1. Programming Language Concepts (CIS 635)
Elsa L Gunter
4303 GITC
NJIT,

2. 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
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3. Variables and Methods
Classes have two kinds of methods and two kinds of variables:
Instance methods and instance variables
Class methods and class variable
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4. Instance Variables and Methods
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
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5. 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
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6. Inheritance
Inheritance allows all variables and methods exported from parent class to be part of a derived class or subclass
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7. Inheritance Export information: public: everybody sees
private: only seen inside class
protected: exported only to subclasses
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8. 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
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9. 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
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10. 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
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11. 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 Elsa L. Gunter

12. Introduction to Java Recently introduced (1995)
Modern and sound design (Gosling)
Very rapid growth (and heavy hype)
Killer application: web-based computing (applets)
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13. 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
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14. 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
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15. Getting Java For MS Windows: Installation instructions found
Installation instructions found
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16. 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)
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17. 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
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18. Sample Java Code (“Hello.java”)
class Hello {
public static void main (String args[]) {
System.out.println ("So at last we get to Java!\n"); }
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19. 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!
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20. Terminolgoy Class: Hello Class method: main
Only class method of class Hello
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21. 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
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22. 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
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23. Java Method Declaration
public static void main (String args [])
public and static describe scope and sharing characteristics of method main
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24. 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)
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25. 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;
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26. Java Built-in Data Types
Size(bits)
Default initial value
boolean 8
false
byte
char 16
‘x0’
short
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27. Java Built-in Data Types
Size(bits)
Default initial value
int 32
long 64
float
0.0F
double
0.0D
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28. 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 Elsa L. Gunter

29. 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 Elsa L. Gunter

30. 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 Elsa L. Gunter