Memory and I/O

In Java

Output

• The simplest way to output to the screen is to use System.out.print( ) and println( );

– Whatever is in the parentheses will be printed on the screen

– It is called the argument of the method call print( )

– If you are printing a string, it must be enclosed by double quotes.

Escape sequences

• Escape sequences are used to format strings (i.e. inside double quotes)

• The backslash (\) is the escape character

• It indicates that the next character in the string has special meaning

– \n means start a new line

– \t means move the cursor to the next tab stop

– \\ means print the backslash

– \” means print the double quote

Formatted output

• System.out.printf(arguemnts) allows the programmer more control over how output looks

• Sysytem.out.printf(“%s\n%s\n”, “Hello”, “World”);

– “%s\n%s\n” is the formatting string

– “Hello” is substituted for the first &s, “World” for the second

• Its major advantages come with floating point numbers, not strings and integers

• These two statements output exactly the same:
Sysytem.out.printf(“%s\n%s\n”, “Hello”, “World”);
Sysytem.out.print(“Hello\nWorld\n”);

Input

• The Scanner class in the new version of Java makes input a lot easier than it used to be.

• How do you use the Scanner class?

1. import java.util.Scanner before the class declaration

2. Declare a Scanner object inside the class declaration
Scanner varName = new Scanner (System.in);

3. Call one of the methods of the Scanner class to input
int num = varName.nextInt( ) returns an int
String str = varName.next( ) returns a String

• The value returned is assigned to a variable by the assignment operator

Program on page 47 of your text

import java.util.Scanner;

public class Addition
{ public static void main( String args[ ] )
{ Scanner input = new Scanner( System.in );

int number1, number2, sum;

System.out.print( "Enter first integer: " );
number1 = input.nextInt( );
System.out.print( "Enter second integer: " );
number2 = input.nextInt( );

sum = number1 + number2;
System.out.printf( "Sum is " + sum );
//System.out.printf( "Sum is %d\n“, sum ); this is the same as the line above
}

}

What happens in main memory

• After your program is compiled, it must be loaded into RAM to be executed

• It is loaded into a section of RAM called static RAM (or sometimes the stack)

• The executable statements are loaded in one part, and room to store any variables declared are stored directly below

• The variable names then correspond to locations (or addresses) in RAM

Variables

• Every variable has:

– A name

• This is the variable name give it by the programmer

– A type

• So far, the only type we have looked at are integers

• The keyword for integers is int

– A size

• On most systems, an integer takes four bytes

– A value

• This can either be assigned by the programmer, or by the user of the program

• If no value is assigned by the programmer, or the user, it will contain the junk that was there from the last use

Loading a program into RAM

• On the whiteboard, we will load the program Addition and its data slots into RAM

Java Arithmetic

• Arithmetic operators in Java are much like in algebra

a + b is addition; a - b is subtraction;
a * b is multiplication; and a/b is division

• Integer division yields an integer;
so 5/2 = 2; The fractional part is discarded

• With integers, you often want to know the remainder after division;

– this is called the mod function, indicated a%b

10%4 = 2; 5%2 = 1; 15%5 = 0

Operator precedence

• What is the difference between
5+3*2 and (5+3)*2

• It is understood that multiplication takes precedence over addition

– But parentheses can override the precedence

• In a programming language the precedence between operators is well defined

– But sometimes easily overlooked

• My advice is to use parentheses if you are in doubt if you need them

– It is never wrong to have them, and often wrong to leave them out.