Getting Started with Java Programming, Slides of Algorithms and Programming

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Java Crash Course

A Tutorial Introduction for C++

Programmers

So what the heck is Java (besides coffee, that

is)?

• A programming language.
  • Syntax and constructs are very similar to C++
• A virtual platform
  • Java Virtual Machine is a software “machine” or “hypothetical

chip”

• Since it’s “virtual”, it can be implemented on any hardware
• Cross-platform distribution achieved via .class binary file of

bytecodes (instead of machine-dependent machine code) 

Write Once, Run Anywhere

• A class library
  • Standard APIs for GUI, data storage, processing, I/O, and

networking.

Mechanics of Writing Java Programs

 Create a Java source file. Must have the .java extension and contain

only one public class.

 Compile the source file into a bytecode file. The Java compiler ,

javac, takes your source file and translates its text into instructions that the Java Virtual Machine (Java VM) can understand. The compiler puts these instructions into a .class bytecode file.

 Run the program contained in the bytecode file. The Java VM is

implemented by a Java interpreter , java. This interpreter takes your bytecode file and carries out the instructions by translating them into instructions that your computer can understand.

Putting it all together

public class Hello { public static void main(String args[]) { System.out.println(“Hello, world!”); } }

1. Put in: Hello.java
2. Compile with: javac Hello.java

 Creates Hello.class

1. Run with: java Hello

Java notes for C++ programmers

• Everything’s an object
  • Every object inherits from java.lang.Object
• No code outside of the class definition!
  • No global variables (use static variables instead)
• Single inheritance only
  • Instead, implement interfaces
• All classes are defined in .java files
  • One top level public class per file
    • The file has to have the same name as the public class!
• Syntax is similar (control structures are very similar).
  • Primitive data types are similar
  • But a bool is not an int
  • To print to stdout, use System.out.println()

Requisite First Program (Application

Version)

Put in HelloWorld.java:

public class HelloWorld {

public static void main(String args[]) {

System.out.println("Hello World");

}

}

So what’s going on?

The Java bytecode and interpreter at work!

• Bytecode is an intermediate representation of

the program (the class file)

• Think of it as the machine-code for the Java Virtual Machine
• The Java interpreter (java) starts up a new

“Virtual Machine”

• The VM starts executing the user’s class by

running its main() method

Requisite First Program (Applet Version)

Put in HelloWorld.java: import java.awt.Graphics; public class HelloWorld extends java.applet.Applet { public void paint(Graphics g) { g.drawString("Hello World“, 35, 15); } }

**Put in test.html:

Test the applet

Test the applet

**

Java Language Basics

• Data types same as in C++ (except bool)
  • bool,char,byte,short,int,long,float,

double,string, etc.

• Operators (same as C++)
  • Assignment: =, +=, -=, *=, …
  • Numeric: +, -, *, /, %, ++, --, …
  • Relational: ==. !=, <, >, <=, >=, …
  • Boolean: &&, ||,!
  • Bitwise: &, |, ^, ~, <<, >>, …
• Control Structures  more of what you expect:

1. conditional: if, if else, switch

2. loop: while, for, do

3.break and continue

Classes, References, & Packages

• Classes and Objects
  • “All Java statements appear within methods, and all methods are defined within classes”.
  • Java classes are very similar to C++ classes (same concepts).
  • Instead of a “standard library”, Java provides a lot of Class implementations or packages
• What are packages?
  • You can organize a bunch of classes and interfaces into a package (or library of classes) - defines a namespace that contains all the classes.
  • Use the import keyword to include the packages you need
    • import java.applet.*;
  • You need to use some java packages in your programs
    • java.awt (Abstract Windowing Toolkit), java.io (for Files, etc.), java.util (for Vectors, etc.)
• References
  • No pointers  everything’s a reference!
  • classes
  • arrays

Exceptions

• When a program carries out an illegal action, an

exception is generated.

• Terminology:
  • throw an exception : signal (in the method header)

that some condition or error has occurred but we

want to pass the buck and not deal with it.

• catch an exception : deal with the error (or whatever)

ourselves inside the function/method.

• Catch it using a try/catch block (next slide).
• In Java, exception handling is necessary (forced

by the compiler  compilation errors!)

Try/Catch/Finally

try {

// code that can throw an exception

} catch (ExceptionType1 e1) {

// code to handle the exception

} catch (ExceptionType2 e2) {

// code to handle the exception

} catch (Exception e) {

// code to handle other exceptions

} finally {

// code to run after try or any catch

}

This block is always run

Defining a Class

• One top level public class per .java file.
  • Typically end up with many .java files for a single

program with at least one containing a static

public main() method (if they’re applications).

• Class name must match the file name!
  • The compiler/interpreter use class names to figure out

what the file name is.

• Classes have these three features:
  • A constructor that’s used to allocate memory for the

object, initiailize its elements, and return a reference to

the object

• Methods (function members) Docsity.com

A Sample Class

public class Point {

public Point(double x, double y) {

this.x = x; this.y=y;

}

public double distanceFromOrigin(){

return Math.sqrt(xx+yy);**

}

private double x,y;

}