Chapter 2: Java Fundamentals cont’d Lecture 4: Monday Sept 11, 2006

Outline 2.1 The Parts of a Java Program 2.2 The print and println Methods, and the Java Standard Class Library 2.3 Variables and Literals 2.4 Primitive Data Types 2.5 Arithmetic Operators 2.6 Combined Assignment Operators 2.7 Conversion Between Primitive Types 2.8 Creating Named Constants with final 2.9 The String Class 2.10 Scope 2.11 Comments 2.12 Programming Style 2.13 Reading Keyboard Input 2.14 Dialog Boxes 2.15 Common Errors to Avoid

Operator Precedence What is the result of: Polynomial = 1+2*3+ 6/2 -2; Is it ? (1+2)*3 + 6/(2-2) 1+(2*3) +(6/2)-2 (1+2)*3 + (6/2)-2

Precedence Rules Always evaluate * , / and % before + and – Always negate before any calculations *, / and % have same precedence + and – have same precedence If equal precedence then evaluate from left to right except for negations where we evaluate from right to left

Precedence examples Polynomial = 1+2*3+ 6/2 – 2; Polynomial has the value of 1+6+3-2=8 Polynomial = –1 + 5 – 2; // 2 Polynomial = –(–3) + –(–5); //8

Grouping with parentheses You can use parentheses to force the evaluation of a formula Examples: x * ( y + z*z ) instead of x*y + z*z x * ( y * ( z + 165 ) + 85 ) – 65 Average = (a +b +c ) /3;

The Math class value = Math.pow( x,y); // now value holds x to the power of y value = Math.sqrt( x); //now value holds the square root of x

Combined Assignment Operators += x += 1; x = x + 1; –= x –= 1; x = x – 1; *= x *= 1; x = x * 1; /= x /= 1; x = x / 1; %= x %= 1; x = x % 1;

2.7 Conversion between Primitive Data Types Before a value is stored in a variable, Java checks the Data Type of the value and the variable If the data types are compatible then Java performs the conversion automatically  No Error If the data types are not compatible then Java issues an error.

2.7 Conversion between Primitive Data Types A widening conversion is the conversion of a small value to a larger one A narrowing conversion is the conversion of a large value to a smaller one double largest float long int short byte smallest

Widening conversion Example 1: Example 2: double x; int y = 10; x = y; int x; short y =2; x= y;

Narrowing Conversion We have to perform casting i.e. the name of the smaller data type is put in parentheses in front of the value Example: int number; double pi = 3.14; number = (int) pi;

Cast operator Used to convert from one primitive data type to another Must be used for narrowing conversions

Example: int pies = 10, people = 4; double piesPerPerson; piesPerPerson = pies /people; piesPerPerson =(double) pies/people; piesPerPerson =pies/(double) people; piesPerPerson=(double)(pies/people); (double)(10/4) = (double)(2) = 2.0 because it is an integer division 10/4 = 2 because it is an integer division 10.0/4 = 2.5 because one of the numbers is a double 10/4.0 = 2.5 because people is double

Mixed Integer Operations The result of an arithmetic operation that involves only byte, short, or int variables is always an int even if both variables are of data type short or byte Example: short x =5, y =7; short z = x+y; // this statement gives an error short z = (short) ( x+y ); //correct

Mixed Integer Operations If one of the operator’s operands is a double then the result of the operation is a double If one of the operator’s operands is a float then the result of the operation is a float If one of the operator’s operands is a long then the result of the operation is a long

Creating named constants with final A named constant is a variable whose value is read-only and cannot be changed To create a named constant add the word final to declaration An initialization value is required when declaring a constant Example: final double INTEREST_RATE = 0.069;

More about named constants When naming a constant, the variable name should be written in all uppercase characters. Math.PI is a constant that holds the value of pi ( i.e. 3.14159 …) Math.PI is already declared and initialized so it ready to use. Example: double area = Math.PI * radius * radius ;

The String class A String literal is any text enclosed in quotations A String is the DataType of a variable that can store String literals Example of a String variable: String name = “CS 0007”; System.out.println( name );

The String class To determine how many letters are stored in a String variable (name) use name.length(); Example: String mycourse = “CS 0007”; int number = mycourse.length();

String methods charAt(index) index is an integer and specifies the character position in the String This method returns the character at the specified position Example: char letter; String myText = “This is my Text”; letter = myText.charAt(8);

T h i s m y e x t String methods myText.length returns 15 because there are 15 characters T h i s m y e x t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 myText.charAt(8) returns m because m is the letter at position 8

String methods toLowerCase() This method returns a new String that has all of the characters of the original String but in lowercase Example: String bigName = “I am BIG!!”; String smallName = bigName.toLowerCase(); // now smallName holds “i am big!!”

String methods toUpperCase() Same as toLowerCase() but it converts all the characters to uppercase Example: String smallName = “I am Big!!”; String bigName = smallName.toUpperCase(); // now bigName holds “I AM BIG!!”

Example: String message = "Java is Great Fun!"; String upper = message.toUpperCase(); String lower = message.toLowerCase(); char letter = message.charAt(2); int stringSize = message.length(); System.out.println(message); System.out.println(upper); System.out.println(lower); System.out.println(letter); System.out.println(stringSize);

Scope The variable scope is the part of the program that has access to it public class Scope { public static void main(String[] args) System.out.println(value); // ERROR! int value = 100; }

Scope public class Scope { public static void main(String[] args){ int number = 100; System.out.println(number); int number = 200; //ERROR }

Homework due Monday Sept 18 Do Exercise 2 in the programming challenges page 113 Print your code and submit it in class