Introduction to Java Java Language Language Introduction to - - PowerPoint PPT Presentation

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Introduction to Introduction to Java Java Language Language

ปรีดา เลิศพงศวิภูษณะ plw@ku.ac.th ภาควิชาวิศวกรรมคอมพิวเตอร คณะวิศวกรรมศาสตร มหาวิทยาลัยเกษตรศาสตร

23 มิถุนายน 2547 ปรีดา เลิศพงศวิภูษณะ (plw@ku.ac.th) 2

What is Java What is Java? ?

Java is a high-level, third generation programming language, like C, Fortran, Smalltalk, Perl and many others. Java is most similar to C. But it is not C. You can use Java to write computer applications that crunch numbers, process words, play games, store data or do any of the thousands of other things computer software can do.

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Java is a Platform Java is a Platform

Java is a platform for application development. Java solves the problem of platform-independence by using byte code. It looks a lot like machine language, but unlike machine language Java byte code is exactly the same on every platform. Java programs that have been compiled into byte code still need an interpreter to execute them on any given platform. The interpreter reads the byte code and translates it into the native language

• f the host machine on the fly.

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Java is Simple Java is Simple

Java was designed to make it much easier to write bug free code. Java has considerably more functionality than C, primarily because of the large class library. Java is easy to read and write. Java makes to providing bug-free code is automatic memory allocation and deallocation. The language is small so it’s easy to become fluent. The language is interpreted so the compile-run-link cycle is much shorter. It’s very difficult to write a Java program that will crash your system.

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Java is Object Java is Object-

• Oriented

Oriented

In object-oriented programs data is represented by objects. Objects have 2 sections, fields (instance variables) and methods. Fields tell you what an object is. Methods tell you what an object does. Object-Oriented programming advantages including:

– Simpler, easier to read programs – More efficient reuse of code – Faster time to market – More robust, error-free code

23 มิถุนายน 2547 ปรีดา เลิศพงศวิภูษณะ (plw@ku.ac.th) 6

Java is Platform Independent Java is Platform Independent

Java was designed to not be cross-platform in source form like C, but also in compiled binary form. Java is compiled to an intermediate form called byte-code. To make Java cross-platform is the elimination of undefined or architecture dependent constructs. A Java program never really executes natively on the host machine. A special native program called the Java interpreter reads the byte code and executes the corresponding native machine Thus to port Java programs to a new platform all that is needed is to port the interpreter and some of the library routines.

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Java is Safe Java is Safe

Java was designed from the ground up to allow for secure execution of code across a network, even then the source of that code was untrusted and possibly malicious. There are no pointers in Java. Cannot access arbitrary addresses in memory. Java implements a robust exception handling mechanism to deal with both expected and unexpected errors. Java, by making it easier to write bug-free code.

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Java is High Java is High-

• Performance

Performance

Java byte code can be compiled on the fly to code that rivals C++ in speed using a “just-in-time compiler”. Working on native-machine-architecture compilers for Java. Does not require a separate interpreter.

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Java is Multi Java is Multi-

• Threaded

Threaded

Java is inherently multi-threaded. The Java environment can ensure that a malicious applet doesn’t steal all of the host’s CPU cycles. A single Java program can have many different threads executing independently and continuously. Hard to find bugs.

23 มิถุนายน 2547 ปรีดา เลิศพงศวิภูษณะ (plw@ku.ac.th) 10

Java is Dynamically linked Java is Dynamically linked

Java does not have an explicit link phase. Java source code is divided into

– .java files, roughly one per each class in program – .class files, containing byte code.

The compiler searches the current directory and directories specified in the CLASSPATH environment variable to find other classes explicitly referenced by name in each source code file. class that were unknown to a program when it was compiled can still be loaded into it at runtime.

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Java is Garbage Collected Java is Garbage Collected

Don’t Need to explicitly allocate or deallocate memory in Java. Memory is allocated as needed, both on the stack and the heap, and reclaimed by the garbage collector. There’s no malloc() , free() or destructor methods. There are constructors and these do allocate memory on the heap, but this is transparent to the programmer. Most Java virtual machines use an inefficient, mark and sweep garbage collector.

23 มิถุนายน 2547 ปรีดา เลิศพงศวิภูษณะ (plw@ku.ac.th) 12

The Hello World Application

class HelloWorld { public static void main (String args[]) { System.out.println("Hello World!"); } }

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Compiling and Running Hello World Compiling and Running Hello World

At command-line prompt and type :

javac nofile.java

Unix prompt and type :

% javac HelloWorld.java % java HelloWorld HelloWorld

Windows type :

C:> javac HelloWorld.java C:> java HelloWorld HelloWorld C:>

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for loops for loops

class Count { public static void main (String args[]) { int i; for (i = 0; i < 50; i = i+1) { System.out.println(i); } } }

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for loops for loops ( (variable inside variable inside) )

class Count { public static void main (String args[]) { for (int i = 0; i < 50; i = i+1) { System.out.println(i); } } }

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Increment and decrement operators Increment and decrement operators

Using ++ for increment and -- for decrement

class Count { public static void main (String args[]) { for (int i = 0; i < 50; i++) { System.out.println(i); } } }

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Print statements Print statements

class PrintArgs { public static void main (String args[]) { for (int i = 0; i < args.length; i++) { System.out.println(args[i]); } } }

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Print statements Print statements II II

You can concatenate arguments to println() with a plus sign (+), for example,

System.out.println(“There are ” + args.length + “ command line arguments”);

Using print() instead of println() does not break the line, for example,

System.out.print(“There are ”); System.out.print(args.length); System.out.print(“ command line arguments”); System.out.println();

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Fibonacci Fibonacci Numbers Numbers

class Fibonacci { public static void main (String args[]) { int low = 1; int high = 0; System.out.println(low); while (high < 50) { System.out.println(high); int temp = high; high = high + low; low = temp; } } }

23 มิถุนายน 2547 ปรีดา เลิศพงศวิภูษณะ (plw@ku.ac.th) 20

Variables and Data Types

There are eight primitive data types in Java:

– boolean – byte – short – int – long – float – double – char

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Variables and Data Variables and Data Types Types 2 2

There are 8 primitive data types in Java:

– boolean true or false – int 89, -945, 37865 – long 89L, -945L, 5123567876L – float 89.5f, -32.5f – double 89.5, -32.5, 87.6E45 – char ‘c’, ‘9’, ‘t’ – String “This is a string literal”

Strings are a reference or object type, not a primitive type. However the Java compiler has special support for strings so this sometimes appears not to be the case.

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Variables and Data Types Variables and Data Types 3 3

class Variables { public static void main (String args[]) { boolean b = true; int low = 1; long high = 76L; long middle = 74; float pi = 3.1415292f; double e = 2.71828; String s = “Hello World!”; } }

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Comments Comments

// This is the Hello World program in Java class HelloWorld { public static void main (String args[]) { /* Now let's print the line Hello World */ System.out.println("Hello World!"); } // main ends here } // HelloWorld ends here

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Command line arguments Command line arguments

class printArgs { public static void main (String args[]) { for (int i = 0; i < args.length; i++) { System.out.println(args[i]); } } } % java printArgs Hello There args[0] is the string "Hello". args[1] is the string "There". args.length is 2.

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Points Points

What is a class?

– Fields say what an object is – Methods say what an object does

class TwoDPoint { double x; double y; }

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Objects Objects

class OriginPrinter { public static void main(String[] args) { TwoDPoint origin; // only declares, does not allocate // The constructor allocates and usually initializes the object

• rigin = new TwoDPoint();

// set the fields

• rigin.x = 0.0;
• rigin.y = 0.0;

// print the two point System.out.println("The origin is at " + origin.x + ", " + origin.y); } // end main } // end OriginPrinter

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Multiple Objects

class TwoPointPrinter { public static void main(String[] args) { TwoDPoint origin; // only declares, does not allocate TwoDPoint one; // only declares, does not allocate // The constructor allocates and usually initializes the object

• rigin = new TwoDPoint();
• ne = new TwoDPoint();

// set the fields

• rigin.x = 0.0;
• rigin.y = 0.0;
• ne.x = 1.0;
• ne.y = 0.0;

// print the two-d points System.out.println("The origin is at " + origin.x + ", " + origin.y); System.out.println("One is at " + one.x + ", " + one.y); } // end main } // end TwoPointPrinter

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Assigns that object to both variables

class EqualPointPrinter { public static void main(String[] args) { TwoDPoint origin1; // only declares, does not allocate TwoDPoint origin2; // only declares, does not allocate // The constructor allocates and usually initializes the object

• rigin1 = new TwoDPoint();
• rigin2 = origin1;

// set the fields

• rigin1.x = 0.0;
• rigin1.y = 0.0;

// print System.out.println("origin1 is at " + origin1.x + ", " + origin1.y); System.out.println("origin2 is at " + origin2.x + ", " + origin2.y); } // end main } // end EqualPointPrinter

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Static Fields

Static or class fields belong to a class, not to an object

class Point { double x; double y; static double xorigin = 0.0; static double yorigin = 0.0; } System.out.println("The origin is at (" + Point.xorigin + ", " + Point.yorigin + ")");

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Methods

Methods say what an object does.

class TwoDPoint { double x; double y; void print() { System.out.println(x + "," + y); } } TwoDPoint origin = new TwoDPoint();

• rigin.x = 0.0;
• rigin.y = 0.0;
• rigin.print();

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Passing Arguments to Methods

class TwoDPoint { double x; double y; void print() { System.out.println("(" + x + "," + y + ")"); } void print(int n) { for (int i = 0; i < n; i++) { System.out.println("(" + x + "," + y + ")"); } } }

There are two different print() methods. One takes an argument. One doesn't. As long as the argument lists can disambiguate the choice, this is allowed. This is called overloading.

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Passing Arguments to Methods 2

TwoDPoint origin = new TwoDPoint();

• rigin.x = 0.0;
• rigin.y = 0.0;
• rigin.print(10);
• rigin.print();
• rigin.print(10L);

CALL SIGNATURE

• rigin.print(int);
• rigin.print();
• rigin.print(long);

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Returning values from methods

class TwoDPoint { double x; double y; void print() { System.out.println("(" + x + "," + y + ")"); } String getAsString() { return "(" + x + "," + y + ")"; } } TwoDPoint origin = new TwoDPoint();

• rigin.x = 0.0;
• rigin.y = 0.0;

String s = origin.getAsString(); System.out.println(s); Better yet, TwoDPoint origin = new TwoDPoint();

• rigin.x = 0.0;
• rigin.y = 0.0;

System.out.println(origin.getAsString());

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Setter methods

Setter methods just set the value of a field (often private) in a class.

class TwoDPoint { double x, y; String getAsString() { return "(" + x + "," + y + ")"; } void setX(double value) { x = value; } void setY(double value) { y = value; } } TwoDPoint origin = new TwoDPoint();

• rigin.setX(0.0);
• rigin.setY(0.0);

System.out.println(origin.getAsString());

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Getter methods Getter methods just return the value of a field in a class.

class TwoDPoint { double x; double y; String getAsString() { return "(" + x + "," + y + ")"; } void setX(double value) { x = value; } void setY(double value) { y = value; } double getX() { return x; } double getY() { return y; } } TwoDPoint origin = new TwoDPoint();

• rigin.setX(0.0);
• rigin.setY(0.0);

System.out.println("The x coordinate is " + origin.getX());

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Constructors

Constructors are special methods that have the same name as their class and no return type.

class TwoDPoint { double x, Y; TwoDPoint(double xvalue, double yvalue) { x = xvalue; y = yvalue; } String getAsString() { return "(" + x + "," + y + ")"; } void setX(double value) { x = value; } void setY(double value) { y = value; } double getX() { return x; } double getY() { return y; } }

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Constructors 2

TwoDPoint origin = new TwoDPoint(0.0, 0.0); System.out.println("The x coordinate is " + origin.getX());

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Shadowing field names and this

class TwoDPoint { double x, y; TwoDPoint(double x, double y) { this.x = x; this.y = y; } String getAsString() { return "(" + this.x + "," + this.y + ")"; } void setX(double x) { this.x = x; } void setY(double y) { this.y = y; } double getX() { return this.x; } double getY() { return this.y; } }

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Arrays

import java.util.Random; class RandomTest { public static void main (String args[]) { int[] ndigits = new int[10]; double x; int n; Random myRandom = new Random(); // Initialize the array for (int i = 0; i < 10; i++) { ndigits[i] = 0; } // Test the random number generator a whole lot for (long i=0; i < 100000; i++) { // generate a new random number between 0 and 9 x = myRandom.nextDouble() * 10.0; n = (int) x; //count the digits in the random number ndigits[n]++; } // Print the results for (int i = 0; i < 10; i++) { System.out.println(i+": " + ndigits[i]); } } }

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Arrays 2

% javac RandomTest.java % java RandomTest 0: 10171 1: 9724 2: 9966 3: 10065 4: 9989 5: 10132 6: 10001 7: 10158 8: 9887 9: 9907 %