Intro to programing with java-lecture 3

Let’s dive
more more in
java

Hello!
I am Mohamed Essam !
You can find me at
mohamedessam.cs@gmail.com
2

“
“Talk is cheap. Show me the
code.”
― Linus Torvalds
3

Last time
Let’s start with the first set of slides
1

Simple java Program
public class Welcome {
public static void main(String[] args) {
// Display message Welcome to Java! A class Block
System.out.println("Welcome to Java!"); a method block
}
}
5
Welcome to Java!
Output

Numeric Operators
The operators for numeric data types include the standard arithmetic
operators:
￮ addition (+),
￮ subtraction (–),
￮ multiplication (*),
￮ division (/),
￮ and remainder (%),
The operands are the values operated by an operator.
7

perform mathematical
computations
￮ Listing 1.3 gives an example of evaluating.
8
public class ComputeExpression
{ public static void main(String[] args)
{
System.out.println((10.5 + 2 * 3) / (45 – 3.5));
}
}
Output
0.39759036144578314

Programing Expectation VS Reality
9

What bug is
The terms "bug" and "debugging" are popularly attributed to Admiral
Grace Hopper in the 1940s. While she was working on a Mark II computer
at Harvard University, her associates discovered a moth stuck in a relay
and thereby impeding operation, whereupon she remarked that they
were "debugging" the system. However, the term "bug", in the sense of
"technical error"
10

Types of errors
1. Syntax error:
Errors that are detected by the compiler are called syntax errors or
compile errors. Syntax errors result from errors in code construction,
such as mistyping a keyword, omitting some necessary punctuation,
or using an opening brace without a corresponding closing brace.
These errors are usually easy to detect because the compiler tells
you where they are and what caused them.
11

Types of errors
2. Runtime error:
Another example of runtime errors is division by zero. This happens
when the divisor is zero for integer divisions.
public class ShowRuntimeErrors {
System.out.println(1 / 0);
}
}
12

Types of errors
3. Logic Errors:
Logic errors occur when a program does not perform the way it was
intended to. Errors of this kind occur for many different reasons.
For example if we have a program prompt a user for input and
detect if the input is negative or positive we will notice that the
program will print the zero as a negative number because it’s less
than one.
public class LogicErrors {
int x=0;
if(x>=1)
{
System.out.println("positive");
}
else if(x<1)
{System.out.println("negative");
}
}
} 13

Augmented Assignment
Operators
￮ The operators +, -, *, /, and % can be combined with the assignment operator to
form augmented operators.
￮ Very often the current value of a variable is used, modified, and then
reassigned back to the same variable.
￮ For example, the following statement increases the variable count by 1:
count = count + 1;
￮ Java allows you to combine assignment and addition operators using
an augmented (or compound) assignment operator.
￮ For example, the preceding statement can be written as count += 1; The
+= is called the addition assignment operator.
14

Operators
15

Operators
16
￮ The augmented assignment operator is performed last after all the
other operators in the expression are evaluated. For example,
￮ x /= 4 + 5.5 * 1.5;
￮ is same as x = x / (4 + 5.5 * 1.5);

Increment and Decrement Operators
￮ The ++ and —— are two shorthand operators for incrementing and decrementing a variable
by
￮ These are handy because that’s often how much the value needs to be changed in many
programming tasks.
￮ For example, the following code increments i by 1 and decrements j by 1. int i = 3, j = 3; i++;
￮ // i becomes 4
￮ j——;
￮ // j becomes 2
￮ These operators can also be placed before the variable.
￮ For example, int i = 3, j = 3; ++i;
￮ // i becomes 4 ——j;
￮ // j becomes 2 ++i increments i by 1 and ——j decrements j by 1.
￮ These operators are known as prefix increment (or preincrement) and prefix decrement (or
predecrement). As you see, the effect of i++ and ++i or i—— and ——i are the same in the
preceding examples. However, their effects are different when they are used in statements
that do more than just increment and decrement.
17

18
Let’s build a Gas station
system

Gas station system
We have a gas station wanted to build a
program to count the final price after fueling
the car with the gasoline.
Hint
the price of the fueling for
￮ Gasoline 95 is 8 $.
￮ Gasoline 92 is 7 $
￮ Gasoline 80 is 5 $
￮ We need a program to get the type of the
gasoline and the number of liters ,
calculate the total price and print it on the
screen.
19

Gas station pseudocode
￮ get the input(Price ,Liters) from the user.
maybe from the machine itself or an admin for the system who can change the
price for liters when it increase.
￮ Store it in two variables.
￮ multiply the price in the liters and store it in a variable.
￮ Print the number of liters and the price on the screen.
20

Variables
￮ According to the integer and floating
variables in the last lecture we have six kinds
with different sizes
￮ byte 1 byte -128 to 127
￮ short 2 bytes -32,768 to 32,767
￮ int 4 bytes -2,147,483,648 to 2,147,483,647
￮ long 8 bytes -9,223,372,036,854,775,808 to 9,223,372,036,854,775,80.
￮ float 4 bytes 7 decimal digits.
￮ double 8 bytes 16 decimal digits.
21

In your opinion ?
￮ Liters
￮ The number liters for cars lies between 30 to 65 and can be a floating
number .
￮ Hint :I have to put f in the end of the number of float like float x=3.1f. or use
double instead .
￮ Gasoline price per liter
￮ because the price of it is between 5 to 8 dollars and byte variable can
store up to 127
￮ Also I don’t need to use short which up to 32,767 or int which are up to
2,147,483,647
￮ Total cost
￮ if we predict that there is a person can fuel his car 65 liters with gasoline
95 which are 8$ will liter the price will be 520 which already can store in
float which are up to 2,147,483,647.
22
￮ Which types of variables I can use for my three variables Liters ,
Gasoline price and Total cost.

public class gasstation {
float x=30.55f;
byte y=8;
float z=x*y;
System.out.println(z);
//system.out.println(liters*gasolineprice);
}
}
23
244.4
Output

public static void main(String[] args) { Bad Design
float x=30.55f;
byte y=8;
float z=x*y;
System.out.println(z);
//system.out.println(liters*gasolineprice);
}
}
24
244.4
Output

How YOU can start writing clean code?
1.Use Descriptive Names
￮ 1. What are variables, classes, and functions? There are many ways
to answer that, but when you really think about it, those things are
nothing more than the interface between a programmer and the
underlying logic of an application.
￮ So when you use unclear and non-descript names for variables,
classes, and functions, you’re essentially obfuscating the
application logic from any programmer who reads the code,
including yourself.
￮ “I’m not a great programmer; I’m just a good programmer with
great habits.”
— Kent Beck
￮ What does a variable named x actually mean? Who knows. You’d
probably have to read the entire chunk of code to reverse
engineer its meaning. On the other hand, the meaning of a
variable like gasolineLiters is instantly recognizable. 26

float gasLiters=30.55f;
byte gasLiterPrice=8;
float totalCost=gasLiters*gasLiterPrice;
System.out.println(totalCost);
}
}
27
244.4
Output

2.Write Good Comments
￮ Write good comments” is the oldest piece of advice in the world of
programming. In fact, as soon as newbies are introduced to
comments, they’re pretty much encouraged to comment as often
as they can.
￮ But it almost feels like we’ve swung too far in the opposite
direction. Newbies, in particular, tend to over-comment –
describing things that don’t need to be described and missing the
point of what a “good comment” actually is.
￮ “Always code as if the guy who ends up maintaining your code will
be a violent psychopath who knows where you live.”
— John Woods
￮ Here’s a good rule of thumb: comments exist to explain WHY a
piece of code exists rather than WHAT the code actually does. If
the code is written cleanly enough, it should be self-explanatory as
to what it does — the comment should shed light on the intention
behind why it was written. 28

float gasLiters=30.55f;
// For Gasoline 95
byte gasLiterPrice=8;
//the total cost of gasLiter which are the multiply of gasLiters * gasLiterPrice
System.out.println(totalCost);
}
}
30
244.4
Output

Prompting the user
for input in java
Enter your name:
31

Prompting the user
for input in java
￮ To use a different gasLiters, you have to
modify the source code and recompile it. Obviously, this is not
convenient, so instead you can use the Scanner class for console
input.
￮ Scanner reader= new Scanner(System.in);
The syntax new Scanner(System.in) creates an object of the Scanner
type. The syntax Scanner input declares that reader is a variable whose
type is Scanner.
￮ So Scanner is class is something similar to the dog class that we
create last lecture to be as a template for us and to take an objects
from it to create multiple kinds of dogs with just one class template,
32

Football Video game use Object
oriented programing for create
players
￮ what happening in Football games indeed they create one
class player and take objects for from it for all players
instead to create class for every player.
33

Prompting the user
for input in java
￮ To invoke a method on an object is to ask the object to perform a
task. You can invoke the nextFloat() method to read a double value
as follows:
float gasLiters=reader.nextFloat();
￮ This statement reads a number from the keyboard and assigns the
number to gasLiters variables.
import java.util.Scanner; // Scanner is in the java.util package
￮ But don’t forget to import the class.
34

Reading Numbers from the
Keyboard
￮ You know how to use the nextDouble() method in the Scanner class
to read a double value from the keyboard. You can also use the
methods listed in Table 2.2 to read a number of the byte, short, int,
long, and float type
35

Prompting the user
for input in java
import java.util.Scanner;
Scanner reader = new Scanner(System.in); // Reading from System.in
int gasLiters= reader.nextFloat(); // Scans the next token of the input as an int.
36

Scanner reader = new Scanner(System.in); // Reading from System.in
// Prompt the user to enter the liters number
System.out.print("Enter the number of liters: ");
float gasLiters=reader.nextFloat();
// Prompt the user to enter a gas price
System.out.print("Enter the gas price : ");
byte gasLiterPrice=reader.nextByte();
System.out.println("the total cost is: "+totalCost);
}
} 37

if Statements
￮ An if-else statement decides the execution path based on
whether the condition is true or false. A one-way if statement
performs an action if the specified condition is true. If the condition
is false, nothing is done. But what if you want to take alternative
actions when the condition is false? You can use a two-way if-else
statement. The actions that a two-way if-else statement specifies
differ based on whether the condition is true or false. Here is the
syntax for a two-way if-else statement:
￮ if (boolean-expression)
￮ {
￮ statement(s)-for-the-true-case; }
￮ else
￮ {
￮ statement(s)-for-the-false-case;
￮ }
38

SimpleIfDemo.java
The program can decide which statements to execute based on a
condition. If you enter a negative value for radius in Listing 2.2,
ComputeAreaWithConsoleInput.java, the program displays an invalid
result. If the radius is negative, you don’t want the program to compute
the area. How can you deal with this situation? Like all high-level
programming languages, Java provides selection statements: statements
that let you choose actions with alternative courses. You can use
selection statement.
if (radius < 0) {
System.out.println("Incorrect input"); }
else {
area = radius * radius * 3.14159;
System.out.println("Area is " + area);
}
39

boolean Data Type
￮ The boolean data type declares a variable with the value either
true or false. How do you compare two values, such as whether a
radius is greater than 0, equal to 0, or less than 0? Java provides six
relational operators (also known as comparison operators), shown
in Table 3.1, which can be used to compare two values (assume
radius is 5 in the table).
40

SimpleIfDemo.java 1
public class SimpleIfDemo
{
Scanner input = new Scanner(System.in);
System.out.println("Enter your grade: ");
byte grade = input.nextByte();
if (grade > =50) System.out.println(“Passed");
else if (grade <50) System.out.println(“failed");
}
}
41

SimpleIfDemo.java 2
public class SimpleIfDemo
{
System.out.println("Enter an integer: ");
int number = input.nextInt();
if (number % 2 == 0) System.out.println(“Even");
else System.out.println(“odd");
}
}
42

SimpleIfDemo.java 4
if (score >= 90.0) System.out.print("A");
else if (score >= 80.0) System.out.print("B");
else if (score >= 70.0) System.out.print("C");
else if (score >= 60.0) System.out.print("D");
else System.out.print("F");
43

Build a program check the Pump
number using if statement before
calculating the total price
44

public class Gasstation {
Scanner input =new Scanner(System.in);
System.out.print("Enter the pumpnumber:");
byte pumpnumber=input.nextByte();
if(pumpnumber==80)
{
System.out.print("Enter the number of gasliters:");
byte pumpprice=5;
float gasliters=input.nextFloat();
float totalcost=gasliters*pumpprice;
System.out.println("The total cost is :"+totalcost);
}
45

else if(pumpnumber==92)
{
byte pumpprice=7;
}
{
byte pumpprice=8;
}
}
}
46

3. Delete Unnecessary Code
We ascribe beauty to that which is simple; which has no superfluous
parts; which exactly answers its end...
— Ralph Waldo Emerson
Less is more. It’s a trite maxim, but sometimes it really is true.
Some of the most exciting improvements I remember making to code
involved removing vast chunks of it. Let me tell you, it’s a good feeling.
47

Logical Operators
￮ The logical operators !, &&, ||, and ^ can be used to create a compound
Boolean expression. Sometimes, whether a statement is executed is
determined by a combination of several conditions. You can use logical
operators to combine these conditions to form a compound Boolean
expression. Logical operators, also known as Boolean operators, operate
on Boolean values to create a new Boolean value.
￮ the not (!) operator, which negates true to false and false to true. Defines
￮ (&&) operator. The and (&&) of two Boolean operands is true if and only if
both operands are true.
￮ (||) operator. The or (||) of two Boolean operands is true if at least one of the
operands is true.
48

Naming Conventions
￮ Sticking with the Java naming conventions makes your programs easy to read and avoids
errors.
￮ Make sure that you choose descriptive names with straightforward
meanings for the variables, constants, classes, and methods in your
program. As mentioned earlier, names are case sensitive. Listed below
are the conventions for naming variables, methods, and classes.
￮ ■ Use lowercase for variables and methods. If a name consists of
several words, concatenate them into one, making the first word
lowercase and capitalizing the first letter of each subsequent word—
for example, the variables radius and area and the method print.
￮ ■ Capitalize the first letter of each word in a class name—for example,
the class names ComputeArea and System.
￮ ■ Capitalize every letter in a constant, and use underscores between
words—for example, the constants PI and MAX_VALUE. It is important to
follow the naming conventions to make your programs easy to read.
49

Named Constants
￮ A named constant is an identifier that represents a permanent value.
￮ The value of a variable may change during the execution of a program, but a
named constant, or simply constant, represents permanent data that never
changes. In our ComputeArea program, p is a constant. If you use it frequently,
you don’t want to keep typing 3.14159; instead, you can declare a constant for p.
Here is the syntax for declaring a constant:
￮ final datatype CONSTANTNAME = value;
￮ A constant must be declared and initialized in the same statement. The word
final is a Java keyword for declaring a constant. For example, you can declare p
as a constan
50

ComputeAreaWithConstant.java
// Scanner is in the java.util package
public class ComputeAreaWithConstant
{ public static void main(String[] args)
{
final double PI = 3.14159;
// Declare a constant // Create a Scanner object
// Prompt the user to enter a radius
System.out.print("Enter a number for radius: ");
double radius = input.nextDouble();
// Compute area
double area = radius * radius * PI;
// Display result
System.out.println("The area for the circle of radius " + radius + " is " +
area); } } 51

public class Gasstation {
final byte pumpprice80=5;
Scanner input =new Scanner(System.in);
if(pumpnumber==80)
{
float totalcost=gasliters*pumpprice80;
}
52

{
}
{
}
}
} 53

if (pumpnumber !=80 && pumpnumber !=92 &&
pumpnumber !=95)
{
main(args);
}
else
{
Continue the previous code
}
54

switch Statements
￮ A switch statement executes statements based on the value of a
variable or an expression. The if statement in Listing 3.5,
ComputeTax.java, makes selections based on a single true or false
condition. There are four cases for computing taxes, which depend on
the value of status. To fully account for all the cases, nested if
statements were used. Overuse of nested if statements makes a
program difficult to read. Java provides a switch statement to simplify
coding for multiple conditions. You can write the following switch
statement to replace the nested if statement in Listing 3.5:
55

Simple java Program
Import java.util.Scanner;
System.out.print("Enter the number: ");
int day=input.nextInt();
switch (day){
case 1:System.out.println("Saturday");
break;
case 2:System.out.println("Sunday");
break;
case 3:System.out.println("Monday");
break;
case 4:System.out.println("Tuesday");
break;
case 5:System.out.println("Wensday");
break;
case 6:System.out.println("Thursday");
break;
case 7:System.out.println("Friday");
break; }}
56

references
￮ Beginning Programming with Java For Dummies, 4th Edition.
￮ Data Structures and Problem Solving Using Java4edWeiss.
￮ fundamentals-of-computer-science-using-java.
￮ Head First Java.
￮ Java The Complete Reference, 7th Edition.
￮ CS50 introduction to Computer Science.
57

Intro to programing with java-lecture 3

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