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Java Classes

In Java, a class represents a single concept.

A Java program must have one class whose name is the same as the program filename.

In the example, the Person class must be declared in a program file named Person.java.

public class Person { public static void main(String[] args) { System.out.println("I am a person, not a computer."); } }

main() Method in Java

In Java, every application must contain a main() method, which is the entry point for the application. All other methods are invoked from the main() method.

The signature of the method is public static void main(String[] args) { }. It accepts a single argument: an array of elements of type String.

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

Print Line in Java

In Java, System.out.println() can print to the console:

  • System is a class from the core library provided by Java
  • out is an object that controls output
  • println() is a method associated with that object that receives a single argument
System.out.println("Hello, world!"); // Output: Hello, world!

Comments in Java

In Java, comments are bits of text that are ignored by the compiler. They are used to increase the readability of a program.

Single line comments are made by using // and multi-line comments are made by starting with /* and ending with */.

// I am a single line comment! /* I am a multi-line comment! */

Whitespace in Java

Whitespace, including spaces and newlines, between statements is ignored.

System.out.println("Example of a statement"); System.out.println("Another statement"); // Output: // Example of a statement // Another statement

Compiling in Java

In Java, when we compile a program, each individual class is converted into a .class file, which is known as byte code.

The JVM (Java virtual machine) is used to run the byte code.

# Compile the class file: javac first.java # Execute the compiled file: java first

Statements

In Java, a statement is a line of code that executes a task and is terminated with a ;.

System.out.println("Hello World"); System.out.println("Java Programming Language");