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Java Basics

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Compiling

A “compiler” is a program that translates from one language to another

Typically from easy-to-read to fast-to-run e.g. from C or Assembly to machine code Java must be (explicitly) compiled before it is

run The Java compiler turns Java source code

(.java) into Java bytecode (.class)

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The Java Platform

The Java Virtual Machine (JVM) is responsible for running bytecode

The idea: bytecode can be interpreted quickly The same bytecode can be interpreted on

any architecture: write once, run anywhere Code (C,C++) compiled to machine code

must be compiled to a specific system

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The Java Language

Created by Sun Microsystems Introduced in 1995, initial popularity grew due

to Internet applications Excitement surrounding Java applets Confusion with Javascript

Steady rise in popularity has continued for “better” programming reasons

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A Historical Interlude: The Java Team Java originally intended to be used on “smart”

consumer electronics Bill Joy

Founded Sun, 1982 Intelligent robots will replace humanity in the near future…

James Gosling (“the father of Java”) University of Calgary grad First JVM, compiler, interpreter also developed Emacs

Patrick Naughton Arrested in late 90s on child predator charges Not mentioned so much as a founding father anymore

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The Java Language (cont’d)

… is a high-level programming language … is very object oriented … is similar to C++ and C … typically compiled to Java bytecode … is often confused with the Java Platform,

but these are two different aspects of “Java”

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Syntax and Semantics

The syntax rules of a language define how we can combine reserved words, symbols, and identifiers

The semantics of a program statement define what the statement means Problem with program syntax = “error” Problem with program semantics = “bug”

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Java Program Structure

A Java program consists of: One or more classes A class contains one or more methods A method contains program statements

We will explore these terms in detail

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Java Program Structure

public class MyProgram

{

}

// comments about the class

class header

class body

Comments can be placed almost anywhere

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Java Program Structure

public class MyProgram

{

}

// comments about the class

public static void main (String[] args)

{

}

// comments about the method

method headermethod body

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Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

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Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

Creates a “class” called HelloWorld Compiled to HelloWorld.class Classes used to define objects… later

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Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

The “main” method is where it starts to run Ignore “public static void” and “String[] args” for

now

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Hello World

public class HelloWorld

{

}

// HelloWorld.java

public static void main (String[] args){

}

System.out.println(“Hello World!”);

Contains one “statement” The System.out.println function comes from

the Java “class library” Ends with a semicolon (all statements do)

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Compiling and Running

Create the file HelloWorld.java in a text editor Compile:

javac HelloWorld.java Run:

java HelloWorld Output:

Hello World!

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Comments

Three kinds of comments:

To simplify: comments are good

// a one-line comment/* a multi-line comment *//** a javadoc comment */

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Reserved Words and Identifiers Reserved words are specified by the

language All Java reserved words are in the text

Identifiers are specified by a programmer Maybe you: e.g. HelloWorld Maybe someone else: e.g. println

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Restrictions and Conventions

Restriction Identifiers can not start with a digit

Conventions Title case for class names: HelloWorld Uppercase for constants: MAX

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White Space Conventions

Idea: make programs easy to read Use consistent indentation Use blank lines and comments to visually

separate methods The fact that it compiles doesn’t make it

right…

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Strong Typing

Java is a “strongly typed” language All variables and values have a specific type Type is known when the program is

compiled…. before it is run So all variables and values must be declared

with a type before being used

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Declaring Variables

Syntax:

Examples: int count1, int count 2; int count = 0; String course1 = “CMPT 126”;

<variable declaration> ::= <type> <declarator>, …. ;<declarator> ::= <identifier><declarator> ::= <identifier> = <expression>

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Assignment

We use the = operator for variable assignment Initialization is a special case

When a value is assigned, the old value is overwritten

In Java, we use the final modifier to declare a variable constant final int MAX_HEIGHT = 6;

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Primitive Data Types in Java

Four integer types: byte, short, int, long

Two floating point types float, double

One of them is for characters char

One of them is for boolean values boolean

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Expressions and Assignment

An expression is a combination of one or more operators and operands

Arithmetic operators: +, -, *, /, % Use the normal order of operations

e.g. int exp = 2 * 5 +7;

count = count + 1;

count++; Boolean operators: &&, ||

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More Assignment Operators

x += y is equivalent to x = x + y Also:

-= *= /= %=

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Data Conversion

Non-matching types can be converted A widening conversion is automatic

e.g. from short to int A narrowing conversion may lose information

e.g. from float to int Three kinds of conversion:

Assignment Promotion Casting

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Assignment Conversion

final int dollars = 6;

double money;

money = dollars;

Only works for widening conversion

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Promotion

int count = 2;

float mass = 18.342;

mass = mass / count;

Passing count to an operator that expects floating point values

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Casting

float mass = 18.342;

int roundedmass = (int) mass;

Casting works for widening and narrowing In this example, decimal part is just lost Note: this does not actually round

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Object Types

The primitive types aren’t really enough Java also allows object types, or classes

Typically capitalized Object variables hold references to objects

The declaration only creates a reference This is different from primitive types

Variables of primitive type hold a value

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Example: String Objects

We have already seen one object type in Java: String

A String object is a list of characterse.g. “Hello world!” or “My name is Aaron”

Can be passed to print or println Can be concatenated using the (+) operator

e.g. “Hello world! ” + “My name is Aaron”

“I can also append numbers, like “ + 2

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Object Instances

We must create a new “instance” of an object to store something Each object type has a constructor (more later) Create instances using the reserved world new

e.g. course = new String(“CMPT 126”); This creates a new String in memory

It stores the characters “CMPT 126” The assignment sets course to refer to this

instance

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References and Instances

String course;

new String(“CMPT 126”)

course = new String(“CMPT 126”);

course:

CMPT 126

CMPT 126course: