Introduction to Java

Introduction

• Present the syntax of Java
• Introduce the Java API
• Demonstrate how to build
• stand-alone Java programs
• Java applets, which run within browsers e.g. Netscape
• Example programs

Why Java?

• It’s the current “hot” language
• It’s almost entirely object-oriented
• It has a vast library of predefined objects and operations
• It’s more platform independent
• this makes it great for Web programming
• It’s more secure
• It isn’t C++

Java Virtual Machine

• The .class files generated by the compiler are not executable binaries
• so Java combines compilation and interpretation
• Instead, they contain “byte-codes” to be executed by the Java Virtual Machine
• other languages have done this, e.g. UCSD Pascal
• This approach provides platform independence, and greater security

HelloWorld (standalone)

• Note that String is built in
• println is a member function for the System.out class

public class HelloWorld {

public static void main(String[] args) {

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

}

}

Comments are almost like C++

• /* This kind of comment can span multiple lines */
• // This kind is to the end of the line
• /**� * This kind of comment is a special� * ‘javadoc’ style comment� */

Primitive data types are like C

• Main data types are int, double, boolean, char
• Also have byte, short, long, float
• boolean has values true and false
• Declarations look like C, for example,
• double x, y;
• int count = 0;

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Expressions are like C

• Assignment statements mostly look like those in C; you can use =, +=, *= etc.
• Arithmetic uses the familiar + - * / %
• Java also has ++ and --
• Java has boolean operators && || !
• Java has comparisons < <= == != >= >
• Java does not have pointers or pointer arithmetic

​

Control statements are like C

• if (x < y) smaller = x;
• if (x < y){ smaller=x;sum += x;}�else { smaller = y; sum += y; }
• while (x < y) { y = y - x; }
• do { y = y - x; } while (x < y)
• for (int i = 0; i < max; i++) sum += i;
• BUT: conditions must be boolean !

Control statements II

• Java also introduces the try statement, about which more later

switch (n + 1) {� case 0: m = n - 1; break;� case 1: m = n + 1;� case 3: m = m * n; break;� default: m = -n; break;�}

Java isn't C!

• In C, almost everything is in functions
• In Java, almost everything is in classes
• There is often only one class per file
• There must be only one public class per file
• The file name must be the same as the name of that public class, but with a .java extension

Java program layout

• A typical Java file looks like:

import java.awt.*;�import java.util.*;

public class SomethingOrOther {� // object definitions go here� . . .�}

This must be in a file named SomethingOrOther.java !

What is a class?

• Early languages had only arrays
• all elements had to be of the same type
• Then languages introduced structures (called records, or structs)
• allowed different data types to be grouped
• Then Abstract Data Types (ADTs) became popular
• grouped operations along with the data

​

So, what is a class?

• A class consists of
• a collection of fields, or variables, very much like the named fields of a struct
• all the operations (called methods) that can be performed on those fields
• can be instantiated
• A class describes objects and operations defined on those objects

Name conventions

• Java is case-sensitive; maxval, maxVal, and MaxVal are three different names
• Class names begin with a capital letter
• All other names begin with a lowercase letter
• Subsequent words are capitalized: theBigOne
• Underscores are not used in names
• These are very strong conventions!

The class hierarchy

• Classes are arranged in a hierarchy
• The root, or topmost, class is Object
• Every class but Object has at least one superclass
• A class may have subclasses
• Each class inherits all the fields and methods of its (possibly numerous) superclasses

An example of a class

class Person {� String name;� int age;

void birthday ( ) {� age++;� System.out.println (name + ' is now ' + age);� }�}

Another example of a class

class Driver extends Person {� long driversLicenseNumber;� Date expirationDate;�}

Creating and using an object

• Person john;�john = new Person ( );�john.name = "John Smith";�john.age = 37;
• Person mary = new Person ( );�mary.name = "Mary Brown";�mary.age = 33;�mary.birthday ( );

An array is an object

• Person mary = new Person ( );
• int myArray[ ] = new int[5];
• or:
• int myArray[ ] = {1, 4, 9, 16, 25};
• String languages [ ] = {"Prolog", "Java"};