Java Brush-Up

Part 1

Printing to the screen

Every Java program starts inside the main method. To show something on the screen, use System.out.println(...).

Your very first program

public class App {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

println prints and then moves to a new line. print (without "ln") stays on the same line.

println vs print

System.out.println("I love Java");  // goes to a new line after
System.out.print("1 ");
System.out.print("2 ");
System.out.print("3 ");
// output: 1 2 3   (all on one line)

You can join text and numbers with +. Be careful: parentheses change the result!

Joining text and numbers

System.out.println("Total value: " + (5 + 10));  // Total value: 15
System.out.println("Total value: " + 5 + 10);    // Total value: 510  (!)
System.out.println("Result: " + 9 / 3);          // Result: 3  (division runs first)
System.out.println(50 - 15);                     // 35  (no quotes = just math)

Remember: text inside "double quotes" is printed exactly as written. Math without quotes is calculated first. When text comes first, Java glues values to it one by one, from left to right.

Part 2

Variables & data types

A variable is a named box that holds a value. You always write: type, name, equals, value.

All the types you used

String learning = "I am learning Java";  // text
int t = 50;                              // whole number
float pi = 3.14f;                        // decimal — needs the letter f
double price = 19.99;                    // decimal — no f needed
boolean isJavaFun = true;                // true or false
char firstLetter = 'J';                  // ONE character, single quotes
long population = 1000000;               // very big whole number

System.out.println(learning);
System.out.println("price = " + price);

You can put the result of math into a variable, then print it with a label.

Store a result, then print it

int x = 5;
int y = 10;
int result = x + y;
System.out.println("Sum result: " + result);   // Sum result: 15

Naming rules

What is allowed

// OK:      myCat   my_name   $money   isAdult   priceOfLipstick
// NOT OK:  2cats   (cannot start with a digit)
// NOT OK:  my-name (dash is not allowed — only _ and $ are)
// Style:   camelCase for names, booleans start with is/has  (isAdult, hasCar)

Remember: float needs an f at the end (3.14f), char uses single quotes 'J', and String uses double quotes "Java".

Part 3

Math operators, % and casting

Java has five math operators: + - * / and %. The last one, modulus, gives the remainder after division — you used it a lot!

The basics

int number = 8;
int square = number * number;           // 64
int cube   = number * number * number;  // 512

int remainder = 23 % 5;                 // 3  (23 / 5 = 4, remainder 3)
System.out.println("23 mod 5 = " + remainder);

Order matters. Multiplication and division run before plus and minus. Parentheses always win.

Precedence

int m = 5 + 20 * 2;    // 45  (20*2 first)
int n = (5 + 20) * 2;  // 50  (parentheses first)

int half = 7 / 2;      // 3, NOT 3.5 — int division throws away the decimals!

Four ways to add 1

counter = counter + 1;
counter += 1;
counter++;
++counter;   // all four do the same thing here

Casting changes one type into another. Going from decimal to int loses the decimal part.

Casting

float price = 12.99f;
int intPrice = (int) price;        // 12 — decimals are cut off, not rounded

double average = (double) 7 / 2;   // 3.5 — cast first, then divide

A classic trick: digits of a number

int n = 47;
int tens = n / 10;   // 4
int ones = n % 10;   // 7

Remember: x % 2 == 0 means x is even. a % b == 0 means a is exactly divisible by b. int divided by int is always an int.

Part 4

User input with Scanner

Scanner lets your program ask the user for a value. First import it, then create it, then read.

Reading a number

import java.util.Scanner;

Scanner kb = new Scanner(System.in);
System.out.print("Please enter a number: ");
int number = kb.nextInt();
System.out.println("You entered: " + number);

Reading text and decimals

String name = kb.nextLine();     // reads a whole line of text
double radius = kb.nextDouble(); // reads a decimal number

double area = 3.14159 * radius * radius;
System.out.println("Circle area: " + area);

Remember: nextInt() for whole numbers, nextDouble() for decimals, nextLine() for text. Always print a friendly message before reading, so the user knows what to type.

Part 5

If / else / else-if

An if lets your program make decisions. You compare values with > < >= <= == and !=.

if alone, and if/else

int shoePrice = 20;
if (shoePrice < 25) {
    System.out.println("I can buy these shoes for sport");
}

int mealCalories = 700;
if (mealCalories > 600) {
    System.out.println("This meal is very high-calorie!");
} else {
    System.out.println("This meal is healthy");
}

For more than two roads, chain conditions with else if. Java checks them from top to bottom and runs only the first one that matches.

else-if ladder

int hourOfDay = 14;
if (hourOfDay < 6) {
    System.out.println("Night time");
} else if (hourOfDay < 18) {
    System.out.println("Daytime");
} else {
    System.out.println("Evening time");
}

Chars are compared with == too. And a very useful pattern from your exercises: set a variable inside the branches, print once at the end.

char comparison + assign-in-branch pattern

char grade = 'A';
if (grade == 'A') {
    System.out.println("Excellent");
} else if (grade == 'B') {
    System.out.println("Good");
} else {
    System.out.println("Invalid grade!");
}

int age = 10;
int ticketPrice;
if (age < 12) {
    ticketPrice = 10;
} else {
    ticketPrice = 20;
}
System.out.println("Ticket price: " + ticketPrice);

Remember: one = assigns a value, two == compares. In an else-if ladder you don't need to repeat the lower bound — if the code reached else if (age < 12), age is already 4 or more.

Part 6

Logical operators: &&, ||, !

Sometimes one condition is not enough. && means AND (both must be true), || means OR (at least one is true), ! means NOT.

Combining conditions

int number = 30;
if (number % 3 == 0 && number % 5 == 0) {
    System.out.println("divisible by both 3 and 5");
}

if (number % 2 == 0 && number % 4 != 0) {
    System.out.println("divisible by 2 but not by 4");   // true for 6, 14, 30...
}

A comparison is a boolean value. You can store it in a variable with a nice name.

Named booleans

int x = 8;
boolean isEven = (x % 2) == 0;        // "is it even?" → true

int age = 70;
boolean isEligibleForRetirement = age > 65;
System.out.println(isEligibleForRetirement);   // true

Two variables in one condition

int temperature = 33;
int humidity = 75;
if (temperature > 30 && humidity > 70) {
    System.out.println("The weather is stifling");
} else if (temperature < 15 && humidity < 30) {
    System.out.println("The weather is dry and cool");
}

Remember: % + logical operators is a power combo: even = % 2 == 0, odd = % 2 != 0, "divisible by a but not b" = % a == 0 && % b != 0.

Part 7

While loops

A while loop repeats code as long as a condition is true. You need three things: a counter, a condition, and a step that changes the counter — or the loop never ends!

The classic shape

int i = 0;
while (i < 5) {
    System.out.println("*");
    i++;                      // never forget this line!
}

Doubling: 1, 2, 4, 8, 16

int number = 1;
int i = 1;
while (i <= 5) {
    System.out.println(number);
    number = number * 2;
    i++;
}

while(true) + break

int i = 0;
while (true) {                // runs forever...
    System.out.println("Hello");
    i++;
    if (i == 5) {
        break;                // ...until break stops it
    }
}

Remember: if you forget i++, the condition never becomes false and the loop runs forever. This was the bug in one of the repo files — the loop body only counted and never printed!

Part 8

For loops

A for loop puts the counter, the condition, and the step all on one line. Best choice when you know how many times to repeat.

The shape: start; condition; step

for (int i = 1; i <= 5; i++) {
    System.out.println(i);        // 1 2 3 4 5
}

for (int i = 20; i >= 2; i -= 2) {
    System.out.println(i);        // 20 18 16 ... 2
}

for (int i = 1; i <= 16; i = i * 2) {
    System.out.println(i);        // 1 2 4 8 16
}

The accumulator pattern: build up a total inside the loop.

Summing with an accumulator

int total = 0;
for (int i = 1; i <= 5; i++) {
    total += i;
}
System.out.println("Total = " + total);   // 15

FizzBuzz — the famous one

for (int i = 1; i <= 30; i++) {
    if (i % 3 == 0) {
        System.out.println("Fizz");
    } else if (i % 5 == 0) {
        System.out.println("Buzz");
    } else {
        System.out.println(i);
    }
}

Remember: the step doesn't have to be i++ — it can be i--, i -= 2, even i = i * 2. And an if inside a loop is how you filter numbers.

Part 9

Strings & string methods

A String has built-in methods — small tools you call with a dot. These are the ones you practiced:

The toolbox

String name = "Thailand";
name.length();          // 8  — how many characters
name.charAt(0);         // 'T' — character at a position (starts at 0!)
name.toUpperCase();     // "THAILAND"
name.toLowerCase();     // "thailand"
name.contains("ai");    // true
name.startsWith("T");   // true
name.endsWith("d");     // true
name.indexOf("a");      // 2  — position of the first 'a'
name.replace('a', '*'); // "Th*il*nd"
name.substring(0, 4);   // "Thai" — from 0 up to (not including) 4

Comparing text: use .equals(), never ==. For "ignore big/small letters" use .equalsIgnoreCase().

equals vs equalsIgnoreCase

String name1 = "HasoMis";
String name2 = "HAsOmiS";
name1.equals(name2);            // false — case matters
name1.equalsIgnoreCase(name2);  // true

Classic exercise: cut an email into pieces

String email = "denizukad@gmail.com";
int atIndex = email.indexOf("@");
String username = email.substring(0, atIndex);   // "denizukad"
String website  = email.substring(atIndex + 1);  // "gmail.com"

Walk through a string, one char at a time

String word = "Java";
for (int i = 0; i < word.length(); i++) {
    System.out.println(word.charAt(i));
}

Remember: positions start at 0. The last character is at length() - 1. In substring(start, end) the start is included, the end is not.

Part 10

Arrays

An array holds many values of the same type in one variable. Each value has a position (index) starting at 0.

Create, read, loop

String[] colours = {"Red", "Blue", "Green", "Yellow"};
System.out.println(colours[0]);        // Red — first element

for (int i = 0; i < colours.length; i++) {
    System.out.println(colours[i]);
}

Filtering with an if inside the loop

int[] numbers = {5, 12, 9, 21, 17, 6};
for (int i = 0; i < numbers.length; i++) {
    if (numbers[i] % 2 == 0) {
        System.out.println(numbers[i]);   // only 12 and 6
    }
}

Backwards, and adding up

int[] evens = {2, 4, 6, 8, 10, 12};
for (int i = evens.length - 1; i >= 0; i--) {
    System.out.println(evens[i]);         // 12 10 8 ...
}

int[] weights = {60, 63, 64, 59, 55, 52};
int total = 0;
for (int i = 0; i < weights.length; i++) {
    total += weights[i];
}

An empty array you fill yourself

int[] a = new int[5];          // 5 empty slots, all 0
for (int i = 0; i < a.length; i++) {
    a[i] = (i + 1) * 2;        // fills with 2, 4, 6, 8, 10
}

Remember: for arrays it is .length with no parentheses (Strings use .length() with parentheses). The last index is length - 1.

Part 11

ArrayList

An ArrayList is like an array that can grow. You add items one by one. For numbers and chars you use the "big" type names: Integer, Double, Character.

Create, add, read

import java.util.ArrayList;

ArrayList<String> cities = new ArrayList<>();
cities.add("Istanbul");
cities.add("Bangkok");
cities.add("Chiang Mai");

cities.size();      // 3   (not .length!)
cities.get(0);      // "Istanbul"
System.out.println(cities);   // [Istanbul, Bangkok, Chiang Mai]

Three ways to loop

// 1. index loop
for (int i = 0; i < cities.size(); i++) {
    System.out.println(cities.get(i));
}

// 2. for-each — the easy one
for (String city : cities) {
    System.out.println(city);
}

// 3. forEach with a lambda
cities.forEach(city -> {
    System.out.println(city);
});

Filtering, Thailand style

ArrayList<Integer> days = new ArrayList<>();
days.add(3); days.add(7); days.add(10); days.add(15); days.add(22);

for (int day : days) {
    if (day > 10) {
        System.out.println("day = " + day);   // 15 and 22
    }
}

Remember: array → .length, String → .length(), ArrayList → .size(). Three different spellings — a classic exam trap!

Part 12

Set & HashMap

A Set is a collection with no duplicates — if you add the same thing twice, it keeps only one.

Set removes duplicates by itself

import java.util.Set;
import java.util.HashSet;

Set<String> cityNames = new HashSet<>();
cityNames.add("Bangkok");
cityNames.add("Bangkok");     // ignored — already there
cityNames.add("Phuket");

for (String city : cityNames) {
    System.out.println(city);  // Bangkok appears only once
}

A HashMap stores pairs: a key and its value. You look values up by key.

put, get, and looping

import java.util.HashMap;

HashMap<String, String> names = new HashMap<>();
names.put("Deniz", "Turkey");
names.put("Somchai", "Thailand");

names.get("Deniz");            // "Turkey"

for (String key : names.keySet()) {
    String value = names.get(key);
    System.out.println(key + " lives in " + value);
}

// or with a lambda:
names.forEach((key, value) -> {
    System.out.println(key + " -> " + value);
});

Remember: List = keeps order, allows duplicates. Set = no duplicates. Map = key → value pairs. put to add, get to read, keySet() to loop.

Part 13

Methods (functions)

A method is a named block of code you can reuse. Instead of copying the same lines again and again, you write them once and call the name.

The template

public static returnType methodName(parameters) { ... }

// simplest: no parameters, returns nothing (void)
public static void printWelcome() {
    System.out.println("Welcome to Java programming!");
}

// with parameters
public static void printDifference(int a, int b) {
    System.out.println("Difference: " + (a - b));
}

public static void main(String[] args) {
    printWelcome();
    printDifference(10, 4);   // Difference: 6
}

A method can also return a value. Then void becomes the value's type, and the method ends with return.

Returning values

public static int multiply(int a, int b) {
    return a * b;
}

public static String checkEvenOdd(int number) {
    if (number % 2 == 0) {
        return "Even";
    } else {
        return "Odd";
    }
}

int result = multiply(4, 3);            // 12
System.out.println(checkEvenOdd(9));    // Odd

Building a result with a loop

public static String repeatLaugh(int times) {
    String result = "";
    for (int i = 0; i < times; i++) {
        result += "ha";
    }
    return result;
}
// repeatLaugh(3) → "hahaha"

The best trick from the Catan exercises: one method's return value becomes another method's parameter.

Chaining methods (Catan style)

public static int calculateKnightPoints(int knightCards) {
    return knightCards;      // each knight = 1 point
}
public static void printKnightPoints(int points) {
    System.out.println("Knight points: " + points);
}

printKnightPoints(calculateKnightPoints(5));   // Knight points: 5

Remember: void = prints or does something, returns nothing. A return type (int, String...) = gives a value back that you can store or pass on. After return, the method stops.

Part 14

Classes & objects

A class is a blueprint. It says what data (fields) something has. From one class you can create many objects with new.

A class and its objects

public class Phone {
    String brand;
    String model;
    int storage;
}

// in main:                 ClassName variableName = new ClassName()
Phone phone1 = new Phone();
phone1.brand = "iphone";
phone1.model = "iphone 13";
phone1.storage = 36;

Phone phone2 = new Phone();
phone2.brand = "samsung";

A constructor lets you fill the fields in one line. Inside it, this.name means "the field of this object".

Constructor + this

public class Employee {
    String name;
    String position;
    int salary;

    public Employee(String name, String position, int salary) {
        this.name = name;
        this.position = position;
        this.salary = salary;
    }
}

Employee employee1 = new Employee("Deniz", "QA", 85000);

To print an object nicely, add a toString() method — then println(object) just works. (IntelliJ can generate it for you.)

toString

@Override
public String toString() {
    return "Employee{name='" + name + "', position='" + position +
           "', salary=" + salary + "}";
}

System.out.println(employee1);
// Employee{name='Deniz', position='QA', salary=85000}

Objects can live inside lists, and everything you learned before still works:

Objects in an ArrayList

List<Country> countries = new ArrayList<>();
countries.add(hungary);
countries.add(vietnam);

for (Country country : countries) {
    if (country.population > 50000) {
        System.out.println(country.capitalCity);
    }
}

Remember: class = the blueprint, object = one real thing made from it. new creates the object. A class can have more than one constructor (with different parameters) — that is called overloading.

Java Brush-Up

Printing to the screen

Practice time

Variables & data types

Naming rules

Practice time

Math operators, % and casting

Practice time

User input with Scanner

Practice time

If / else / else-if

Practice time

Logical operators: &&, ||, !

Practice time

While loops

Practice time

For loops

Practice time

Strings & string methods

Practice time

Arrays

Practice time

ArrayList

Practice time

Set & HashMap

Practice time

Methods (functions)

Practice time

Classes & objects

Practice time