Introduction to Java

Olusegun Oyetade — Wed, 04 Feb 2026 19:42:44 +0000

Java is a popular programming language invented by James Gosling in 1995. It is a programming language used to develop mobile apps, web apps, games and much more.
Java is a programming language is object oriented (Class and Object).

how to install Java Development Kit on your PC;
There are open source like java openjdk, adoptium, intelliJ

checking if java is installed on your PC

for compiler:
javac -version

for the language:
java -version

Java file
// public is an access modifier , other type can be default, private, protected
// class - blue print
public class Main{

// void: return nothing
// System.out.println() for printing to our console

public static void main(String[] args){
        System.out.println("hello world"); // String must be wrapped in double quote ""
    }

}

Extension is .java
flow: javac Main.java will give us Main.class

Java variables

public class Main{

public static void main(String[] args){

Data types
// categories of data types:
// 1. primitives
// 2. reference

  // primitives: numbers, fractions, boolean, char
  // whole numbers: integers, short, byte, long

  // declare a variable: e.g variable name here is myAge and age
  // assign a value to the variable
  // the = is the assignment operator
  // every statement must end with ;
  int age = 89;
  byte myAge = 127;  // byte start -128 to 127  , naming is called camelCase
  long myLong = 98765456L;
 // float and double: double has higher precision than float, default double
 double height = 1.8;
 float width = 90.89F;
 float my_width = 90.89f; // snake case
  float PI = 90.78F; // pascal

To print to console
// sout
System.out.println(age);

Concatenation
// + is an artihmetic operator but can be used to concatenate 2 variables together
System.out.println("my age is " + age);

// char
char grade = 'A';
System.out.println(grade);

// boolean
boolean  isTall = false;
System.out.println(isTall);


// String is reference type and inbuilt
String fname = "Ayomide";
String lname = "Fashola";
System.out.println(fname + " " + lname);

// difference between primitives and reference
// primitives - starts with lower while Reference starts with upper


// for every class there are methods
int res = fname.length(); // .name means you are calling a method, a method will have .()
System.out.println(res);
    }

}

Java conditionals

public class Main {

// control flow aka conditionals
public static void main(String[] args) {

boolean isRaining = true;

if(isRaining){
System.out.println("go to bed");
}else{
System.out.println("go to school");
}

boolean isMarried = true;
int weddingAge = 6;
boolean willingTomarry = true;

boolean gr = true == true;
System.out.println("is true == to true: " + gr);

System.out.println(true && false);
System.out.println(true && true);
System.out.println(6 < 6);

if(isMarried == true && weddingAge < 6){
System.out.println("not qualified for falentine");
} else if(willingTomarry == true){
System.out.println("you will be considered");
} else{
System.out.println("your matter tire us");
}
}

Java loop

public class Main {

public static void main(String[] args) {

// loops

// int i = 0;
// i++; // i = i + 1; // increment operator
// i++;
// System.out.println(i);

// System.out.println(i < 5);

// DRY - do not repeat yourself
// FOR LOOP
for(int i = 0; i < 5; i++){
  System.out.println("ayomide");
}


// while loop and do while loop

}
}

Java Arrays
public class Main {

public static void main(String[] args) {

// Arrays 
int age = 90;
// index based/ access elements in arrays
//                   0,1,2 ..... n
int[] studentsAge = {1,2,3,4,5,67,90};

System.out.println(studentsAge[2]);
System.out.println("the numbers of elements is " + studentsAge.length);

for(int j = 0; j < studentsAge.length; j++ ){
  System.out.println("index is " + j + " and the value is " + studentsAge[j]);
}

for (int elements : studentsAge) {

  System.out.println("i am number  " + elements);
}

String name = "ronaldo";
String nameToUpper = name.toUpperCase();
System.out.println(nameToUpper); // String methods

}
}
Thank you

DEV Community: Olusegun Oyetade

Introduction to Java