwise AND
| Bitwise OR
ˆ Bitwise exclusive OR
˜ One’s compliment

<< Shift Left >> Shift Right
>>> Shift Right with zero fill

Other Operators

Increment and Decrement Operators:

++ and −−
Example:

++x; x++; –x; x–;

Conditional Operators:

exp1 ? exp2 : exp3

Example:

x = (a > b) ? a : b;

This is the same as:

if (a > b)

x = a;

else

x = b;

Mathematical Functions

Java supports mathematical functions such as cos, sin, log using the Math
class, defined in the java.lang package.
The functions should be used as follows:

Math.method_name();

Example:

double y, z;

y = Math.sqrt(x);

z = Math.cos(y);

Note: Refer to Java documentation for a complete list of methods supported.

Mathematical Functions – Example

// MathSqrtExample.java – Compute the square root of a number

class MathSqrtExample {

public static void main(String args[]) {

double x = 4;

double y;

y = Math.sqrt(x);

System.out.println(“The square root of ” + x + ” is ” + y);

}

Program Output:

The square root of 4.0 is 2.0

Flow of Control

Keyword

Flow of Control: Refers to branching and looping mechanisms in the Java
language.

Most branching and looping statements are controlled by boolean expressions:

A boolean expression evaluates to either true or false.

The primitive type boolean may only take the values true or false

Branching

Java supports the following branching statements:

if-else statement

multi-way if-else statement

switch statement

two way decision expression

The if-else and multi-way if-else Statements

if (Boolean_Expression) {

Yes_Statements

} else {

No_Statements

}

if (Boolean_Expression) {

Yes_Statements_1

} else if (Boolean_Expression) {

Yes_Statements_2

} else if (Boolean_Expression) {

Yes_Statements_3

} else {

Statements_For_All_Other_Possibilities

}

The if-else Statement – Example

//IfElseExample.java – Demonstrates if-else control flow

public class IfElseExample {

public static void main(String args[]){

int i = 5;

if (i < 10) { System.out.println("i is less than 10"); } else { System.out.println("i is greater than or equal to 10"); } } } Program Output: i is less than 10 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 52 / 67 The switch Statement The switch statement supports multi-way branching : switch (Controlling_Expression) { case Case_Label_1: Statement_Sequence_1 break; case Case_Label_2: Statement_Sequence_2 break; case Case_Label_n: Statement_Sequence_n break; default: Default_Statement Sequence break; } Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 53 / 67 The switch Statement - Example //SwitchExample.java - Demonstrates the switch statement public class SwitchExample { public static void main (String args[]) { char x = 'b'; switch(x) { case 'a': System.out.println("Chracter = a"); break; case 'b': System.out.println("Character = b"); break; default: System.out.println("Other character"); break; } } } Program Output: Character = b Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 54 / 67 The Two-way Decision Statement - Example // TwoWayExample.java - Demonstrates two way decision making class TwoWayExample { public static void main(String args[]) { int i = 5; int flag; flag = (i < 10) ? 0 : 1; System.out.println("Flag = " + flag); } } Program Output: Flag = 0 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 55 / 67 Loops Loops in Java are similar to those in other high-level languages. The code that is repeated in a loop is called the body of the loop. Each repetition of the loop body is called an iteration of the loop. Loops can be nested similar to other programming languages. Java supports the following looping statements. while statement do-while statement for statement Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 56 / 67 The while and do-while Statements while (Boolean_Expression) { Statement_1 Statement_2 .... Statement_Last } do { Statement_1 Statement_2 ... Statement_Last } while (Boolean_Expression); Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 57 / 67 The while Statement - Example // WhileExample.java - Demonstrates the use of the while statement public class WhileExample { public static void main(String args[]) { int sum = 0, n = 1; while (n <= 10) { sum = sum + n; n = n + 1; } System.out.println("Sum = " + sum); } } Program Output: Sum = 55 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 58 / 67 The do-while Statement - Example // DoWhileExample.java - Demonstrates the use of the do-while statement public class DoWhileExample { public static void main(String[] args) { int sum = 0, n = 1; do { sum = sum + n; n = n +1; } while ( n <= 10); System.out.println("Sum = " + sum); } } Program Output: Sum = 55 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 59 / 67 The for Statement for (Initialize_Expressions; Terminate_Expression; Update_Expressions) { Statement_1 Statement_2 .... Statement_Last } Contains three types of expressions within the parentheses related to controlling variables: Initialize_Expressions - determine how the control variable or variables are initialized or declared and initialized before the first iteration Terminate_Expression - determines when the loop should end, based on the evaluation of a Boolean expression before each iteration Update_Expressions - determine how the control variable or variables are updated after each iteration of the loop body Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 60 / 67 The for Statement - Example // ForExample.java - Demonstrates the for loop public class ForExample { public static void main(String[] args) { for (int n = 0; n < 5; n++) { System.out.println(" n = " + n); } } } Program Output: n = 0 n = 1 n = 2 n = 3 n = 4 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 61 / 67 The break Statement The break statement causes to exit the loop (while, do or for). 1 while(.....) { 2 ........ 3 if (condition) 4 break; // Will go to line 7 5 ........ 6 } 7 .... 1 loop1: while(.....) { 2 ....... 3 loop2: while (.....) { 4 ........ 5 if (condition) 6 break loop1; // Will go to line 11 7 ........ 8 } 9 .......... 10 } 11 ............. Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 62 / 67 The break Statement - Example // BreakExample.java - Demonstrates the use of the break statement public class BreakExample { public static void main(String[] args) { loop1: for (int i = 0; i < 3; i++) { loop2: for (int j = 0; j < 3; j++) { System.out.println("i=" + i + " j=" +j); if (j == 1) break loop1; } } } } Program Output: i=0 j=0 i=0 j=1 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 63 / 67 The continue Statement The continue statement skips the rest of the statements in the loop (while, do or for). 1 while (.....) { 2 ........ 3 ........ 4 if (condition) 5 continue; // Will goto line 1 6 ........ 7 ........ 8 } 9 ............. Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 64 / 67 The continue Statement - Example // ContinueExample.java - Demonstrates the use of the continue statement public class ContinueExample { public static void main(String[] args) { for (int i = 0; i < 3; i++) { for (int j = 0; j < 3; j++) { if (j == 1) continue; System.out.println("i=" + i + " j=" +j); } } } } Program Output: i=0 j=0 i=0 j=2 i=1 j=0 i=1 j=2 i=2 j=0 i=2 j=2 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 65 / 67 Learning Outcomes Upon completion of this topic you will be able to: Identify some of the key Java features Understand the following in context of Java: I Identifiers, Data Types, Variables and Constants I Operators and Expressions I Flow of control Write simple Java programs Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 66 / 67 References Absolute Java by Water Savitch (Fourth Edition), Chapters 1 & 3 Shanika Karunasekera SWEN20003 c© University of Melbourne 2020 67 / 67

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