Programming in Java I

Lecture Notes, class 4 for csc211.

Prepared by: Anthony Larrain

These notes serve as an outline to the lecture, they are not intended to be complete. You should be reading the assigned sections. During class I may include or omit certain topics.

The Scanner Class

• Provides an alternative to JOptionPane for getting input.
• The Scanner class lives in package java.util
• Can use a Scanner object to read data from the standard input or console.
• The Scanner class is NOT available to versions of Java previous to 5.0.
• A Scanner object will convert an input value to a primitive type or String.
• When creating a Scanner object we pass as an argument the object System.in
• The following piece of code will allow the user to read an integer from the console.

Scanner read = new Scanner(System.in);
int age      = read.nextInt();

• Should provide the user with instructions (prompt) on what to enter.

Scanner read = new Scanner(System.in);
System.out.print("Enter your age  ");
int age      = read.nextInt();

• Some methods of the Scanner class

Return Value	Method Name
int	nextInt()
double	nextDouble()
String	next()
String	nextLine()

See PoundsToKilograms.java and SimpleTax.java

Using JOptionPane for Output

• Class JOptionPane has a static or class method named showMessageDialog that pops up an output dialog box.
• Method
showMessageDialog returns void or nothing
• Method
showMessageDialog requires at least two arguments

JOptionPane.showMessageDialog(null, "greetings");

• Keyword null is used to indicate the center of the screen.
• The above code produces the following dialog box.

Control Flow

Unless directed otherwise the statements of a method are executed sequentially, one after the other, starting with the first. This is called sequential execution . There are a few ways in Java to alter the program flow so that the next statement executed, may be other than the next one in sequence. This is called transfer of control

3 ways to alter the flow of control

• Invoking a method.

When a method is called (invoked) control jumps to the code that defines the method. When the method is finished control returns to the place in the calling method where the method was invoked.

• Using a Selection Structure. (Decisions)

  Deciding which statement(s) to execute next. The decision is based on a boolean expression , which is an expression that evaluates to either true or false .

• Using a Repetition Structure (Looping)

Loops allow us to execute a statement or group of statements over and over again based on some boolean expression.

The if Statement

The if statement has the form.

  if ( condition is true )
     execute this-statement;
 

The condition enclosed in parentheses must be an expression that evaluates to true or false. Such an expression is called a boolean expression.

The if-else Statement

 if( condition )
   statementA;
 else
   statementB;
 

If the condition is true we execute statementA, if condition is false execute statementB.

Comparison Operators

• Equality operators

  ==      equal to
  !=     not equal to
  

• Relational operators

  <    less than
  <=   less than or equal to
  >    greater than
  >=   greater than or equal to
  

The comparison operators:

• Have lower precedence than the arithmetic operators.
• Return a boolean value.(True or False)

See COperators.java

More Operators

Shorthand Assignment Operators

• The Assignment Operator ( = ) , gives a variable a value.

• +=

                int sum = 30;
                sum += 5;      now sum is 35
                Equivalent to  sum = sum + 5;
          

• -=

                int a = 5;
                a -= 2;         a is 3
                Equivalent to   a = a - 2;
          

• *=

                int x = 20;
                x *= 3;         x is now 60
                Equivalent to   x = x * 3;
          

• /=

                int z = 40;
                z /= 2;         z is now 20
                Equivalent to   z = z / 2;
          

© Anthony Larrain 2004 - 5

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