Classes, methods, and conditional statements

1
Classes, methods, and conditional statements

• Were past the basics. These are the roots.

2
What is a class?

• A class is an object variable.
• Classes can be divided into subclasses that
  inherit some properties of the superclass.
• Built into the Java language are a variety of
  classes that contain useful functions and
  calculating tools.
• Well work more with classes later after we learn
  about methods.

3
The main class

• In Java, the main class is the actual program
  itself. It is executable.

import extra. public class Graduation
public static void main(String args)
Std.out.println( This is a very simple
program. )
public can be accessed by all parts of the
program class signifies that the code between
the braces belongs to a class Graduation
gives the class a name, always begins with a
capital letter.
4
What is a method?

• A method is a smaller block of code designated to
  do something.
• It usually takes input and gives output, but
  sometimes it doesnt need one or the other (or
  either).
• A method is like a mini-program that can be run
  over and over.
• Methods are sometimes called subroutines or
  functions.

5
The main method

• In Java, the main method is where the computer
  begins to read code when the program runs.

import extra. public class Graduation
public static void main(String args)
Std.out.println( This is a very simple
program. )
public can be accessed by all parts of the
program static the function is permanent void
the function gives no output to the computer
itself main the function is the programs main
function String args we are coding using text
6
Methods in action

• Write a program to do the following
• The user will input two double variables, a and
  b, representing the lengths of the arms of a
  right triangle
• The program must find the value of the hypotenuse
  squared, that is, a2b2
• How could we write a program to do this?

7
A solution
import extra. public class Graduation
public static void main(String args)
double a double b double
cSquared Std.out.println( Please input a.
) a Std.in.readDouble() Std.out.println(
Please input b. ) b Std.in.readDouble()
cSquared Math.pow(a,2) Math.pow(b,2) Std.o
ut.println( c2 equals cSquared)
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A better solution use a method

• Whenever there is an operation that we perform
  repeatedly, it is helpful to place it into its
  own method.

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How to write a method

• Heres the structure of a methodmodifier
  resulttype name (inputs) calculations return
  
• Heres what our method for squaring might look
  likeprivate double square (double d) double
  dSquared Math.pow(d2) return dSquared

private because only our class needs to be able
to use it double because the return (or output)
is a double
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Implementing a method

• Place the code for the method itself inside the
  class, either before or after the main method

• To call a method from the main class, write the
  methods name, followed by the methods argument
  (input) in parens.
• Exampledouble aSquared square(a)

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A better solution the code

• import extra.
• public class Graduation
• private double square (double d) double
  dSquared Math.pow(d2) return dSquared
• public static void main(String args)
• double a
• double b
• double cSquared
• Std.out.println( Please input a. )
• a Std.in.readDouble()
• Std.out.println( Please input b. )
• b Std.in.readDouble()
• cSquared square(a) square(b)

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Your turn

• Write a method (not a program) that takes an
  integer as its argument (input), halves the
  integer, and returns (outputs) a double as the
  answer.
• Your method should be private.

13
A solution

• private double half (int i) double halfOfI
  i / 2.0 return halfOfI

Notice we divided an integer by a double
(i/2.0). If we had divided an integer by an
integer (i/2), the resulting integer would be
truncated, and so would the double. Remember
the argument is input, the return is output
14
Conditional statements!

• Conditional statements are the heart of
  programming, they allow the program to react
  differently to different inputs

15
The while statement

• Suppose we want to make a program that counts to
  ten.
• We want our program to start counting at one, but
  we want it to know to stop counting, too!
• How do we get it to stop counting at ten?

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Writing a while statmenet

• int counter 1
• while ( counter lt 10 )
• Std.out.println( counter )
• counter

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Mechanics of a while statement

• If the condition inside the parens () is TRUE,
  the code inside the braces is executed.
• If the condition inside the parens () is FALSE,
  then the code inside the braces is NOT
  executed.
• int counter 1
• while ( counter lt 10 )
• Std.out.println( counter )
• counter

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The if statement

• Revist the Graduation program that calculated the
  years left you had in high school.
• It would be prudent to put in code that prevents
  the input of a number greater than 12.

19
Writing an if statmenet

• The last line in our code was this
• lastGrade Std.in.readInt()
• Heres what a good if statement would look like
• if ( lastGrade gt 12 )
• Std.out.println(Youve already graduated.)
• else int yearsLeft
• yearsLeft 12 - lastGrade
• ...

20
Mechanics of an if statement

• If the condition inside the parens () is TRUE,
  the code inside the braces is executed.
• If the condition inside the parens () is FALSE,
  then the code inside the braces is NOT
  executed. Instead, the code inside the braces
  after else is executed. (The else and its braces
  are optional.)
• if ( lastGrade gt 12 )
• Std.out.println(Youve already graduated.)
• else int yearsLeft
• yearsLeft 12 - lastGrade
• ...

21
The for loop

• Suppose we want to make a program that prints the
  squares of the first ten integers
• We could use a while loop, but we could also use
  a for loop

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Writing a for loop

• int counter
• for ( counter 1 counter lt10 counter )
• Std.out.println( (counter counter) )

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Mechanics of a for loop

• There are THREE statements in the parens(), and
  there are semicolons after the first two.
• The FIRST statement is executed the first time
  through the loop, and the THIRD statement is
  executed at the beginning of each subsequent trip
  through the loop.
• If the SECOND is true, then the code inside the
  braces is executed. If it is FALSE, then the
  code inside the braces is NOT executed.
• int counter
• for ( counter 1 counter lt10 counter )
• Std.out.println( (counter counter) )

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Its your choice!

• while loop
• int counter 1
• while ( counter lt 10 )
• Std.out.println( ( counter counter ) )
• counter

• for loop
• int counter
• for ( counter 1 counter lt10 counter )
• Std.out.println( (counter counter) )

25
Combining methods and conditional statements!

• Write a program that asks a user for two
  integers a base and an exponent.
• The program should check to make sure that the
  exponent is at least 1.
• Then, the program should use a method (not
  Math.pow) to do the multiplying. Some kind of
  loop should be used.