Java program files must have the extension .java.
The root name of a Java program file must match the name of the public class defined in that file.
All Java program files must start with the definition of a public class.
A class can act as the starting point for execution only if it has implemented a method with the signature public static void main(String[] args).
The javac terminal command is used to compile a Java program by passing it the name of a Java source code file.
The java terminal command is used to launch compiled Java programs by passing it the name of a compile class that implements a public static void main(String[] args) method.
Both javac and java commands are provided with the JDK.
Other methods may be defined in a class that encapsulate and execute an arbitrary number of Java statements.
Naming a method with the concept that it implements and defining it to take necessary parameters to perform its computation is a good example of abstraction in object oriented programming.
Java rarely needs whitespace to be used when writing source code. Nevertheless, you should use whitespace to format your source code so that it is easy to understand. This is an obligation of all programmers.
All new objects may be instantiated in Java by invoking its constructor using the new keyword.
In spite of its literal notation, String is a class that may be instantiated using its constructor and the new keyword.
An object’s encapsulated behavior is accessed by adding the dot-operator (.) after an object, a field or method name, and parentheses with optional comma-separated parameter values.
String objects have many methods that you may invoke to operate on a String's internal data.
String objects are immutable. All modifications to a String object’s internal data results in the creation of a new String object.
The Scanner class is a utility that is provided by Java in its Core Library for reading data from an input stream like a file or the terminal. The input stream to read is specified as an argument to the Scanner constructor.
The Scanner class is installed with the Java Core Library.
The Scanner class is not available by default. You must import it first from the Core Library by adding the following command to the top of your Java program file: import java.util.Scanner;.
The Scanner class has many useful methods for testing if there is input waiting to be read from its input stream and for reading and parsing input tokens to produce a desired data type.
Remember to invoke the Scanner object’s .close() method when done using it to avoid resource leakage.
The Scanner is very useful for making your programs more generally useful by reading input data before performing calculations.
Random is another useful class provided in the Java Core Library that generates pseudorandom numbers.
Like Scanner, to use Random you must first invoke import java.util.Random; at the top of your source code file.
The Random class a constructor has many useful methods for generating random numbers drawn from uniform and normal distributions.
Pseudorandom numbers generated may have a boolean type or different primitive numerical types.
To be useful in practical applications, the range of pseudorandom numbers generated often must be scaled and translated.
Class libraries not included in the Java Core Library may be distributed as Java ARchive (JAR) files, which can be as simple as a zip archive containing .class files.
The Java compiler command (javac) and launcher command (java) must be told where to find a JAR file containing classes referenced in your program. Use the -cp option (short for --class-path) to inform the command where to find your classes.
