Designing Programs

Out-Lab 4, Chapter 4

An overview of Object Oriented programming

• OOP has two types of classes

– Encapsulate classes: Those that encapsulate data and the methods that act on it together

• The function is to create a new type

– Application classes: Those with a main method.

• The function is to execute a program

• For the first three chapters, we worked mostly with creating a new type with a class

• For the next few chapters we will be working on tools to help you write methods in either type of class

Types in Java

• Primitive types

– int, double, char, boolean

• Types created by classes

– The Java library has hundreds of prewritten classes

– Programmers can create their own types of the library does not contain exactly what they want

• The Java API lists all the predefined classes and their methods

• Examples of prewritten classes

– String object, Scanner objects

– Objects of classes can call methods to work on objects of their type

Application classes

• Contain a main method to begin execution of the program

• May contain other methods.

• These methods should be marked static

– This is because they are not encapsulated in a class to manipulate data.

Out-Lab 4 Figuring Gross Pay

• This lab is to write a Java program that will determine the gross pay for employees.

• We’ll use pseudocode with top-down design to figure out how to write this program.

• Top-down design says we formulate the bigger problems first, then worry about the smaller problems later

Top-down design

• First, of course, we have to understand our problem

• Look at the instructions: (it is often good to start with I/O)

– The program should prompt the user for the number of hours worked and the pay rate for an indefinite number of employees

– After you have the data for one employee, you should call the method that calculates the gross pay to get the gross pay for that employee.

– The gross pay for each employee should be output before prompting for the data for the next employee

– The company you are writing this program for pays straight time for the first 40 hours worked by each employee and time and a half for all hours worked in excess of 40 hours

Compound assignment operators

• One of the most common operations the computer does is to add a value to a variable, then store it back in the variable
var = var + num;

• This is so common there is a shortcut for typing it

var += num;

• There are other compound assignment operators

var - = num;

var *= num;

var /=num;

var %=num

Increment & decrement Operators

• Increment operators add one to a variable

– They can be either prefix increment or postfix increment

– Prefix adds one, then completes the statement

– Postfix completes the statement, then adds one

– Example num = 8; var = 4;
num = ++var + 10; what is the value of num?

num = var++ + 10; what is the value of num?

• Pre and post fix decrement operators act the same way

The Conditional Operator

• There is a shortcut for if/else statements

• It is optional to use, but you need to know how to read it in code

• Standard if/else statement
if (x > size)
y = 10;
else y = 15;

• The shortcut that means exactly the same
(x>size) ?: y=10 : y=15;

• (condition) ?: do_if_true: do_if_false

Write this program

• We want to program a calculator (ours will do only addition and subtraction)

• The user should be able to enter an expression like 576 + 943, or 456 -345, and our program should give the answer.

• The user should be prompted to ask if they want to do another problem.

• Do first in pseudocode

• Then implement in the simplest way possible, then compile that much