05. Java Loops Methods and Classes

Loops, Methods, Classes
Using Loops, Defining and Using
Methods, Using API Classes,
Exceptions, Defining Classes
Svetlin Nakov
Technical Trainer
www.nakov.com
Software University
http://softuni.bg

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1. Loops
 while, do-while, for, for-each
2. Methods
 Defining Methods
 Invoking Methods
3. Using the Java API Classes
4. Exception Handling Basics
5. Defining Simple Classes
Table of Contents

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 The "Java Basics" course is NOT for absolute beginners
 Take the "C# Basics" course at SoftUni first:
https://softuni.bg/courses/csharp-basics
 The course is for beginners, but with previous coding skills
 Requirements
 Coding skills – entry level
 Computer English – entry level
 Logical thinking
Warning: Not for Absolute Beginners

Loop: Definition
 A loop is a control statement that repeats the execution of a
block of statements
 May execute a code block fixed number of times
 May execute a code block while given condition holds
 May execute a code block for each member of a collection
 Loops that never end are called an infinite loops
while (condition) {
statements;
}
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While Loop
 The simplest and most frequently used loop
 The repeat condition
 Returns a boolean result of true or false
 Also called loop condition
while (condition) {
statements;
}
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While Loop – Example: Numbers 0…9
int counter = 0;
while (counter < 10) {
System.out.printf("Number : %dn", counter);
counter++;
}
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Using the break Operator
 The break operator exits the inner-most loop
public static void main(String[] args) {
int n = new Scanner(System.in).nextInt();
// Calculate n! = 1 * 2 * ... * n
int result = 1;
while (true) {
if (n == 1)
break;
result *= n;
n--;
}
System.out.println("n! = " + result);
}
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Do-While Loop
 Another classical loop structure is:
 The block of statements is repeated
 While the boolean loop condition holds
 The loop is executed at least once
do {
statements;
}
while (condition);
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Product of Numbers [N..M] – Example
 Calculating the product of all numbers in the interval [n..m]:
Scanner input = new Scanner(System.in);
int n = input.nextInt();
int m = input.nextInt();
int number = n;
BigInteger product = BigInteger.ONE;
do {
BigInteger numberBig = new BigInteger("" + number);
product = product.multiply(numberBig);
number++;;
}
while (number <= m);
System.out.printf("product[%d..%d] = %dn", n, m, product);
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 The classical for-loop syntax is:
 Consists of
 Initialization statement
 Boolean test expression
 Update statement
 Loop body block
For Loops
for (initialization; test; update) {
statements;
}
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 A simple for-loop to print the numbers 0…9:
For Loop – Examples
for (int number = 0; number < 10; number++) {
System.out.print(number + " ");
}
 A simple for-loop to calculate n!:
long factorial = 1;
for (int i = 1; i <= n; i++) {
factorial *= i;
}
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 continue bypasses the iteration of the inner-most loop
 Example: sum all odd numbers in [1…n], not divisors of 7:
Using the continue Operator
int n = 100;
int sum = 0;
for (int i = 1; i <= n; i += 2) {
if (i % 7 == 0) {
continue;
}
sum += i;
}
System.out.println("sum = " + sum);
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 The typical for-each loop syntax is:
 Iterates over all the elements of a collection
 The element is the loop variable that takes sequentially all
collection values
 The collection can be list, array or other group of elements of
the same type
For-Each Loop
for (Type element : collection) {
statements;
}
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 Example of for-each loop:
 The loop iterates over the array of day names
 The variable day takes all its values
 Applicable for all collection: arrays, lists, strings, etc.
For-Each Loop – Example
String[] days = { "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "Sunday" };
for (String day : days) {
System.out.println(day);
}
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 Loops can be nested (one inside another)
 Example: print all combinations from TOTO 6/49 lottery
Nested Loops
for (int i1 = 1; i1 <= 44; i1++)
for (int i2 = i1 + 1; i2 <= 45; i2++)
for (int i3 = i2 + 1; i3 <= 46; i3++)
for (int i4 = i3 + 1; i4 <= 47; i4++)
for (int i5 = i4 + 1; i5 <= 48; i5++)
for (int i6 = i5 + 1; i6 <= 49; i6++)
System.out.printf("%d %d %d %d %d %dn",
i1, i2, i3, i4, i5, i6);

Methods
Defining and Invoking Methods

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 Methods are named
pieces of code
 Defined in the class
body
 Can be invoked
multiple times
 Can take parameters
 Can return a value
Methods: Defining and Invoking
private static void printAsterix(int count) {
for (int i = 0; i < count; i++) {
System.out.print("*");
}
System.out.println();
}
int n = 5;
for (int i = 1; i <= n; i++) {
printAsterix(i);
}
}

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Methods with Parameters and Return Value
static double calcTriangleArea(double width, double height) {
return width * height / 2;
}
Scanner input = new Scanner(System.in);
System.out.print("Enter triangle width: ");
double width = input.nextDouble();
System.out.print("Enter triangle height: ");
double height = input.nextDouble();
System.out.println("Area = " + calcTriangleArea(width, height));
}
Method names in Java
should be in camelCase

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 Recursion == method can calls itself
Recursion
int n = 5;
long factorial = calcFactorial(n);
System.out.printf("%d! = %d", n, factorial);
}
private static long calcFactorial(int n) {
if (n <= 1) {
return 1;
}
return n * calcFactorial(n-1);
}

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 Java SE provides thousands of ready-to-use classes
 Located in packages like java.lang, java.math, java.net,
java.io, java.util, java.util.zip, etc.
 Using static Java classes:
 Using non-static Java classes
Build-in Classes in the Java API
LocalDate today = LocalDate.now();
double cosine = Math.cos(Math.PI);
Random rnd = new Random();
int randomNumber = 1 + rnd.nextInt(100);

Using the Java API Classes
Live Demo

Exception Handling Basics
Catch and Throw Exceptions

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 In Java exceptions are handled by the try-catch-finally
construction
 catch blocks can be used multiple times to process different
exception types
Handling Exceptions
try {
// Do some work that can raise an exception
} catch (SomeException ex) {
// Handle the caught exception
} finally {
// This code will always execute
}

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Handling Exceptions – Example
String str = new Scanner(System.in).nextLine();
try {
int i = Integer.parseInt(str);
System.out.printf(
"You entered a valid integer number %d.n", i);
}
catch (NumberFormatException nfex) {
System.out.println("Invalid integer number: " + nfex);
}
}

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 A method in Java could declare "throws SomeException"
 This says "I don't care about SomeException", please re-throw it
The "throws …" Declaration
public static void copyStream(InputStream inputStream,
OutputStream outputStream) throws IOException {
byte[] buf = new byte[4096]; // 4 KB buffer size
while (true) {
int bytesRead = inputStream.read(buf);
if (bytesRead == -1)
break;
outputStream.write(buf, 0, bytesRead);
}
}

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 When we use a resource that is expected to be closed, we use the
try-with-resources statement
Resource Management in Java
try(
BufferedReader fileReader = new BufferedReader(
new FileReader("somefile.txt"));
) {
while (true) {
String line = fileReader.readLine();
if (line == null) break;
System.out.println(line);
} catch (IOException ioex) {
System.err.println("Cannot read the file ".);
}

Defining Simple Classes
Using Classes to Hold a Set of Fields

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 Classes in Java combine a set of named fields / properties
 Defining a class Point holding X and Y coordinates:
Defining Classes in Java
class Point {
private int x, y;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
public int getX() {
return x;
}
public void setX(int x) {
this.x = x;
}
public int getY() { … }
public void setY(int y) { … }
}

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 Eclipse provides tools
for automatically
generate constructors
and getters / setters
for the class fields
Defining Classes in Eclipse

Defining Simple Classes
Live Demo

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 Java supports the classical loop constructs
 while, do-while, for, for-each
 Similarly to C#, JavaScript, PHP, C, C++, …
 Java support methods
 Methods are named code blocks
 Can take parameters and return a result
 Java supports classical exception handling
 Through the try-catch-finally construct
 Developers can define their own classes
 With fields, methods, constructors, getters, setters, etc.
Summary

?
https://softuni.bg/courses/java-basics/
Loops, Methods, Classes

37
 This course (slides, examples, demos, videos, homework, etc.)
is licensed under the "Creative Commons Attribution-
NonCommercial-ShareAlike 4.0 International" license
 Attribution: this work may contain portions from
 "Fundamentals of Computer Programming with Java" book by Svetlin Nakov & Co. under CC-BY-SA license
 "C# Basics" course by Software University under CC-BY-NC-SA license
License

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