compile code;
javac com/neco/Main.java
run bit code;
src java com.neco.Main

Primitive types

int myNum = 5; // Integer (whole number) float myFloatNum = 5.99f; // Floating point number char myLetter = 'D'; // Character boolean myBool = true; // Boolean String myText = "Hello"; // String

A primitive type has always a value, while non-primitive types can be null.

Shorthand Reassignment

+= x += 3 x = x + 3
-= x -= 3 x = x - 3
*= x *= 3 x = x * 3
/= x /= 3 x = x / 3

Concatenate Strings in Java

String result = str1 + str2;

how to work string in java

char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.' };
String helloString = new String(helloArray);
System.out.println(helloString);

How to work primitive and reference

Array

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
int[] myNum = {10, 20, 30, 40};

Primitives the default value for arrays is zero
for String array default value is 'nnull'

Widening Casting

    int myInt = 9;

    double myDouble = myInt; // Automatic casting: int to double

Ternary Operator

int time = 20;
String result = (time < 18) ? "Good day." : "Good evening.";

Java For Each Loop

for (type variableName : arrayName) {
  // code block to be executed
}

Java Break

for (int i = 0; i < 10; i++) {
      if (i == 4) {
        break;
      }
      System.out.println(i);
    }

output= 0,1,2,3

Multidimensional Arrays

int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
System.out.println(myNumbers[1][2]); // Outputs 7

Java Methods

Create a Method

public class Main {
  static void myMethod() {
    // code to be executed
  }
    
 static void myMethod(String fname, int age) {
    System.out.println(fname + " is " + age);
  }
}

  static int myMethod(int x, int y) {
    return x + y;
  }

static means that the method belongs to the Main class and not an object of the Main class.
void means that this method does not have a return value. You will learn more about return values later in this chapter

Java Block Scope

A block of code refers to all of the code between curly braces {}.

public class Main {
  public static void main(String[] args) {

    // Code here CANNOT use x

    { // This is a block

      // Code here CANNOT use x

      int x = 100;

      // Code here CAN use x
      System.out.println(x);

    } // The block ends here

  // Code here CANNOT use x

  }
}

--------
    public static void main(String[] args) {
	// write your code here


        if (true){
            int x =10;
        }

        System.out.println(x); // cant use x heir

    }

Java Recursion

Recursion is the technique of making a function call itself. This technique provides a way to break complicated problems down into simple problems

public class Main {
  public static void main(String[] args) {
    int result = sum(10);
    System.out.println(result);
  }
  public static int sum(int k) {
    if (k > 0) {
      return k + sum(k - 1);
    } else {
      return 0;
    }
  }
}

What OOP?

Object-Oriented Programming (OOP) is a programming approach that uses objects to structure and organize code.
Objects are instances of classes, which contain data and the methods that operate on that data
OOP is based on the principles of encapsulation, inheritance, and polymorphism,

Creating Object

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj1 = new Main();  // Object 1
    Main myObj2 = new Main();  // Object 2
    System.out.println(myObj1.x);
    System.out.println(myObj2.x);
  }
}

Diffirece Static vs. Public

we created a static method, which means that it can be accessed without creating an object of the class, unlike public, which can only be accessed by objects:

public class Main {
  // Static method
  static void myStaticMethod() {
    System.out.println("Static methods can be called without creating objects");
  }

  // Public method
  public void myPublicMethod() {
    System.out.println("Public methods must be called by creating objects");
  }

  // Main method
  public static void main(String[] args) {
    myStaticMethod(); // Call the static method
    // myPublicMethod(); This would compile an error

    Main myObj = new Main(); // Create an object of Main
    myObj.myPublicMethod(); // Call the public method on the object
  }
}

Java Constructors

A constructor in Java is a special method that is used to initialize objects.
It can be used to set initial values for object attributes:

// Create a Main class
public class Main {
int x;

// Create a class constructor for the Main class
public Main() {
  x = 5;
}

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

Note that the constructor name must match the class name, and it cannot have a return type (like void).

//filename: Main.java
public class Main {
  int modelYear;
  String modelName;

  public Main(int year, String name) {
    modelYear = year;
    modelName = name;
  }

  public static void main(String[] args) {
    Main myCar = new Main(1969, "Mustang");
    System.out.println(myCar.modelYear + " " + myCar.modelName);
  }
}

---------

  public Main(int year, String name) {
    modelYear = year;
   String modelName = name;
  } // we get error

Java Modifiers

meaning that it is used to set the access level for classes, attributes, methods and constructors.

##Access Modifiers

Public; The class is accessible by any other class
Private: The code is only accessible within the declared class

Non-Access Modifiers

For classes, you can use either final or abstract:
abstract;The class cannot be used to create objects

Java Encapsulation

meaning of Encapsulation, is to make sure that "sensitive" data is hidden from users.

public class Person {
  private String name; // private = restricted access

  // Getter
  public String getName() {
    return name;
  }

  // Setter
  public void setName(String newName) {
    this.name = newName;
  }
}

Java Packages

To use a class or a package from the library, you need to use the import keyword:

import package.name.Class;   // Import a single class
import package.name.*;   // Import the whole package
---
import java.util.Scanner;
//In the example above, java.util is a package, while Scanner is a class of the java.util package.

Java Inheritance

subclass (child) - the class that inherits from another class
superclass (parent) - the class being inherited from

class Vehicle {
  protected String brand = "Ford";        // Vehicle attribute
  public void honk() {                    // Vehicle method
    System.out.println("Tuut, tuut!");
  }
}

class Car extends Vehicle {
  private String modelName = "Mustang";    // Car attribute
  public static void main(String[] args) {

    // Create a myCar object
    Car myCar = new Car();

    // Call the honk() method (from the Vehicle class) on the myCar object
    myCar.honk();

    // Display the value of the brand attribute (from the Vehicle class) and the value of the modelName from the Car class
    System.out.println(myCar.brand + " " + myCar.modelName);
  }
}

Polymorfism

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

class Pig extends Animal {
  public void animalSound() {
    System.out.println("The pig says: wee wee");
  }
}

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

class Main {
  public static void main(String[] args) {
    Animal myAnimal = new Animal();  // Create a Animal object
    Animal myPig = new Pig();  // Create a Pig object
    Animal myDog = new Dog();  // Create a Dog object
    myAnimal.animalSound();
    myPig.animalSound();
    myDog.animalSound();
  }
}

Enums

An enum is a special "class" that represents a group of constants (unchangeable variables, like final variables).

public class Main {
  enum Level {
    LOW,
    MEDIUM,
    HIGH
  }

  public static void main(String[] args) {
    Level myVar = Level.MEDIUM; 
    System.out.println(myVar);
  }
}

Java Date and Time

Java does not have a built-in Date class, but we can import the java.time package to work with the date and time API.

√import java.time.LocalDate; // import the LocalDate class
 
 public class Main {
   public static void main(String[] args) {
     LocalDate myObj = LocalDate.now(); // Create a date object
     System.out.println(myObj); // Display the current date
   }
 }

Diffirence Array & ArrayList

The difference between a built-in array and an ArrayList in Java, is that the size of an array cannot be modified

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> cars = new ArrayList<String>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");
    System.out.println(cars);
  }
}

Java Exceptions - Try...Catch

Example
public class Main {
  public static void main(String[ ] args) {
    try {
      int[] myNumbers = {1, 2, 3};
      System.out.println(myNumbers[10]);
    } catch (Exception e) {
      System.out.println("Something went wrong.");
    }
  }
}

Java File Handlig

import java.io.File;  // Import the File class

File myObj = new File("filename.txt"); // Specify the filename

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java-core's Introduction

compile code;

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