Run Java from terminal
CODE
javac Main.java
java Main
Welcome to Java!
CODE
class Main {
public static void main (String[] args){
System.out.println("Welcome to Java!");
}
}
Variables
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
// 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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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)
CODE
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
CODE
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()
CODE
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
CODE
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
CODE
public class Main {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder("Hi");
sb.append(" there!");
System.out.println(sb.toString());
}
}
Null Pointer
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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
CODE
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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