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Java Basics
1
Compiling
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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
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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
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Created by Sun Microsystems
Introduced in 1995, initial popularity grew due
to Internet applications
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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
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Java originally intended to be used on “smart”
consumer electronics
Bill Joy
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James Gosling (“the father of Java”)
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Founded Sun, 1982
Intelligent robots will replace humanity in the near future…
University of Calgary grad
First JVM, compiler, interpreter
also developed Emacs
Patrick Naughton
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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)
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… 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
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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
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Problem with program syntax = “error”
Problem with program semantics = “bug”
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Java Program Structure
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A Java program consists of:
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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
//
comments about the class
public class MyProgram
{
class header
class body
Comments can be placed almost anywhere
}
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Java Program Structure
//
comments about the class
public class MyProgram
{
//
comments about the method
public static void main (String[] args)
{
method body
method header
}
}
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Hello World
//
HelloWorld.java
public class HelloWorld
{
public static void main (String[] args)
{
System.out.println(“Hello World!”);
}
}
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Hello World
//
HelloWorld.java
public class HelloWorld
{
public static void main (String[] args)
{
System.out.println(“Hello World!”);
}
}
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Creates a “class” called HelloWorld
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Compiled to HelloWorld.class
Classes used to define objects… later
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Hello World
//
HelloWorld.java
public class HelloWorld
{
public static void main (String[] args)
{
System.out.println(“Hello World!”);
}
}
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The “main” method is where it starts to run
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Ignore “public static void” and “String[] args” for
now
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Hello World
//
HelloWorld.java
public class HelloWorld
{
public static void main (String[] args)
{
System.out.println(“Hello World!”);
}
}
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Contains one “statement”
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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
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Create the file HelloWorld.java in a text editor
Compile:
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Run:
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javac HelloWorld.java
java HelloWorld
Output:
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Hello World!
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Comments
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Three kinds of comments:
// a one-line comment
/* a multi-line
comment */
/** a javadoc comment */
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To simplify: comments are good
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Reserved Words and Identifiers
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Reserved words are specified by the
language
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All Java reserved words are in the text
Identifiers are specified by a programmer
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Maybe you: e.g. HelloWorld
Maybe someone else: e.g. println
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Restrictions and Conventions
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Restriction
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Identifiers can not start with a digit
Conventions
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Title case for class names: HelloWorld
Uppercase for constants: MAX
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White Space Conventions
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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
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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
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Syntax:
<variable declaration> ::= <type> <declarator>, …. ;
<declarator> ::= <identifier>
<declarator> ::= <identifier> = <expression>
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Examples:
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int count1, int count 2;
int count = 0;
String course1 = “CMPT 126”;
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Assignment
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We use the = operator for variable
assignment
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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
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final int MAX_HEIGHT = 6;
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Primitive Data Types in Java
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Four integer types:
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Two floating point types
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float, double
One of them is for characters
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byte, short, int, long
char
One of them is for boolean values
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boolean
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Expressions and Assignment
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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++;
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Boolean operators: &&, ||
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More Assignment Operators
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x += y is equivalent to x = x + y
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Also:
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-=
*=
/=
%=
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Data Conversion
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Non-matching types can be converted
A widening conversion is automatic
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A narrowing conversion may lose information
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e.g. from short to int
e.g. from float to int
Three kinds of conversion:
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Assignment
Promotion
Casting
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Assignment Conversion
final int dollars = 6;
double money;
money = dollars;
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Only works for widening conversion
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Promotion
int count = 2;
float mass = 18.342;
mass = mass / count;
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Passing count to an operator that expects
floating point values
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Casting
float mass = 18.342;
int roundedmass = (int) mass;
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Casting works for widening and narrowing
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In this example, decimal part is just lost
Note: this does not actually round
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Object Types
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The primitive types aren’t really enough
Java also allows object types, or classes
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Object variables hold references to objects
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Typically capitalized
The declaration only creates a reference
This is different from primitive types
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Variables of primitive type hold a value
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Example: String Objects
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We have already seen one object type in
Java: String
A String object is a list of characters
e.g. “Hello world!” or “My name is Aaron”
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Can be passed to print or println
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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
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We must create a new “instance” of an object
to store something
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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
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It stores the characters “CMPT 126”
The assignment sets course to refer to this
instance
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References and Instances
String course;
course:
new String(“CMPT 126”)
CMPT 126
course = new String(“CMPT 126”);
course:
CMPT 126
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