Download An Introduction to Java

An Introduction to Java
By Nicholas Policelli
Basic Program Structure
public class ClassName
{
public static void main(String[] args)
{
program statements
}
user defined methods
}
Basic Program Structure
 Classes can be public or private
 Methods can be either public or static
 Public methods can be called from any class
 Static methods cannot be used on objects
 Void indicates that the method does not return a value
 The array args stores command line arguments
Basic Program Structure
 The source code must have the same name as the public class
and have a .java
 During compilation the Java compiler produces a bytecode
file which is given a .class extension
 Execution: java (class name)
Data Types
 Integers: int, short, long, byte
 Floating-Point Types: float, double
 The Character Type: char
 The Boolean Type: boolean (values: true, false)
Operators and Strings
 Arithmetic Operators: +, -, *, /, %
 Relational Operators: ==, !=, <, >, <=, >=
 Logical Operators: &&, ||, !
 Strings are standard Java class.
 Concatenation uses +
Variables
 Variables in Java need to be declared.You need to initialize
variables before you use them.
Examples:
int n=5;
System.out.println(n);
String s=”Hello”;
String t=”world”;
System.out.println(s+” “+t);
If, If Else, For ,and While Loops
 If and if else follow the C++ conventions.
if(…)
{
…
}
 While and For loops also follow the same C++ conventions
Arrays
 Standard class in Java
 Declared by specifying the type enclosed in []
 All elements may be listed by enclosing in {} and separated
with commas.
 If one array variable is copied into another array variable,
both variables refer to the same array
 As in C++ and Perl array indices run from 0 up to the length
of the array minus one
 arrayName.length can be used to find the length of the array
Array Example
int n=5;
int[] numbers = new int[n];
for(int i=0; i<numbers.length; i++)
numbers[i]=n-i;
Arrays.sort(numbers);
for(int i=0; i<numbers.length; i++)
System.out.println(numbers[i]);
Object Oriented Programming in Java
 Program structure with user defined classes
 The basic structure of Java programs with user defined classes:
public class ClassName
{
public static void main(String[] args)
{
program statements
}
user defined methods
}
user defined classes
Class Definitions
 The basic structure of Java classes:
class NameOfClass
{
constructors
methods
fields
}
Objects and Object Variables
 Java constructors construct and initialize objects.
 Constructors have the same name as the class.
 Objects are constructed by an application of a constructor
and new.
 Object variables store the reference to the object, not the
object itself.
Example:
Date D=new Date(); // An object of a Date class is
constructed and its reference
System.out.println(D); // is stored inside variable D. Next we
print out the object.
Class Fields and Methods
 Class fields define the state of an object.
 Constructors define and initialize class fields.
 Accessor methods access class fields of an object.
 Mutator methods are used to modify class fields.
Parameter Passing
 Two kinds of method parameters
 Primitive Types
 Object References
 In both cases the variable is passed by value,
 For primitive types the parameter will contain the primitive
type
 For Object References the parameter will contain the
reference, not the object
References
 Example classes and more can be found in the Java resources
on the course website