Java Foundations: Basic Syntax, Conditions, Loops

Java Foundations
Basic Syntax, I/O,
Conditions, Loops
and Debugging

Your Course
Instructors
Svetlin Nakov
George Georgiev

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Basic Syntax, I/O, Conditions,
Loops and Debugging
Java Introduction

Table of Contents
1. Java Introduction and Basic Syntax
2. Comparison Operators in Java
3. The if-else / switch-case Statements
4. Logical Operators: &&, ||, !, ()
5. Loops: for, while, do-while
6. Debugging and Troubleshooting
7

Java: Introduction
and Basic Syntax

Java – Introduction
 Java is modern, flexible, general-purpose
programming language
 Object-oriented by nature, statically-typed, compiled
 In this course will use Java Development Kit (JDK) 12
 Later versions will work as well
9
static void main(String[] args) {
// Source Code
}
Program
entry
point

 IntelliJ IDEA is powerful IDE for Java and other languages
 Create a project
Using IntelliJ Idea
10

Declaring Variables
 Defining and Initializing variables
 Example:
11
{data type / var} {variable name} = {value};
int number = 5;
Data type
Variable name
Variable value

Console I/O
Reading from and Writing to the Console

Reading from the Console
 We can read/write to the console,
using the Scanner class
 Import the java.util.Scanner class
 Reading input from the console using
13
import java.util.Scanner;
…
Scanner sc = new Scanner(System.in);
String name = sc.nextLine(); Returns String

Converting Input from the Console
 scanner.nextLine() returns a String
 Convert the string to number by parsing:
14
…
String name = sc.nextLine();
int age = Integer.parseInt(sc.nextLine());
double salary = Double.parseDouble(sc.nextLine());

 We can print to the console, using the System class
 Writing output to the console:
 System.out.print()
 System.out.println()
Printing to the Console
15
System.out.print("Name: ");
String name = scanner.nextLine();
System.out.println("Hi, " + name);
// Name: George
// Hi, George

 Using format to print at the console
 Examples:
16
Using Print Format
Placeholder %s stands for string
and corresponds to name
Placeholder %d stands
for integer number and
corresponds to age
String name = "George";
int age = 5;
System.out.printf("Name: %s, Age: %d", name, age);
// Name: George, Age: 5

 D – format number to certain digits with leading zeros
 F – format floating point number with certain digits after the
decimal point
 Examples:
int percentage = 55;
double grade = 5.5334;
System.out.printf("%03d", percentage); // 055
System.out.printf("%.2f", grade); // 5.53
Formatting Numbers in Placeholders
17

 Using String.format to create a string by pattern
 Examples:
String name = "George";
int age = 5;
String result = String.format(
"Name: %s, Age: %d", name, age);
System.out.println(result);
// Name: George, Age 5
Using String.format(…)
18

 You will be given 3 input lines:
 Student name, age and average grade
 Print the input in the following format:
 "Name: {name}, Age: {age}, Grade {grade}"
 Format the grade to 2 decimal places
Problem: Student Information
19
John
15
5.40
Name: John, Age: 15, Grade: 5.40

…
String name = sc.nextLine();
double grade = Double.parseDouble(sc.nextLine());
System.out.printf("Name: %s, Age: %d, Grade: %.2f",
name, age, grade);
Solution: Student Information
20

Comparison Operators
==, !=, <, >, <=, >=
==

Comparison Operators
22
Operator Notation in Java
Equals ==
Not Equals !=
Greater Than >
Greater Than or Equals >=
Less Than <
Less Than or Equals <=

 Values can be compared:
Comparing Numbers
2
3
int a = 5;
int b = 10;
System.out.println(a < b);
System.out.println(a > 0);
System.out.println(a > 100);
System.out.println(a < a);
System.out.println(a <= 5);
System.out.println(b == 2 * a);
// true
// true
// false
// false
// true
// true

The if-else Statement
Implementing Control-Flow Logic

 The simplest conditional statement
 Test for a condition
 Example: Take as an input a grade and check if the student
has passed the exam (grade >= 3.00)
The if Statement
25
double grade = Double.parseDouble(sc.nextLine());
if (grade >= 3.00) {
System.out.println("Passed!");
}
In Java, the opening
bracket stays on the
same line

 Executes one branch if the condition is true and another,
if it is false
 Example: Upgrade the last example, so it prints "Failed!",
if the mark is lower than 3.00:
The if-else Statement
26
The else
keyword stays
on the same
line, after the }
if (grade >= 3.00) {
System.out.println("Passed!");
} else {
// TODO: Print the message
}

 Write a program that reads hours and minutes from the console
and calculates the time after 30 minutes
 The hours and the minutes come on separate lines
 Example:
Problem: I Will be Back in 30 Minutes
27
23
59 0:29
1
46 2:16
0
01 0:31
11
32 12:02
11
08 11:38
12
49 13:19

Solution: I Will be Back in 30 Minutes (1)
28
int hours = Integer.parseInt(sc.nextLine());
int minutes = Integer.parseInt(sc.nextLine()) + 30;
if (minutes > 59) {
hours += 1;
minutes -= 60;
}
// Continue on the next slide

Solution: I Will be Back in 30 Minutes (2)
29
if (hours > 23) {
hours = 0;
}
if (minutes < 10) {
System.out.printf("%d:%02d%n", hours, minutes);
} else {
System.out.printf("%d:%d", hours, minutes);
}
%n goes on
the next line

The Switch-Case Statement
Simplified if-else-if-else

 Works as sequence of if-else statements
 Example: read input a number and print its corresponding month:
The switch-case Statement
31
int month = Integer.parseInt(sc.nextLine());
switch (month) {
case 1: System.out.println("January"); break;
case 2: System.out.println("February"); break;
// TODO: Add the other cases
default: System.out.println("Error!"); break;
}

 By given country print its typical language:
 English -> England, USA
 Spanish -> Spain, Argentina, Mexico
 other -> unknown
Problem: Foreign Languages
32
England English
Spain Spanish

Solution: Foreign Languages
33
// TODO: Read the input
switch (country) {
case "USA":
case "England": System.out.println("English"); break;
case "Spain":
case "Argentina":
case "Mexico": System.out.println("Spanish"); break;
default: System.out.println("unknown"); break;
}

Logical Operators
Writing More Complex Conditions
&&

 Logical operators give us the ability to write multiple
conditions in one if statement
 They return a boolean value and compare boolean values
Logical Operators
35
Operator Notation in Java Example
Logical NOT ! !false -> true
Logical AND && true && false -> false
Logical OR || true || false -> true

 A theatre has the following ticket prices according to the age of
the visitor and the type of day. If the age is < 0 or > 122,
print "Error!":
Problem: Theatre Promotions
36
Weekday
42
18$ Error!
Day / Age 0 <= age <= 18 18 < age <= 64 64 < age <= 122
Weekday 12$ 18$ 12$
Weekend 15$ 20$ 15$
Holiday 5$ 12$ 10$
Holiday
-12

Solution: Theatre Promotions (1)
37
String day = sc.nextLine().toLowerCase();
int price = 0;
if (day.equals("weekday")) {
if ((age >= 0 && age <= 18) || (age > 64 && age <= 122)) {
price = 12;
}
// TODO: Add else statement for the other group
}
// Continue…

38
else if (day.equals("weekend")) {
if ((age >= 0 && age <= 18) || (age > 64 && age <= 122)) {
price = 15;
} else if (age > 18 && age <= 64) {
price = 20;
}
} // Continue…

39
else if (day.equals("holiday")){
if (age >= 0 && age <= 18)
price = 5;
// TODO: Add the statements for the other cases
}
if (age < 0 || age > 122)
System.out.println("Error!");
else
System.out.println(price + "$");

 A loop is a control statement that repeats
the execution of a block of statements.
 Types of loops:
 for loop
 Execute a code block a fixed number of times
 while and do…while
 Execute a code block while given condition is true
Loop: Definition
41

For-Loops
Managing the Count of the Iteration

 The for loop executes statements a fixed number of times:
For-Loops
43
Initial value
Loop body
for (int i = 1; i <= 10; i++) {
System.out.println("i = " + i);
}
Increment
Executed at
each iteration
The bracket is
again on the
same line
End value

 Print the numbers from 1 to 100, that are divisible by 3
 You can use "fori" live template in Intellij
Example: Divisible by 3
44
for (int i = 3; i <= 100; i += 3) {
System.out.println(i);
}
Push [Tab] twice

 Write a program to print the first n odd numbers and their sum
Problem: Sum of Odd Numbers
45
5
1
3
5
7
9
Sum: 25
3
1
3
5
Sum: 9

Solution: Sum of Odd Numbers
46
int n = Integer.parseInt(sc.nextLine());
int sum = 0;
for (int i = 1; i <= n; i++) {
System.out.println(2 * i - 1);
sum += 2 * i - 1;
}
System.out.printf("Sum: %d", sum);

While Loops
Iterations While a Condition is True

 Executes commands while the condition is true:
int n = 1;
while (n <= 10) {
System.out.println(n);
n++;
}
While Loops
48
Loop body
Condition
Initial value
Increment the counter

 Print a table holding number*1, number*2, …, number*10
Problem: Multiplication Table
49
int number = Integer.parseInt(sc.nextLine());
int times = 1;
while (times <= 10) {
System.out.printf("%d X %d = %d%n",
number, times, number * times);
times++;
}

Do…While Loop
Execute a Piece of Code One or More Times

 Like the while loop, but always executes at least once:
int i = 1;
do {
System.out.println(i);
i++;
} while (i <= 10);
Do ... While Loop
51
Loop body
Condition
Initial value
Increment
the counter

 Upgrade your program and take the initial times from the console
Problem: Multiplication Table 2.0
52
int number = Integer.parseInt(sc.nextLine());
int times = Integer.parseInt(sc.nextLine());
do {
System.out.printf("%d X %d = %d%n",
number, times, number * times);
times++;
} while (times <= 10);

Debugging the Code
Using the InteliJ Debugger

 The process of debugging application includes:
 Spotting an error
 Finding the lines of code that cause the error
 Fixing the error in the code
 Testing to check if the error is gone
and no new errors are introduced
 Iterative and continuous process
Debugging the Code
5
4

 Intellij has a
built-in debugger
 It provides:
 Breakpoints
 Ability to trace the
code execution
 Ability to inspect
variables at runtime
Debugging in IntelliJ IDEA
5
5

 Start without Debugger: [Ctrl+Shift+F10]
 Toggle a breakpoint: [Ctrl+F8]
 Start with the Debugger:
[Alt+Shift+F9]
 Trace the program: [F8]
 Conditional breakpoints
Using the Debugger in IntelliJ IDEA
56

 A program aims to print the first n odd numbers and their sum
Problem: Find and Fix the Bugs in the Code
57
int n = Integer.parseInt(sc.nextLine());
int sum = 1;
for (int i = 0; i <= n; i++) {
System.out.print(2 * i + 1);
sum += 2 * i;
}
System.out.printf("Sum: %d%n", sum);
10

 …
 …
 …
Summary
58
 Declaring variables
 Reading from / printing to the console
 Conditional statements allow implementing
programming logic
 Loops repeat code block multiple times
 Using the debugger

 …
 …
 …
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