Basic

Basic Java Programs

A basic Java program starts with a class definition, which contains a main method.
The main method is where the program starts executing:

public static void main(String[] args) {
  // your code goes here
  System.out.println("Hello World");
}

• The main method is a static method, which means it can be called without creating an instance of the class.
• args is used to pass command-line arguments to the program (the first element is the name of the program itself)

Variables and Data Types

int age = 20;
System.out.println(age);

int age = 20;              // Integer (whole number)
float myWeight = 55.5f;    // 4 bytes Floating point number
double myWeight2 = 55.5;   // 8 bytes Floating point number
String name = "Hiep";      // String
char firstLetter = 'H';    // Character
boolean isMale = true;     // Boolean

Data Types

Condition

If else

int age = 20;
if (age < 13) {
  System.out.println("Children");
} else if (age < 18) {
  System.out.println("Adolescents");
} else {
  System.out.println("Adults");
}

Ternary operator

int age = 20;
String result = (age < 18) ? "No good" : "OK";
System.out.println(result);

Switch

int age = 15;
switch (age) {
  case 15:
    System.out.println("class 10");
    break;
  case 16:
    System.out.println("class 11");
    break;
  default:
    System.out.println("class 12");
}

Loop

While

int i = 0;
while (i < 5) {
  System.out.println(i);
  i++;
}

Do While

int i = 0;
do {
  System.out.println(i);
  i++;
} while (i < 5);

For

for (int i = 0; i < 5; i++) {
  /* if (i == 4) {
    continue;
  } */
  System.out.println(i);
  /* if (i == 4) {
    break;
  } */
}

For-Each

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
  System.out.println(i);
}

Method

Also known as function.
Code once, use many times.

public class Main {
  // fname, age is Parameters
  static void printAge(String fname, int age) {
    System.out.println(fname + " is " + age);
  }

  // Calculate sum
  static int calSum(int x, int y) {
    // return int value
    return x + y;
  }

  public static void main(String[] args) {
    // "Hiep", 20 is Arguments
    printAge("Hiep", 20);

    int sum = calSum(5, 8);
    System.out.println(sum);
  }
}

Overloading

Scope

Other

Scanner

Scanner scanner = new Scanner(System.in);

System.out.print("Enter name: ");
String name = scanner.nextLine();

System.out.print("Enter age: ");
int age = scanner.nextInt();

System.out.print("Enter weight: ");
double weight = scanner.nextDouble();

scanner.nextLine(); // ignore Enter (new line)
System.out.print("Enter country: ");
String country = scanner.nextLine();

System.out.print("Is male (true or false): ");
boolean isMale = scanner.nextBoolean();

scanner.close(); // Close the scanner

// Using String.format() to format the string
String result = String.format("Result:\nName: %s, Age: %d, Weight: %.2f, Country: %s, Is male: %b", name, age, weight, country, isMale);
System.out.println(result);

String format

File file = new File("input.txt");
Scanner scanner = new Scanner(file);

while (scanner.hasNextLine()) {
  String line = scanner.nextLine();
  System.out.println(line);
}

scanner.close();

Casting

double age = 20.6d;
int ageInt = (int) age; // Outputs 20

String ageText = "20";
int ageValue = Integer.parseInt(ageText);

String weightText = "50.6";
double weightValue = Double.parseDouble(weightText);

String methods

String str = "Hello, world!";
int length = str.length(); // length is 13
int index = str.indexOf('o'); // index is 4
int lastIndex = str.lastIndexOf("o"); // lastIndex is 8
char ch = str.charAt(0); // ch is 'H'
String subStr = str.substring(7, 12); // subStr is "world"
String subStrEnd = str.substring(7); // subStrEnd is "world!"
String lowerCaseStr = str.toLowerCase(); // lowerCaseStr is "hello, world!"
String upperCaseStr = str.toUpperCase(); // upperCaseStr is "HELLO, WORLD!"
String str2 = "   Hello, world!   ";
String trimmedStr = str2.trim(); // trimmedStr is "Hello, world!"
String replacedStr = str.replace("o", "x"); // replacedStr is "Hellx, wxrld!"

Character methods

char c = 'A';
boolean isLetter = Character.isLetter(c); // true
char upperCh = Character.toUpperCase(c); // 'A'
String str = Character.toString(c); // "a"
char ch = '9';
boolean isDigit = Character.isDigit(ch); // true

Math

Math.max(5, 10); // 10
Math.min(5, 10); // 5
Math.round(5.5); // 6
Math.floor(5.5); // 5.0
Math.ceil(5.5);  // 6.0
Math.sqrt(64);   // 8

Math.random();
int randomNum = (int)(Math.random() * 101);  // 0 to 100

Arrays

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"}; // fixed-size
System.out.println("First car: " + cars[0]); // Volvo

System.out.println("Cars length:" + cars.length); // 4

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

for (String i : cars) {
  System.out.println(i);
}

Enum

Enums