Java

First steps

Java Installation

JRE
JDK
java
javac

There are several implementation of Java, one currently controlled by Oracle and it can be downloaded from Oracle.com but as far as I know there are various problematic licensing restrictions. A better choice is to download the Open JDK.

Java Download Open JDK

On MS Windows install it by unipping the .zip file and then configuring the PATH to include the bin/ directory.

Before we even write a line of code we would like to make sure that both the java compiler and the java runtime is accessible for us. So we open a command line window or a Power Shell window and we type in javac --version and java --version respectively.

On MS Windows

> javac --version
javac 14.0.2

> java --version
openjdk 14.0.2 2020-07-14
OpenJDK Runtime Environment (build 14.0.2+12-46)
OpenJDK 64-Bit Server VM (build 14.0.2+12-46, mixed mode, sharing)

Java Installation on Ubuntu Linux

sudo apt install openjdk-24-jdk-headless

$ javac -version
javac 24-ea

$ java --version
openjdk 24-ea 2025-03-18
OpenJDK Runtime Environment (build 24-ea+16-Ubuntu-1)
OpenJDK 64-Bit Server VM (build 24-ea+16-Ubuntu-1, mixed mode, sharing)

An older version

$ javac -version
javac 1.8.0_252

$ java -version
openjdk version "1.8.0_252"
OpenJDK Runtime Environment (build 1.8.0_252-8u252-b09-1ubuntu1-b09)
OpenJDK 64-Bit Server VM (build 25.252-b09, mixed mode)

Java from Oracle

Download Java from Oracle
JRE - Java Runtime Environment (Java Virtual Machine JVM)
JDK - Java Development Kit (Java Compiler javac)

$ java -version
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

$ javac -version
javac 1.8.0_60

Hello World in Java

System.out.println
System
println

Printing Hello World on the screen is a bit more complex in Java than in many of the so-called scripting languages, but still it is probably the most simple thing you can do in the language. So let's see it.

Create a file with a name like HelloWorld.java . The extension .java is important , so is the name. In the file we create a class with the same name as the name of the file.

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

$ javac HelloWorld.java       # Compiles the file and creates HelloWorld.class
$ java HelloWorld
Hello World

Java Data Types

String
int
double
boolean
true
false

Primitive types such as int, float, boolean, etc. Or reference types, such as strings, arrays, or objects.

42          - int
1_234       - int
true, false - boolean
"string"    - string in double quotes
'x'         - char in single quotes

'x' == "x"  - execption as cannot compare char with string.

"abc".getClass() works and prints class java.lang.String, but does not work on any of the other data types

public class CheckDataTypes {
    public static void main(String[] args) {
        // https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
        System.out.println("Checking Data Types");
        String h = "Hello";
        String w = "World";
        System.out.println(h);     // Hello
        System.out.println(w);     // World
        String full = h + " " + w;
        System.out.println(full);  // Hello World


        // int is 32 bit sigend between -2^31 and 2^31-1
        int val1 = 19;
        int val2 = 23;
        int val3 = val1 + val2;
        System.out.println(val3);  // 42

        // double 64-bit IEEE 754 floating point.
        double db1 = 19.3;
        double db2 = 22.7;
        double db3 = db1 + db2;
        System.out.println(db3);  // 42.0

        boolean b = true;
        System.out.println(b);  // true
        b = ! b;
        System.out.println(b);  // false


        if (!b) {
            System.out.println("yeah");
            System.out.println("works");
        }
        // Actually if there is only a single statement in the block you don't even need the curly braces, but I would recommend this practice:
        if (!b)
            System.out.println("this too");

        // because of this:
        if (b)
            System.out.println("in condition");
            System.out.println("NOT in condition"); // this is not inside the condition even though the indentation seems to suggest that.

    }
}

javac CheckDataTypes.java
java CheckDataTypes

Checking Data Types
Hello
World
Hello World
42
42.0
true
false
yeah
works
this too
NOT in condition

Java Variable declaration

int
boolean
char

With and without initialization.

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

        int aNumber;
        boolean aBoolean;
        char aCharacter;

        int otherNumber = 12;
        boolean otherBoolean = true;
        char otherCharacter = 'x';

        System.out.println(otherNumber);
        System.out.println(otherBoolean);
        System.out.println(otherCharacter);
    }
}

javac Variables.java
java Variables

12
true
x

Java comments

//
/*
// single-line comment
/* multi-line comment */

Java prompt - read from STDIN

InputStreamReader
BufferedReader
readLine

import java.io.*;

class Prompt{
    public static void main(String[] args) throws IOException{
        System.out.print("Your name: ");
        InputStreamReader converter = new InputStreamReader(System.in);
        BufferedReader in = new BufferedReader(converter);
        String name = in.readLine();
        System.out.println("Your name is: " + name);
    }
}

javac Prompt.java
java Prompt

Convert string to number

Integer
parseInt

public class Convert {
    public static void main(String[] args) {
        String number_str = "23";
        int a_number      = 19;
        System.out.println(number_str);              // 23
        System.out.println(number_str + a_number);   // 2319

        int real_number = Integer.parseInt(number_str);
        System.out.println(real_number + a_number);  // 42
    }
}

javac Convert.java
java Conver

Add Command line values

args
length

public class AddCmdLineArgs{
    public static void main(String[] args){
        if (args.length != 2) {
            System.out.println("Must have exactly 2 values on the command line");
            System.exit(1);
        }
        System.out.println("First: " + args[0]);
        System.out.println("Second: " + args[1]);
        System.out.println(args[0] + args[1]);
    }
}

javac AddCmdLineArgs.java
java AddCmdLineArgs Foo Bar

Add Command line integers

args
Integer
parseInt

public class AddCmdLineNumbers{
    public static void main(String[] args){
        if (args.length != 2) {
            System.out.println("Must have exactly 2 values on the command line");
            System.exit(1);
        }
        System.out.println("First: " + args[0]);
        System.out.println("Second: " + args[1]);
        System.out.println(Integer.parseInt(args[0]) + Integer.parseInt(args[1]));
    }
}

javac AddCmdLineNumbers.java
java AddCmdLineNumbers 19 23

Java for-loop

public class ForLoop  {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            System.out.println(i);
        }
    }
}

$ javac ForLoop.java $ java ForLoop

Java while-loop

while

public class WhileLoop  {
    public static void main(String[] args) {
        int i = 1;
        while (i <= 10) {
            System.out.println(i);
            i++;
        }
    }
}

Java command line arguments

public class CmdLineArgs{
    public static void main(String[] args){
        System.out.println("The following command line arguments were passed:");
        for (int i=0; i < args.length; i++){
            System.out.println("arg[" + i + "]: " + args[i]);
        }
    }
}

javac CmdLineArgs.java
java CmdLineArgs --name "Foo Bar" --email foo@bar.com

Concatenate strings in Java

Use the + operator

public class ConcatStrings {
    public static void main(String[] args) {
        String h = "Hello";
        String w = "World";
        String full = h + " " + w;
        System.out.println(full);  // Hello World
    }
}

$ javac ConcatStrings.java
$ java ConcatStrings

Use class

class Greeting {
    public static void hello() {
       System.out.println("Hello World");
    }
}

public class GreetingsA {
    public static void main(String[] args) {
        System.out.println("Hello");
        Greeting.hello();
    }
}

javac GreetingsA.java
java GreetingsA

Hello
Hello World

Method signature

class Greeting {
    public static void hello() {
       System.out.println("Hello World");
    }
    public static void hello(String message) {
       System.out.println("Hello World " + message);
    }
}

public class GreetingsB {
    public static void main(String[] args) {
        System.out.println("Hello");
        Greeting.hello();
        Greeting.hello("Brave New World");
    }
}

javac GreetingsB.java
java GreetingsB

Hello
Hello World
Hello World Brave New World

Use Java class from another class

public class Calculator {
    public static Integer add(Integer a, Integer b) {
        if (a == 10) {
            return 10;
        }
        return a+b;
    }

}

public class UseCalc {
    public static void main(String[] args) {
        Number a = Calculator.add(2, 3);
        System.out.println(a);
        Number b = Calculator.add(10, 3);
        System.out.println(b);
    }
}

cd examples/mymath
rm -f *.class; javac Calculator.java UseCalc.java ; java UseCalc

Calc

public class Calc {

  public static void main(String[] args) {
    Number c = add(2, 3);
    System.out.println(c);
  }

  public static Integer add(Integer a, Integer b) {
    /* System.out.println(a);
    System.out.println(b); */
    return a+b;
  }
}

Cat

import java.io.*;

/**
    This class demonstrates how to read a line of text from the keyboard
    Based on http://www.abbeyworkshop.com/howto/java/readLine/ReadLine.java
*/
public class Cat{
    public static void main(String[] args){
        try {
            String CurLine = ""; // Line read from standard in

            InputStreamReader converter = new InputStreamReader(System.in);
            BufferedReader in = new BufferedReader(converter);

            CurLine = in.readLine();
            while (CurLine != null){
                System.out.println(CurLine);
                CurLine = in.readLine();
            }
        }catch (IOException e){
            System.out.println("IO Exception\n");
        }
    }
}

Grep

import java.io.*;
import java.util.regex.*;

/**
    This class demonstrates how to read a line of text from the keyboard
    Based on http://www.abbeyworkshop.com/howto/java/readLine/ReadLine.java
*/
public class Grep{
    public static void main(String[] args){
        try {
            Pattern pattern = Pattern.compile(args[0]);

            String line = ""; 

            InputStreamReader converter = new InputStreamReader(System.in);
            BufferedReader in = new BufferedReader(converter);

            line = in.readLine();
            while (line != null){
                Matcher matcher = pattern.matcher(line);
                if(matcher.find()) {
                    System.out.println(line);
                }
                line = in.readLine();
            }
        }catch (IOException e){
            System.out.println("IO Exception\n");
        }catch (ArrayIndexOutOfBoundsException e){
            System.out.println("Usage: java Grep REGEX   (and then type strings on STDIN, Ctr-D to exit)\n");
        }
    }
}

Read file

import java.io.*;

/**
    Source: http://www.abbeyworkshop.com/howto/java/readFile/ReadFile.java
*/

public class ReadFile{
    public static void main(String[] args){
        try {
			
			/*	Sets up a file reader to read the file passed on the command
				line one character at a time */
			FileReader input = new FileReader(args[0]);
            
			/* Filter FileReader through a Buffered read to read a line at a
			   time */
			BufferedReader bufRead = new BufferedReader(input);
			
            String line; 	// String that holds current file line
            int count = 0;	// Line number of count 
            
            // Read first line
            line = bufRead.readLine();
            count++;
            
			// Read through file one line at time. Print line # and line
            while (line != null){
                System.out.println(count+": "+line);
                line = bufRead.readLine();
                count++;
            }
            
            bufRead.close();
			
        }catch (ArrayIndexOutOfBoundsException e){
            /* If no file was passed on the command line, this expception is
			generated. A message indicating how to the class should be
			called is displayed */
			System.out.println("Usage: java ReadFile filename\n");			

		}catch (IOException e){
			// If another exception is generated, print a stack trace
            e.printStackTrace();
        }
        
    }// end main
}

Read line

import java.io.*;

class ReadLine{
    public static void main(String[] args) throws IOException{
        String CurLine = ""; // Line read from standard in
        
        System.out.println("Enter a line of text (type 'quit' to exit): ");
        InputStreamReader converter = new InputStreamReader(System.in);
        BufferedReader in = new BufferedReader(converter);

        
        while (!(CurLine.equals("quit"))){
            CurLine = in.readLine();
            
            if (!(CurLine.equals("quit"))){
                System.out.println("You typed: " + CurLine);
            }
        }
    }
}

import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;

public class ReadLines {
    public static void main(String[] args) throws IOException {
        if (args.length != 1) {
            System.out.println("Must have exactly 1 value on the command line");
            System.exit(1);
        }
        String filename = args[0];


        BufferedReader inputStream = null;

        try {
            inputStream = new BufferedReader(new FileReader(filename));

            String line;
            while ((line = inputStream.readLine()) != null) {
                System.out.println(line);
            }
        } finally {
            if (inputStream != null) {
                inputStream.close();
            }
        }
    }
}

Character stream
read bytestream
reads characters byte-by-byte

Regex

import java.io.*;
import java.util.regex.*;

public class Regex{
    public static void main(String[] args){
        {
            String line = "STRING"; 
            Pattern pattern = Pattern.compile("S.R");

            Matcher matcher = pattern.matcher(line);
            if(matcher.find()) {
                System.out.println(line);
                System.out.println("Found '" + matcher.group() +
                               "' starting at index " + matcher.start() +
                               " and ending at index " + matcher.end() + ".");
 
            }else{
                System.out.println("*** NO MATCH ***");
            }
        }
	}
}

Regex Test harness

import java.io.*;
import java.util.regex.*;

/*
 * Source: http://java.sun.com/docs/books/tutorial/extra/regex/example-1dot4/RegexTestHarness.java
*/

public final class RegexTestHarness {

    private static String REGEX;
    private static String INPUT;
    private static BufferedReader br;
    private static Pattern pattern;
    private static Matcher matcher;
    private static boolean found;

    public static void main(String[] argv) {
        initResources();
        processTest();
        closeResources();
    }

    private static void initResources() {
       try {
           br = new BufferedReader(new FileReader("regex.txt"));
       }
       catch (FileNotFoundException fnfe) {
            System.out.println("Cannot locate input file! "+fnfe.getMessage());
            System.exit(0);
        }
       try {
           REGEX = br.readLine();
           INPUT = br.readLine();
       } catch (IOException ioe) {}

        pattern = Pattern.compile(REGEX);
        matcher = pattern.matcher(INPUT);

        System.out.println("Current REGEX is: "+REGEX);
        System.out.println("Current INPUT is: "+INPUT);
    }

    private static void processTest() {
        while(matcher.find()) {
            System.out.println("I found the text \"" + matcher.group() +
                               "\" starting at index " + matcher.start() +
                               " and ending at index " + matcher.end() + ".");
            found = true;
        }

        if(!found){
            System.out.println("No match found.");
        }
    }

    private static void closeResources() {
       try{
           br.close();
       }catch(IOException ioe){}
    }
}

Split a string

StringTokenizer

import java.util.*;
/**
 *  This class demonstrates how to use the StringTokenizer class to split apart
 *  strings of text.
 *  Source: http://www.abbeyworkshop.com/howto/java/splitString/SplitString.java
 */
class SplitString {
    public static void main(String[] arguments) {
        StringTokenizer ex1, ex2; // Declare StringTokenizer Objects
        int count = 0;

        String strOne = "one two  three      four   five";
        ex1 = new StringTokenizer(strOne); //Split on Space (default)

        while (ex1.hasMoreTokens()) {
            count++;
            System.out.println("Token " + count + " is [" + ex1.nextToken() + "]");
        }

        count = 0;     // Reset counter

        String strTwo = "item one,item two,item three,item four"; // Comma Separated
        ex2 = new StringTokenizer(strTwo, ",");  //Split on comma

        while (ex2.hasMoreTokens()) {
            count++;
            System.out.println("Token " + count + " is [" + ex2.nextToken() + "]");
        }
    }
}

Java: Run external program

InputStream
Runtime
getRuntime
exec

//import java.io.BufferedReader;
import java.io.InputStream;
import java.io.IOException;

class Runls {
    public static void main(String[] args) {
        try
        {
            // Command to create an external process
            String command = "ls";
            InputStream iStream = null;

            // Running the above command
            Runtime run  = Runtime.getRuntime();
            Process proc = run.exec(command);
            iStream = proc.getInputStream();
            int line;
            while ((line = iStream.read()) != -1) {
                System.out.println(line);
            }

        }
        catch (IOException e) {
            e.printStackTrace();
        }
   }
}

Method Overloading

class MethodOverloading {
    public static void main(String[] args) {
        System.out.println("main");
        MethodOverloading.hello();
        MethodOverloading.hello("Brave New World");
        MethodOverloading.hello(42);
    }
    public static void hello() {
       System.out.println("hello");
    }
    public static void hello(String message) {
       System.out.println("hello with message: " + message);
    }
    public static void hello(int answer) {
        System.out.println("the answer is: " + answer);
     }
}

$ javac MethodOverloading.java
$ java MethodOverloading

main
hello
hello with message: Brave New World
the answer is: 42

Public instance attributes

public class TryPoint {
    public static void main(String[] args) {
        Point a = new Point(2, 3);
        System.out.println(a);
        System.out.println(a.x);
        a.x = 7;
        System.out.println(a.x);
        //System.out.println(a.getX);
    }
}

public class Point {
    public int x;
    private int y;
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Getter for private instance attributes

public class TryPoint {
    public static void main(String[] args) {
        Point a = new Point(7, 3);
        System.out.println(a.getX());
    }
}

public class Point {
    private int x;
    private int y;
    public Point(int x, int y) {
        this.x = x;
        this.y = y;

    }

    public int getX() {
        return this.x;
    }
}

Setter for private instance attributes

public class TryPoint {
    public static void main(String[] args) {
        Point a = new Point(7, 3);
        System.out.println(a.getX());
        a.setX(23);
        System.out.println(a.getX());

    }
}

public class Point {
    private int x;
    private int y;
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int getX() {
        return this.x;
    }

    public void setX(int x) {
        this.x = x;

    }
}

Compile and Run in shell

{% embed include file="src/examples/java/run.sh)

Static or Class attribute

public class Bike {
    private static int count = 0;
    public static int getCount() {
        return Bike.count;
    }
    public Bike() {
        Bike.count++;
    }
    public void finalize() {
        Bike.count--;
        System.out.println("Finalize");
    }
}

public class RunBike {
    public static void bike_shop() {
        System.out.println(Bike.getCount());
        Bike a = new Bike();
        System.out.println(Bike.getCount());
        Bike b = new Bike();
        System.out.println(Bike.getCount());
        Bike c = new Bike();
        System.out.println(Bike.getCount());
        b = null;
        //System.gc();
        System.out.println(Bike.getCount());
    }
    public static void main(String[] args) {
        bike_shop();
        System.out.println(Bike.getCount());
    }
}

// The finalize methods is supposed to be similar to a destructor, but it might be never called in the life of the program. So we cannot rely on it.

inheritance

public class Person {
    public String name;

    public Person(String full_name) {
        name = full_name;
    }
}

public class Worker extends Person {
    public int id;

    public Worker(String full_name, int id_number) {
        super(full_name);
        id = id_number;
    }
}

public class TryClass {
    public static void main(String[] args) {
        System.out.println("Hello");
        Person p1 = new Person("Joe");
        System.out.println(p1);        // Person@2a139a55
        System.out.println(p1.name);   // Joe

        Person p2 = new Person("Jane");
        System.out.println(p2);        // Person@15db9742
        System.out.println(p2.name);   // Jane

        p1.name = "Joseph";
        System.out.println(p1.name);   // Joseph
        System.out.println(p2.name);   // Jane

        // System.out.println(p2.id);   // will not compile

        Worker w1 = new Worker("Zorg", 23);
        System.out.println(w1.name);   // Zorg
        System.out.println(w1.id);     // 23
    }
}

Combination

public class Point {
    private int x;
    private int y;

    public Point(int a, int b) {
        x = a;
        y = b;
    }

    public String showCoord() {
        return "(" + x + ", " + y + ")";
    }

    public void move(int dx, int dy) {
        x += dx;
        y += dy;
    }

    public int getx() {
        return x;
    }
    public int gety() {
        return y;
    }
}

import java.lang.Math;

public class Line {
    private Point a;
    private Point b;

    public Line(Point aa, Point bb) {
        a = aa;
        b = bb;
    }

    public String showLine() {
        return a.showCoord() + "___" + b.showCoord();
    }

    public double length() {
        return Math.pow(Math.pow(a.getx()-b.getx(), 2) + Math.pow(a.gety()-b.gety(), 2), 0.5);
    }
}

public class RunCode {
    public static void main(String[] args) {
        System.out.println("Hello");
        Point p1 = new Point(2, 3);
        System.out.println(p1);
        // System.out.println(p1.x);  //  x has private access in Point
        System.out.println(p1.showCoord());
        p1.move(3, 7);
        System.out.println(p1.showCoord());

        Point p2 = new Point(2, 6);
        System.out.println(p2.showCoord());

        Line line = new Line(p1, p2);
        System.out.println(line.showLine());
        System.out.println(line.length());
    }
}

Other

Other examples

public class PrintAdd {
    public static void main(String[] args) {
        int a = 3;
        int b = 5;
        System.out.println("The sum: " + (a+b));
    }
}

public class Characters {
    public static void main(String[] args) {
        char t = '0';
        char x = (char) 97;
        System.out.println(t);
        System.out.println(x);
    }
}

Regex

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class MyRegex {
    public static void main(String[] args) {
        String[] emails = {"abc@corporate.com", "demo-id12@corporate.com", "demo-id12@other.com"};
        Pattern pattern = Pattern.compile("demo-id[0-9]+@corporate\\.com$");
        for (int i=0; i < emails.length; i++) {
            System.out.println(emails[i]);
            Matcher matcher = pattern.matcher(emails[i]);
            if (matcher.find()) {
                System.out.println("OK");
            }
        }
    }
}

Split

public class MySimpleSplit {
    public static void main(String[] args) {
        String planetNames = "Mercury,Venus,Earth,Mars";
        System.out.println(planetNames);
        System.out.println(planetNames.length());

        String[] planets = planetNames.split(",");
        System.out.println(planets[0]);
        System.out.println(planets.length);
        System.out.println("");
        for (int i=0; i < planets.length; i++) {
            System.out.println(planets[i]);
        }
    }
}

Substring

public class MySubstring {
    public static void main(String[] args) {
        String text = "The black cat climbed the greeen tree.";
        System.out.println(text);
        int textLength = text.length();
        System.out.println(textLength);

        // substring(from)
        String tail = text.substring(textLength-5);
        System.out.println(tail);

        // substring(from, to)
        String word = text.substring(textLength-5, textLength-1);
        System.out.println(word);
    }
}

Mixing types in add

+ is overloaded in Java so if one side is a string and the other is an integer, the integer will be automatically converted to a string and then + will be concatenation.

public class MixedAdd {
    public static void main(String[] args) {
        String a = "19";
        String b = "23";
        int c = 23;
        double d = 2.3;
        float f = 2.3f;
        System.out.println("The sum: " + (a+b));
        System.out.println("The sum: " + (a+c));
        System.out.println("The sum: " + (a+d));
        System.out.println("The sum: " + (a+f));
        System.out.println("The sum: " + (f+a));


        String line = "-";
        // MixedAdd.java:16: error: bad operand types for binary operator '*'
        // System.out.println(line * 23);
        System.out.println(line.repeat(23));
    }
}

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