Programming in Java: Basic Concepts

Code structure in Java

1. Source code file - *.java files
2. Class definition is put inside a source code file
3. A Class definition consists of methods and statements

public class Dog {
  void bark() {
    statement1;
    statement2;
  }
}

Compilation to bytecode

You can compile your program using source code compiler for JDK in CLI:

javac <YourClassName>.java

Classpath

-classpath tells to the compiler where to find third-party classes:

javac -classpath lib.jar HelloWorld.java

And you also should add the classpath while running the program:

java -classpath lib.jar:hw.jar HelloWorld

You can also read the compiled bytecode using the following tool:

javap -v <YourClassName>.class

Running Java program

To run the program use JVM and invoke main class with main() method:

java <YourMainClass>

Run Java program from jar:

java -jar <nameofyourarchave>.jar
java -jar hw.jar

java -classpath <nameofyourarchave>.jar <NameOfMainClass>
java -classpath hw.jar HelloWorld

Java Archive or jar

jar is basically a zip file which contains all classes of the program and one special file - manifest. Manifest (META-INF/MANIFEST.MF) strores meta information about archive, specifically about main class

There is a special tool for working with java archaves - jar.

To create a jar use the following command:

jar cfe <nameofyourarchave>.jar <NameOfMainClass> <AllFilesYouWantToAddToArchave>
jar cfe hw.jar HelloWorld HelloWorld.class

To see what is inside of jar without unpacking:

jar tf <nameofyourarchave>.jar
jar tf hw.jar

Unpack the archive:

jar xf hw.jar

Anatomy of a class (1)

Execution of the program in JVM starts from a specially-written method:

  public static void main (String[] args) {
    // your code goes here
  }

Every Java application has to have at least one class, and at least one main method (not one main per class; just one main per application)

When you run your program, you're really running a class

In the Java programming language, every application must contain a main method whose signature is:

public static void main(String[] args)

The modifiers public and static can be written in either order (public static or static public), but the convention is to use public static as shown above. You can name the argument anything you want, but most programmers choose "args" or "argv".

The main method is similar to the main function in C and C++; it's the entry point for your application and will subsequently invoke all the other methods required by your program.

The main method accepts a single argument: an array of elements of type String.

public static void main(String[] args)

This array is the mechanism through which the runtime system passes information to your application. For example:

java MyApp arg1 arg2

Each string in the array is called a command-line argument. Command-line arguments let users affect the operation of the application without recompiling it. For example, a sorting program might allow the user to specify that the data be sorted in descending order with this command-line argument:

-descending

The "Hello World!" application ignores its command-line arguments, but you should be aware of the fact that such arguments do exist.

Basic Syntax

Statements

int x = 3;
String name = "Dirk";
x = x * 17;
System.out.print("x is " + x);
double d = Math.random();
// this is a comment

Loops

while (x > 12) {
   x = x - 1;
}
for (int x = 0; x < 10; x = x + 1) {
   System.out.print("x is now " + x);
}

Conditions

if (x == 10) {
   System.out.print("x must be 10");
} else {
   System.out.print("x isn't 10");
}
if ((x < 3) & (name.equals("Dirk"))) {
   System.out.println("Gently");
}
System.out.print("this line runs no matter what");

References

1. Head First Java, 2nd Edition
2. Java Programming Basics by Simon Roberts
3. Lesson: A Closer Look at the "Hello World!" Application