Basic of Java

Structure of Java

• All java programs must have at least one class definitions.
• Main method is the entry point of our java application.
• In the example below HelloWorld is the class for overall java program and is named after the .java file.
• The main function is where the program starts. The main method takes in a list of Strings when they are given during the running of the program.

\\ arguments given when running java
java MyProgram arg1 arg2 arg3

\\ general example
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Data types

• primitive types
  • int
  • float
  • char
  • boolean
• non-primitive types
  • String(capitalized when used)

How to print

System.out.print("Hello World")

Switch statement (similar to if-statement)

int day = 2;
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other day");
}

For-loops

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

Do-while loop

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

Arrays

Initializing an array

int[] myNumbers = {10, 20, 30, 40};
System.out.println(myNumbers[0]); // Outputs 10

OOP in Java

Instantiating objects (both ways work)

public class Main {
    public static class MyClass {
    int x = 5;
    }
    public static void main(String[] args) {
        MyClass myObj = new MyClass();
        System.out.println(myObj.x);
    }
}

class MyClass {
    int x = 5;
}
public class Main {
    
    public static void main(String[] args) {
        MyClass myObj = new MyClass();
        System.out.println(myObj.x);
    }
}

Output

5

Inheretene

Dog inheretes the animalSound() method from the aminal class but redefines it by

class Animal {
    public void animalSound() {
        System.out.println("The animal makes a sound");
    }
}

class Dog extends Animal {
    public void animalSound() {
        System.out.println("The dog says: bow wow"); // redefine what animalSount() does
    }
}

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.animalSound();
    }
}

Polymorphism

The ability of a variable, function, or object to take on multiple forms. Here we see that we can instantiate a Dog object from the Animal class.

Animal myAnimal = new Dog(); // Polymorphism
myAnimal.animalSound(); // Outputs: The dog says: bow wow

Encapsulation

The technique of making the fields in a class private and providing access via public methods. Because name is a private attribute, it can only be accessed via getName() and setName(), rather than through the instantiated object directly.

public class Person {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String newName) {
        name = newName;
    }
}