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CS111: PROGRAMMING LANGUAGE II Computer Science Department

Lecture 1(b): Java Basics (I)

Lecture Contents 2



Java basics  Input/output  Variables

 Expressions

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Your first Java program.. 3

// indicates a comment. class keyword Java is case sensitive braces { , } delimit a class body main Method

“Everything must be in a class” There are no global functions or global data. Computer Science Department

Basic language elements 4

Text I/O 5



Standard output.  Flexible

OS abstraction for output.  In Java, applications use the standard output object (System.out) to display text on terminal.

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Command line output 6

Multiple line O/P Formatting output

public class TestIO { public static void main(String[] args) { System.out.println(“Welcome to java”); System.out.println(“Welcome to \n java”); System.out.print(“Welcome to”); System.out.println(“java”); System.out.printf(“%s\n%s\n“, “Welcome to”,”java”); }

} } Computer Science Department

Common escape sequences 7

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Printf Conversion-Characters 8

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Strings & Text 9

String msg1 = new String( “Hello” ); String msg2 = “Hello” ; String msg3 = “Year “ + 2005; //valid?? Computer Science Department

Standard Input 10

 

input is received from Terminal window. Input entered while program is executing.

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Reading values 11

 

use : import java.util.Scanner; Define an object of the Scanner class: Scanner input = new Scanner(System.in);



input values: num1 = input.nextInt();



Display after calculation: System.out.printf(“the square is : %d \n”, num1*num1);

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example 12

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example 13

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variables 14



Declaration, memory allocation, initialization data type

variable name

int total = 0; int count, temp, result; Multiple variables can be created in one declaration

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Constants 15

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

A “constant variable” is an identifier that is similar to a variable except that it holds one value for its entire existence Why constants:   



give names to otherwise unclear literal values facilitate changes to the code prevent inadvertent errors

In Java: final double PI = 3.14159265;

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Arithmetic Expressions 16

 

An expression is a combination of operators and operands Arithmetic expressions (we will see logical expressions later) are essentially special methods applied to numerical data objects: compute numeric results and make use of the arithmetic operators: Addition Subtraction Multiplication Division Remainder

+ * / % Computer Science Department

Assignment-related Operators 17

 

Increment and decrement operators: ++, -Assignment operators: +=, -=, *=, /= these three expressions have the same effect

count

=

count + 1;

count

+= 1;

count

++;

these two expressions have the same effect

count

=

count - 10;

count

-= 10; Computer Science Department

Operator Precedence 18

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What is the order of evaluation in the following expressions? a + b + c + d + e 1 2 3 4

a + b * c - d / e 3 1 4 2

a / (b + c) - d % e 2 1 4 3 a / (b * (c + (d - e))) 4 3 2 1 Computer Science Department

How Do Data Conversions Happen? 19



Implicitly:  occurs

automatically  uses widening conversion,  Examples

:

4.0 / 8 (which / is it: double/double, float/float, int/int) 4 / 8.0 (which / is it: double/double, float/float, int/int) 4 + 5 / 9 + 1.0 + 5 / 9 / 10.0 (what is the value?)

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How Do Data Conversions Happen? 20



Explicitly: Casting  widening

/ narrowing conversions  Examples: double MyResult; MyResult = 12.0 / 5.0; //OK int myInt = (int) MyResult; // truncation MyResult = (double)myInt/3.0;

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Be careful!! 21

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Example: in 1996, Ariane 5 rocket exploded after takeoff because of bad type conversion.

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Class Math 22



All Math class methods are static 



Each is called by preceding the name of the method with the class name Math and the dot (.) separator

Method arguments may be constants, variables or expressions

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Example 24



Solve quadratic equation ax2 + bx + c = 0. public class Quadratic { public static void main(String[] args) { double a,b,c,d; // input coefficient values.. // calculate roots d = Math.sqrt(b*b - 4.0*a*c); double root1 = (-b + d) / (2.0*a); double root2 = (-b - d) / (2.0*a); // print them out System.out.println(root1); System.out.println(root2); } } Computer Science Department

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That’s all for today….. Text Book: Chap 2

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