Introduction to JavaLecture Notes 3

Variables

l A variable is a name for a location in memory used to hold a value. In Java data declaration is identical to c/c++

int n; int m = 5; double x;

Variables

with one modification, instead of const Java has final: final int N = 10; meaning that the value of N cannot be changed anymore. Java is a strongly typed language, the value on the rhs

(right hand side) must be compatible with the type on the lhs.

int x = 2.3; is illegal, however double x = 2; is ok.

Types

Java has TWO general types: primitive types and reference types. There are, correspondingly, two kinds of data values that can be stored in variables, or passed as arguments, or returned by methods, and operated on: primitive values (or data) and reference values. Primitive data - numbers and characters. In the declaration

int n; char ch = 'a'; int and char are types. Reference data - objects which

are defined by a class, for example String str = "15-111";

Primitive Types

A primitive type is predefined by the Java programming language and named by its reserved keyword. There are

the char type; 4 integer types: byte (1 byte), short (2 bytes), int (4

bytes), long (8 bytes), and char (2 bytes); 2 floating types: float (4 bytes), double (8 bytes); the boolean type, an example, boolean a;

Booleans have only two values: true and false.

Reference Types

There are three kinds of reference types: class types, interface types, and array types. In the following example of class

declaration public class Demo { int x;} Demo is a class type, and x is a field (or an instance

variables). How would you use the class type? To create an object. An object is instantiated (created) class. We need references to point to objects. Once we implement a class Demo, we implement a

new type, so that we can declare Demo tmp;

An Example

import javax.swing.JOptionPane;public class Example1 { public static void main(String[] args) { String s1, s2; int n1, n2, max;

// prompt for integers s1 = JOptionPane.showInputDialog("Enter an integer"); s2 = JOptionPane.showInputDialog("Enter another integer");

// convert these strings to integers n1 = Integer.parseInt(s1); n2 = Integer.parseInt(s2);

// find the maximum max = Math.max(n1,n2); // print the message and the max value JOptionPane.showMessageDialog(null, "The biggest is " + max); System.exit(0); } }

Exceptions

If the input string cannot be converted to an integer, showInputDialog() throw an exception. We will discuss exceptions in details much later.

try { n1 = Integer.parseInt(s1); } catch (NumberFormatException a) { System.out.println("Tne number is not integer'); System.exit( 0 ); }

Operator new

To create an object we use the new operator. Integer n; //declaration n = new Integer(5); //instantiation

The act of creating an object is called instantiation. The above two steps can be done in one

Integer n = new Integer(5); String str = new String("This is a string");

The new method calls a constructor, which is a special method of a class responsible for initialization.

Operator new

Note, specially for String, you can avoid calling new. It's done implicitely. So the following operations are identical

String str = "This is a string"; String str = new String("This is a string"); Here is another example. There is a class DecimalFormat in

the package java.text which formats the unexact numbers. DecimalFormat precision = new DecimalFormat( "0.00" );

The class DecimalFormat has a method format, that formats the number to a particular form. Calling

System.out.println(precision.format(1.2345) );

will print "1.23".

Control Flows

if-statement as in c++ if (ch != 's' && total < MAX) count++; //same as count = count + 1; else count=/2; //same as count = count/2;block statements

if (ch != 's' && total < MAX) { count++; System.out.println("for debugging"); } else count=/2;

Control Flows

conditional operator as in c++. Here is the above example written as the conditional operator.

count = (ch != 's' && total < MAX) ? count+1 : count/2;

The conditional operator is a great tool to separate plural and single forms. Here is an example from the previous exercise (an

exercise on converting miles to kilometers):

JOptionPane.showMessageDialog(null, ml + " mile" + ((ml==1)?"":"s") + " is " + km + " kilometers")

Control Flows

while-loop as in c++

while (true) // cannot be while(1) { System.out.println("a number"); System.exit(0); }

One of the most common use of the while-loop is to validate input.

Control Flows

switch-statement as in c++

switch (token) { case 'a': count1++; break; case 'b': count2++; break; default: System.out.println("error..."); }