Introduction to Java Lecture Notes 3

1. Introduction to JavaLecture Notes 3

2. Variables • 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;

3. 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.

4. 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";

5. 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.

6. 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;

7. 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); } }

8. 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 ); }

9. 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.

10. 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".

11. 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;

12. 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")

13. 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.

14. Control Flows switch-statement as in c++ switch (token) { case 'a': count1++; break; case 'b': count2++; break; default: System.out.println("error..."); }