Java Basics

Olubiyi Grace — Mon, 09 Feb 2026 20:49:31 +0000

Run Java from terminal

javac Main.java
java Main

Welcome to Java!

class Main {
public static void main (String[] args){
System.out.println("Welcome to Java!");
   }
}

Variables

class Main{
    static void main(String[] args){
        byte age = 27;
        short dailyIncome = 32678;
        int annualIncome = 12156216;
        long phoneNumber = 2348137963060L;
        float height = 1.83F;
        String name = "Grace"
        double weight = 80.8;
        char incomeCurrency = '$';
        boolean worksRemotely = true;

        System.out.println(age);
        System.out.println(dailyIncome);
        System.out.println(annualIncome);
        System.out.println(phoneNumber);
        System.out.println(height);
        System.out.println(weight);
        System.out.println(incomeCurrency);
        System.out.println(worksRemotely);

        System.out.println(((Object) height).getClass().getSimpleName());
        System.out.println(((Object) worksRemotely).getClass().getSimpleName());
    }
}

String Methods

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

      String name = "Olubiyi Grace";
        System.out.println(name.toLowerCase());
        System.out.println(name.toUpperCase());
        System.out.println(name.length());
    }
}

Enum

public class Main {
    enum TRANSACTION{
    SUCCESS,
    PENDING,
    FAILED
}
    public static void main(String[] args) {

        TRANSACTION transactionStatus = TRANSACTION.PENDING;
        System.out.println(transactionStatus);
        System.out.println(transactionStatus.compareTo(TRANSACTION.SUCCESS));
    }
}

Condition

class Main {
    public static void main(String[] args) {
    int ageA = 27;
    int ageB = 25;

    if (ageA > ageB){
        System.out.println("I am older");
    } else if (ageA < ageB){
        System.out.println("You are older");
    } else {
        System.out.println("We are age mates.");
    }
  }
}

Switch

class Main{
    static void main(String[] args){
int age = 27;
switch (age) {
      case 25 -> System.out.println("Not my age");
      case 26 -> System.out.println("Add 1 and you will be correct.");
      case 27 -> System.out.println("You got it right! This is my age.");
      default -> System.out.println("Why not ask me instead of taking a guess.");
        }
   }
}

For Loop

public class Main {
    public static void main(String[] args) {
       int age = 27;

       for(int x = 0; i < age; x++){
           System.out.println("Age is " + x);
       }
    }
}

While Loop

public class Main {
    public static void main(String[] args) {
     //while example1
        int x = 9;
    while (x < 15){
        System.out.println(x);
        x++;
        }
//while example2
                 int x = 9
              while (x > 3){
                 System.out.println(x);
                 x--;
               }
    }
}

Do/while Loop

// do/while example1
           int x = 8;
        do{
            System.out.println(x);
             x++;
             }
           while (x < 12);
// do/while example2
                     int x = 12;
                     do {
                       System.out.println(x);
                       x++;
                     }
                  while (x >15);        //this will still print despite having a false condition. That is what differentiate it from do loop.

Function

public class Main {
    public static void main(String[] args) {
        int response = addSum(2, 3);
        System.out.println(response);
        System.out.println(((Object) response).getClass().getSimpleName());
    }
    public static int addSum(int a, int b) {
        return a + b;
    }
}

Atomicity

AtomicInteger atomicInt = new AtomicInteger(0);

atomicInt.incrementAndGet(); // ++count
atomicInt.getAndIncrement(); // count++

atomicInt.decrementAndGet(); // --count

atomicInt.addAndGet(5);      // count += 5

int value = atomicInt.get(); // get the current value

boolean updated = atomicInt.compareAndSet(5, 10); 
// if current value == 5, update to 10

Array

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

        int[] arrays = {1, 2, 3, 4};
        for (int item : arrays) {
            System.out.println("Item :" + item);
        }
    }
}

Class and Object

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

        Person personOne =new Person("joe", 78);
        personOne.speak();
    }

}

 class Person{
    private final String name;
    private final int age;

     public Person(String name, int age) {
         this.name = name;
         this.age = age;
     }

     void speak (){
        System.out.println("Name is " + name + " and age is " + age);
    }
}

Inheritance

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

        Person personOne =new Person("joe", 78);
        personOne.speak();
        personOne.scream();
    }

}

class Scream {
    void scream(){
        System.out.println("Ahhhhhhh!");
    }
}

 class Person extends Scream{
    private final String name;
    private final int age;

     public Person(String name, int age) {
         this.name = name;
         this.age = age;
     }

     void speak (){
        System.out.println("Name is " + name + " and age is " + age);
    }
}

Interface

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

        Person personOne = new Person("joe");
        personOne.scream();
    }

}

interface Scream {
    void scream();
}

class Person implements Scream {

    private final String name;

    public Person(String name) {
        this.name = name;
    }

    @Override
    public void scream() {
        System.out.println(name + " screamed, Ahhhhh!!!");
    }
}

Static Method

public class Main {
    public static void main(String[] args) {
        double pi = Utils.PI;
        int square = Utils.square(3);
        System.out.println(pi);
        System.out.println(square);

    }

}


class Utils {
    static final double PI = 3.14;

    static int square(int x) {
        return x * x;
    }
}

This

public class Main {
    public static void main(String[] args) {
        Bark dog = new Bark("woof");
        System.out.println(dog.action);
        System.out.println(dog.play());

    }

}

class Bark{
    String action;

    public Bark(String action) {
        this.action = action; // refers to Bark
    }

    boolean play(){
        System.out.println("Joe is " + this.action);
        return false;
    }
}

Constructor Overloading

public class Main {
    public static void main(String[] args) {
        Bark dog = new Bark("woof");

    }

}

class Bark {
    String name;
    String color;

    public Bark(String name, String color) {
        this.name = name;
        this.color = color;
    }

    public Bark(String name){
        this.name = name;
    }
}

Override (Polymorphism)

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.sound();

    }

}

class Animal {
    void sound(){
        System.out.println("woof");
    }
}

class Dog extends Animal{
    @Override
    void sound(){
        System.out.println("Barking...");
    }
}

Abstract

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.sound();

        Animal n = new Animal(); // abstract class cannot be instantiated
    }

}

abstract class Animal {
    void sound(){
        System.out.println("woof");
    }
}

class Dog extends Animal{
    @Override
    void sound(){
        System.out.println("Barking...");
    }
}

super()

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

}


class Animal {
    Animal() {
        System.out.println("Animal created");
    }
}

class Dog extends Animal {
    Dog() {
        super(); // calls Animal constructor
        System.out.println("Dog created");
    }
}

Wrapper Class

public class Main {
    public static void main(String[] args) {
        Integer num = 90;
        Float f1 = 90.90F;      //use wrapper class if you are working with collections, JSON serialization, null-checking
        System.out.println(num.equals(90)); //stick with primitives if you need performance and don't need object features.
    }
//  Wrapper classes provide helper methods (e.g., Integer.parseInt(), Double.compare()).
}

String Builder

public class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("Hi");
        sb.append(" there!");
        System.out.println(sb.toString());
    }
}

Null Pointer

import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

import static java.lang.Character.getName;

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

        try {
            //Avoiding null NPE
            String myName = "";
            int[] myNumbers = new int[10];
            List<String> items = new ArrayList<>();
            System.out.println(myName);
            System.out.println(myNumbers);
            System.out.println(items);

            if (myName != null) {
                System.out.println("printing this");
            }

            String possiblyNullName = "";
            String nullName = possiblyNullName != null ? possiblyNullName : "Unknown";
            System.out.println(nullName);


            String name = null;
            System.out.println(name.length()); //💥 NullPointerException

            int[] numbers = null;
            System.out.println(numbers.length); // 💥 NPE
        } catch (NullPointerException e) {
            System.out.println(e.getMessage());
        }

    }

}

List

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> animals = new ArrayList<>(); //List<Object>
        animals.add("monkey");
        animals.add("lion");
        System.out.println(animals);
        for (String animal: animals){
            System.out.println("I am a/an " + animal);
        }
    }

}

HashMap

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> response = new HashMap<>();
        response.put("message", "success");
        response.put("status", "ongoing");
        System.out.println(response);
        System.out.println(((Object) response).getClass().getSimpleName());
    }

}
LinkedMap

import java.util.LinkedHashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        // Create a LinkedHashMap
        LinkedHashMap<String, String> capitals = new LinkedHashMap<>();

        // Add key-value pairs
        capitals.put("Nigeria", "Abuja");
        capitals.put("USA", "Washington D.C.");
        capitals.put("France", "Paris");
        capitals.put("Japan", "Tokyo");

        // Iterate in insertion order
        System.out.println("Countries and their Capitals (in insertion order):");
        for (Map.Entry<String, String> entry : capitals.entrySet()) {
            System.out.println(entry.getKey() + " → " + entry.getValue());
        }

        // Access a value
        System.out.println("\nCapital of Japan is: " + capitals.get("Japan"));

        // Check if a key exists
        if (capitals.containsKey("France")) {
            System.out.println("France is in the map.");
        }

        // Remove an entry
        capitals.remove("USA");
        System.out.println("\nAfter removing USA:");
        System.out.println(capitals);
    }
}

Stream

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);

        List<Integer> response = numbers.stream()
                .filter(i -> i % 2 == 0)
                .toList();

        System.out.println(response);
    }

}

Stream

import java.util.Arrays;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        List<Integer> response = numbers.stream()
                .filter(i -> i % 2 == 0)
                .toList();

        System.out.println(response);
    }

}

Exception

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

        try {
            int response = 10 / 0;
            System.out.println(response);
        }catch (ArithmeticException e){
            System.out.println("Oops!, Error");
        }
    }

}

I/O

import java.io.*;

public class Main {
    public static void main(String[] args) {
        try {
            BufferedReader reader = new BufferedReader(new FileReader("C:\\Users\\CAPTAIN ENJOY\\Downloads\\JavaNote\\src\\text.txt"));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();

            BufferedWriter writer = new BufferedWriter(new FileWriter("C:\\Users\\CAPTAIN ENJOY\\Downloads\\JavaNote\\src\\text.txt"));
            writer.write("ner");
            writer.close();
        } catch (IOException e) {
            System.out.println(e.getMessage());
        }
    }
}

Lambda

import java.io.*;
import java.util.Arrays;
import java.util.List;

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

        List<String> names = Arrays.asList("john", "wick", "doe");
        names.forEach(name -> System.out.println("my name is " + name));
    }
}

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Java Basics