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Programming Language
Concepts (CIS 635)
Elsa L Gunter
4303 GITC
NJIT,
www.cs.njit.edu/~elsa/635
Object-Oriented Programming
•
•
•
•
Class is a form of abstract type
Instances of class called objects
Operators of class called methods
Applying a method to an object in
the class called message passing
– Messages take object as implicit
argument
Copyright 2002 Elsa L. Gunter
Variables and Methods
• Classes have two kinds of
methods and two kinds of
variables:
–Instance methods and instance
variables
–Class methods and class variable
Copyright 2002 Elsa L. Gunter
Instance Variables and
Methods
• Instance variables
– Instance variables hold state of single
object
– Different objects have different
(copies of) instance variables
• Instance methods:
– Instance methods act on a single
object, usually acting on instance
variables
Copyright 2002 Elsa L. Gunter
Class Variables and Methods
• Class variables hold state that is
shared in common among all
objects of class
• Class methods act on attributes of
the whole class through class
variables
• Class methods can’t access
instance variables
Copyright 2002 Elsa L. Gunter
Inheritance
• Inheritance allows all variables
and methods exported from
parent class to be part of a
derived class or subclass
Copyright 2002 Elsa L. Gunter
Inheritance
• Export information:
–public: everybody sees
–private: only seen inside class
–protected: exported only to
subclasses
Copyright 2002 Elsa L. Gunter
Inheritance
• If a method is added to a
subclass with the same name
(and usually signature) as in the
parent class the new version
overrides inherited version in
subclass
Copyright 2002 Elsa L. Gunter
Inheritance Polymorphism and
Dynamic Binding
• Method of superclass may be
overridden in subclass
• Dynamic binding: when a method of
superclass is applied to object of
subclass, methods of subclass are used
in computing result, instead of methods
of superclass
• Java has dynamic binding by default
• C++ method must be marked as virtual
for dynamic binding to be used with it
Copyright 2002 Elsa L. Gunter
Virtual (or Abstract) Methods
• Virtual method: has declaration but no
function body
• Subclass must override all virtual
methods with methods with fully
concrete bodies to be instantiated
Copyright 2002 Elsa L. Gunter
Single Versus Multiple
Inheritance
• Single inheritance: Class may only be a
subclass of one parent
• Multiple inheritance: Class may be
immediate subclass of two or more
parents
• Problem with multiple inheritance:
class A:B,C {…}
What if we have both B.m and C.m?
Which method is A.m?
Answer in most languages: B.m
Copyright 2002 Elsa L. Gunter
Introduction to Java
• Recently introduced (1995)
• Modern and sound design
(Gosling)
• Very rapid growth (and heavy
hype)
• Killer application: web-based
computing (applets)
Copyright 2002 Elsa L. Gunter
Introduction to Java
• Originally designed for smart
appliances such as interactive
TVs and “set-top boxes”
–Priorities: small, secures,
portable
–Makes it good for running on the
web
Copyright 2002 Elsa L. Gunter
Platform Independence
• Machine-independent semantics
and security model
• Key concept: Java Virtual Machine
– Each platform implements Java
Virtual Machine
– Java compiles to Java byte code that
runs on (is interpreted by) Java
Virtual Machine
Copyright 2002 Elsa L. Gunter
Getting Java
• For MS Windows:
– http://java.sun.com/j2se/1.3/ or
– http://www.cis.njit.edu/~elsa/635/j2sd
k-1_3_0_02-win.exe
• Installation instructions found
– http://java.sun.com/j2se/1.3/installwindows.html
Copyright 2002 Elsa L. Gunter
Getting Started With Java
• Will assume you have adjusted
your PATH variable as
described in instructions
• Start up editor (eg Notepad)
• Create file called “Hello.java”
–Use quotes so that editor doesn’t
add its own extension (like .txt)
Copyright 2002 Elsa L. Gunter
Getting Started With Java
• Put code on next slide in file
and save
• In MS_DOS window in same
directory, run javac Hello.java
to compile code
• In MS_DOS window in same
directory, run java Hello
Copyright 2002 Elsa L. Gunter
Sample Java Code
(“Hello.java”)
class Hello {
public static void main
(String args[]) {
System.out.println
("So at last we get to
Java!\n");
}
}
Copyright 2002 Elsa L. Gunter
Sample Java Code
(“Hello.java”)
To execute:
C:\My
Documents\635\java>javac
Hello.java
C:\My
Documents\635\java>java
Hello
So at last we get to Java!
Copyright 2002 Elsa L. Gunter
Terminolgoy
• Class: Hello
• Class method: main
– Only class method of class Hello
Copyright 2002 Elsa L. Gunter
Terminolgoy
•
•
•
•
•
Object: System
Method of System: out
Object: System.out
Method of System.out: println
Object: “So at last we get to
Java!“ of class String of
immutable strings
Copyright 2002 Elsa L. Gunter
Java Declarations
• Declarations state type and
other attributes of identifier
private int i, j;
• Declaration has three parts:
–Modifiers: private
–Type: int
–List of identifiers: i, j
Copyright 2002 Elsa L. Gunter
Java Method Declaration
public static void main
(String args [])
• public and static describe
scope and sharing characteristics
of method main
Copyright 2002 Elsa L. Gunter
Java Method Declaration
• Input of main is an array of strings
(objects of class String)
• Return type of main is void, i.e.
no return value
• Formal input parameter is args
(not actually ever used here)
Copyright 2002 Elsa L. Gunter
Initialization
• If an identifier is declared without
being initialized, Java used default
initialization
• To initialize identifier, follow its
declaration by = and an expression:
int lo = 1;
int hi = 1;
Copyright 2002 Elsa L. Gunter
Java Built-in Data Types
Data
type
Size(bits) Default
initial
value
boolean 8
false
byte
8
0
char
16
‘x0’
short
16
0
Copyright 2002 Elsa L. Gunter
Java Built-in Data Types
Data
type
int
long
float
double
Copyright 2002 Elsa L. Gunter
Size(bits) Default
initial
value
32
0
64
0
32
0.0F
64
0.0D
boolean and char Literals
• true and false are not
(andcannot be cast into) integers
• Character literals are enclosed in
single quotes: ‘a’
• Uses same convention for special
characters as C: \n for newline,
\\ for backslash, \’ for single
quote
Copyright 2002 Elsa L. Gunter
Assignment and Equality
• Assignment operator: =
• Assigns value to identifier
• Value and type of assignment operation
is resulting value and type of left-hand
identifier
int nA, nB; nB = (nA = 4) / 2
• Do not confuse with binary boolean
equality operator ==
Copyright 2002 Elsa L. Gunter
Boolean Operators
• Comparison operators >, <, >=,
<=, ==, != return boolean values,
not integers
• Logical boolean operations:
– And: E1 & E2 ( && for short circuit)
– Or: E1 | E2 ( || for short circuit)
– Exclusive Or: E1 ^ E2
Copyright 2002 Elsa L. Gunter