Learn Java

In this free online tutorial you will learn step by step the basic features and capabilities of Java and how to use them.
Be sure you checked [Understand programming] and [Learn programming] sections.

Below we have a nice Editor that you can use to experiment as you learn. You can Copy&Paste the code, but never forget to tweak it and see what happens. This is one of the recipes to learn fast.
Also there is a link inside you can click to popup this Editor as a floating window.

We will start with very basic even trivial examples, and then slowly towards more complex scenarios.

First we need to go trough a lot of dry stuff and mumbo jumbo before we start coding, but be patient that is the nature of Java, why make it easy if it can be hard 😉
Get used to it. You need to be persistent if you want to learn Java.

Java program

A Java program consist of Classes defined in files that use extension .java. Every Java :class can have a method called main() that will be executed whenever you run a compiled program (Understand programming). Yes in Java you use a compiler to compile the source code that you wrote in the .java file to a bytecode file, which can then be run on the Java virtual machine.

If you red Learn programming section which is non mandatory but nice to read ;), especially when we talk about Java, you will have an idea of what is a :class and a :method.

Hello World

I always start with the obligatory Hello world example. Here is how it looks in Java :

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

----
Output:

Hello World!

-----
//We can also compile it on the command line ..
> javac hello.java
//... and then run the compiled program with JVM..
> java hello
//, but you will most probably use IDE as Eclipse

There is a lot going on here. In what follows I will mention many concepts now but will explain them later in their respective sections.

The main thing happening is printing a String on the screen. A String is a sequence characters i.e. text surrounded by double quotes. In this case “Hello World!”

• System: is a class defined in the java.lang package. (Packages are used to organize code)
• out: is an instance of PrintStream, which is a public field of the System class.
• println() is a :method that print its arguments plus new line at the end.

This code is surrounded by the method called main(), which is executed by default when you run the program.
And finally this method is surrounded by the class definition.

Now lets go in reverse.

Every .java file is a :class which contains :methods and :attributes.

• Attributes are :variables, which contain values.
• Methods are encapsulated piece of code that do stuff on behalf of the object

Let’s find out what those terms mean :

Values

Values aka Literals are the atoms of any programming language and includes things such as Numbers, Characters and :Strings.

Variables

Variables are called that because they can vary. In the same way a Box can hold objects a variable can hold different values at different times.

Java is a static :type language so the type of variables have to be explicitly defined. The types are divided to two categories Primitive and reference based.

• Primitive type names are all lowercase and always must have value.
• reference data type names start with upper case, are full blown :objects and can be assigned a null value.

Why would the designers of Java complicate things ?

The reason is that a primitive types are closer to the “machine” i.e.occupy less memory and are faster to use. It complicates matters for programmers but if used in the right places they can speed up a piece of code.
For the same reason Java is statically typed language. It sucks for you, but makes for a faster programs.

You can use the following primitive data types and their reference based equivalents :

Lets see some examples :

public class Variables {
public static void main(String[] args){
   
    //primitive types
    byte b = 25;
    int i = 2356;
    float f = 55.334f;
    double d = 55.334d;
    char c = 'c';

    //reference types
    Short sr = 5;
    Integer ir = 344;
    Float fr = 2.76f;
    Double dr = 5.89;
    Character cr = 'C';
    
    String str = "hello";
    System.out.println(str);

    int changed = 3;//define
    System.out.println(changed);
    changed = 5;// reassign
    System.out.println(changed);
    changed = i;//accepts int <=> int
    System.out.println(changed);
    changed = b;//accepts int <= byte
    System.out.println(changed);
    changed = ir;//accepts int <= Integer
    System.out.println(changed);        
    //doesn't accept
    // changed = f; // int != float    
}

-----
Output:

hello
3
5
2356
25
344

public class HelloWorld {
  public static void main(String[] args) {
    
    String num = "123";
    //convert String to Integer
    Integer inum = Integer.parseInt(num);
    //... and then back to String
    String str = inum.toString();

    //convert reference Double to int
    Double dbl = 123.45d;
    int ival = dbl.intValue();
    System.out.println("Double("+dbl+") => int : " + ival);

    //convert primitive double to int
    double dbl2 = 89.12d;
    int ival2 = new Double(dbl2).intValue();
    System.out.println("double("+dbl2+") => int : " + ival2);
    
    //casting
    int ival3 = (int) dbl2;
    System.out.println("cast:"+dbl2+" => int : " + ival3);
  }
}

-----

Double(123.45) => int : 123
double(89.12) => int : 89
cast:89.12 => int : 89

public class Strings {
  
    //shortcut
    public static <ARG> void say(ARG arg) { System.out.println(arg); }
    
    public static void main(String[] args) {
        String str = "hello world";
        String exc = "!";

        say("substring: " + str.substring(6));
        say("substr 0-5: " + str.substring(0,5));
        say("char @pos 6: " + str.charAt(6));
        say("concat : " + str.concat(exc));
        say( "hello in str ? : " + str.contains("hello"));
        say("isEmpty() ? : " + str.isEmpty());
        say("startsWith(hello) ? : " + str.startsWith("hello"));
        say("indexOf(wo): " + str.indexOf("wo"));
        say("replace() : " + str.replace("hello","howdy"));
        say("toUpperCase() : " + str.toUpperCase());

    }
}

-------

substring: world
substr 0-5: hello
char @pos 6: w
concat : hello world!
hello in str ? : true
isEmpty() ? : false
startsWith(hello) ? : true
indexOf(wo): 6
replace() : howdy world
toUpperCase() : HELLO WORLD

Control statements

We will next look into the constructs that helps us control the flow of execution.

Conditionals

Because solving problems by writing programs requires taking different code paths based on conditions, programming languages have to provide capability to handle them. Java is no exception to this rule.

We thus have :

import java.lang.Math;   

public class IfElse {
  //shortcut
  public static <ARG> void say(ARG arg) { System.out.println(arg); }
  
  public static void main(String[] args) {

    int five = 5;
    int two = 2;
    //lets mix them up
    Integer seven = 7;
    
    //multiply by 10 to change the range to 0 .. 10
    Double random_double = Math.random() * 10;
    //convert to match the types
    int random = Math.round(random_double.floatValue());
    //no conversion when the types match
    long random_long = Math.round(random_double);
    
    say("Generated random number : " + random);
    
    if (five < seven) say("5 is smaller than 7");
    if (five < random) {
      say("5 is smaller than " + random);
    } else {
      say("5 is bigger than " + random);
    } 
    
    if (random > two && random < seven) {
      say(random + " is between 2 and 7");
    } else {
      say(random + " is outside the range 2 .. 7");
    } 

  }
}

Generated random number : 5
5 is smaller than 7
5 is bigger than 5
5 is between 2 and seven
5 is smaller than 7
5 is smaller than 7

import java.lang.Math;   

public class IfElseIf {
  //shortcut
  public static <ARG> void say(ARG arg) { System.out.println(arg); }
  
  public static void main(String[] args) {

    int zero = 0;
    int three = 3;
    int five = 5;
    int seven = 7;
    int ten = 10;

    //multiply by 10 to change the range to 0 .. 13
    Double random_double = Math.random() * 13;
    //convert to match the types
    int random = Math.round(random_double.floatValue());

    say("Generated random number : " + random);
    
    if (random >= zero && random < three) {
      say(random + " is between 0 and 3");
    } else if (random >= three && random < five) {
      say(random + " is between 3 and 5");
    } else if (random >= five && random < seven) {
      say(random + " is between 5 and 7");
    } else if (random >= seven && random < ten) {
      say(random + " is between 7 and 10");
    } else {//default
      say(random + " is outside the range 0 .. 10");
    } 

  }
}

-----

Generated random number : 9
9 is between 7 and 10

Switch-case

Switch-case is another conditional control structure. Why another one ?

While if-else is binary true/false decision, while switch-case is multi choice selection. Something like a simplified if-elseif.

What we can have is an expression that returns a value and depending on this value different option or path of action is selected.

public class SwitchCase {
  
  //shortcut print
  public static <ARG> void say(ARG arg) 
                  { System.out.println(arg); }

  public static void main(String[] args) {
    
    //casting
    int random = (int) Math.round(Math.random() * 7);
    
    say("Generated number : " + random);
    
    switch (random) {
      case 1 : say("Monday"); break;
      case 2 : say("Tuesday"); break;
      case 3 : say("Wednesday"); break;
      case 4 : say("Thursday"); break;
      case 5 : say("Friday"); break;
      case 6 : say("Saturday"); break;
      case 7 : say("Sunday"); break;
      default : say("out of range");
    }
  }
}

-----
Output:

Generated number : 5
Friday

-----
Output:

Loops

There is always a need to do repetitive things. Instead of repeating the code mindlessly we will “surround” this block of code by a condition that will control how many times or how long the block will be executed.

public class Loops {
  
  //shortcut print w/o new line
  public static <ARG> void say(ARG arg) 
                   { System.out.print(arg); }
  // new line
  public static void nl() 
                  { System.out.println(""); }

  public static void main(String[] args) {
    
    //counting from 1 to 5
    for (int i = 1; i < 6 ; i++ ) 
                        say(i + ",");
    nl();
    
    int i = 1;//initialize
    while ( i < 6) {
      say(i + ",");
      i++;
    }
    nl();
    
    String[] nums = { "one", "two",
            "three", "four", "five" };
    //for each loop
    for (String num : nums) {
      say(num + ", "); 
    }
    nl();
  }
}

-----
Output:

1,2,3,4,5,
1,2,3,4,5,
one, two, three, four, five, 

-----
Output: