ICS 23 Fundamental Data Structures Introduction to Java

1
ICS 23 Fundamental Data StructuresIntroduction
to Java

• Section 1
• Overview of Java

2
Java Introduction

• Visual Café for editing, executing, debugging
  Java
• You should learn this on your own during Lab Z.
• Applications and applets
• Applets run within a web browser where a java
  virtual machine is installed.
• Applications stand-alone programs executed from
  Visual Café, or command prompt
• Cgt java myApplication
• We will only implement applications in ICS 23.
• Assume you know C

3
Why Java?

• Platform Independent
• write once, run anywhere (in theory)
• Object-Oriented
• Everything must be in class
• Easy to Learn
• a lot like C

4
Why is Java platform independent?

• Ordinary (C / C) Program
• Your source code -gt (compiler) -gt Executable File
  (native machine code)
• Java Program
• Your source code -gt (compiler) -gt Java bytecode
  (platform independent) -gt (Interpreter)

5
.java and .class files

• Java source code is in .java files
• Each .java file can define one or more classes
  class Cat . . .
• When a .java file is compiled, one or more .class
  files are created
• The .class files are executed by the program java

6
Similarities between Java and C

• Operators - / - / !
  --
• Function declaration void func(int x)
• flow control if else for while do
  return switch
• classes class Cat . . .
• object creation Cat felix new Cat()
• array reference syntax cats10 felix
• member reference syntax
• felix.meow() //member function
• Color c felix.eyeColor //variable

7
Java is not C 1

• No explicit pointer type variables, all
  references to objects are done by using implicit
  references.
• Cat felix // C
• Cat felix // Java
• No for dereferencing a pointer type or
  specifying reference for variable or function
  arguments.
• felix felix // C, not Java
• In Java, all objects are manipulated by
  reference, so there is no need for syntax.

8
Java is not C 2

• All executable code is in a class
• no global variables
• no non-class functions
• Automatic garbage collection.
• Classes have constructors but no destructors.
• When all references to a class no longer exist,
  the class can be garbage collected -- the
  memory it occupies is returned to the free pool
• first Cat object can now be garbage collected

Cat f new Cat() // object allocated f new
Cat() // another obj. allocated
9
Java is not C 3

• No multiple inheritance.
• No operator overloading.
• One exception operator for string
  concatenation.
• String s I studied hours hours
• No preprocessor.
• No define, include, and ifdef ...
• No default parameter values.

10
A Simple Java Application

• //FirstSample.java
• public class FirstSample
• public static void main(String args)
• System.out.println(Hello, World!)

11
Useful Java Programming Hints

• Observe good programming styles from C/C.
• Each file can contain at most one class declared
  as public.
• The name of the public class matches the name of
  the file.
• public class MyClass should be in file
  MyClass.java.
• Java is case sensitive. Double check the case.

12
Arrays

• Array Declaration
• int numbers or int numbers
• Array Creation
• numbers new int 2
• int numbers 3, 2, 1
• Dog pound new Dog3
• pound0 new Dog(Rex, Dog.Collie)
• Array length
• int num numbers.length

13
Strings

• String is a (special) Java class.
• String Creation
• String greeting Hello
• String Concatenation
• String greeting1 He
• String greeting2 llo
• String greeting greeting1 greeting2
• Substring
• String greeting Hello
• String s greeting.substring(0, 4) //Hell

14
Strings (contd)

• Testing for Equality
• .equals() tests for same contents
• operator tests for same memory location
• String a Hello, b He llo
• String c a
• if (a.equals(b))
• System.out.println(Same contents.)
• if (a ! b)
• System.out.println(Different address.)
• if (a c)
• System.out.println(Same address.)

15
Interface - a kind of class

• An interface defines a set of functions that a
  class will implement.
• An interface can be considered as a purely
  abstract class with abstract methods.
• Provides add-on features to a class.
• Eliminates the need for multiple inheritance.

16
Interface Declaration

• interface Drivable
• boolean startEngine()
• void stopEngine()
• class Automobile implements Drivable
• public boolean startEngine()/do something
  here/
• public void stopEngine()/do something. here/
• //
• //somewhere in your program
• Automobile auto new Automobile()
• Drivable vehicle //Declare a variable of
  Drivable
• vehicle auto
• vehicle.startEngine()

17
Notes on Using Interfaces

• A class can implement one or more interfaces.
• If a class implements an interface, it must
  implement all the methods defined in the
  interface.
• An interface can extend another interface.
• interface Container extends Comparable

18
Modifier Keyword static

• Methods, variables and nested classes can be
  declared static.
• Static methods, variables or nested classes
  belong to the (enclosing) class as a whole, not
  any particular class instance.
• Static methods or variables are accessed by
• className.variableName
• className.methodName(.)
• Math.PI //in java.lang package
• System.out // in java.lang package
• Math.pow(3, 2)

19
Modifier Keyword abstract

• An abstract method is a prototype subclasses must
  implement.
• abstract void eat (String name) // Java
• virtual void eat (String name) 0 // C
• A class that contains one or more abstract
  methods must be explicitly declared as an
  abstract class
• abstract class animal
• // . . .
• abstract void eat (String name)
• A class can be declared as abstract even though
  it has no abstract methods. An abstract class
  cannot be instantiated.

20
Exception Handling

• An exception indicates an unusual or error
  condition.
• Exception handling in Java separates the normal
  control flow from error handling flow.

21
Catch an Exception

• // This static method converts String to int
• java.lang.Integer.parseInt(String s)
• throws NumberFormatException
• // my program
• int x 0
• try
• x java.lang.Integer.parseInt(inputString)
• catch(NumberFormatException nfe)
• System.out.println(Darn, nfe)) x -1
• // do something with x

22
Hierarchy of Throwable class

• Java.lang.Object
• --java.lang.Throwable
• --java.lang.Error
• --(abnormal condition, not caught)
• --java.lang.Exception
• --java.lang.RuntimeException
• --(normal error, not caught)
• --(various exceptions that must be caught)