Java Programming Notes

[last updated: 2019-06-13]
go to: Java
go to: SDR Study
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Content here comes from several sources:
tecmint.com
tutorialspoint.com
guru99.com
w3schools.com/java
stackoverflow.com/
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(go to:) Program Statements:
System Output:
 Write to console:
  System.out.println("text to print");
 (go to:) Write a File:
(go to:) Calling a class from within another class:
(go to:) Importing Classes:
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• Definitions of terms:
• Everything in Java is an object.
• Objects have attributes and methods.
• A Java object is a combination of data and procedures that operate/work on the data.
• Objects have states and behaviors. The state of an object is stored in fields (variables), while its behavior is specified in methods (functions).
• Objects are created from templates known as classes. That is, a class must be defined before you can create an object.
• In Java, an object is created using the keyword "new".
  There are three steps to creating a Java object:
     Declaration of the object
     Instantiation of the object
     Initialization of the object
• A class is an entity that determines how an object will behave and what the object will contain. It is an object constructor, a blueprint or a set of instructions to build a specific type of object.
• ...
• Notes on general structure:
• The starting point of a Java programming project is the source file. Source files have a .java extension.
• Each source file must have one and only one public class definition.
• The filename of the source file must exactly match the name of the public class that is defined in it.
• Basic (minimal) structure of a source file:
     public class class0 {
        [statements] }
• Statements in a class or method definition must end with a semicolon.
• The curly braces of a class definition enclose the statements that specify the class.
  Within the public class definition, there may be other, private classes defined.
• Within the curly braces of a class definition, there must be one or more methods defined.
  Each public class must have a 'main' method.
  (It appears this is not completely true. However it does seem to be true if you intend/expect to be able to directly execute the class, eg. by using a $ java ClassName statement.)
• Nominal format for specifying a 'main' method:
     public class class0 {
        public static void main(String[] args) {
           System.out.println("Hello"); }
      }
  
  In this example, the method name is "main", and it is defined as a "public" method, meaning it can be accessed by anyone.
  The return type is "void" which means nothing is returned.
  "Strings[]" means the arguments for the method main are a String array, and the name of the argument array is "args."
  "Static" will be defined later.
• Program execution flow:
• When you execute a Java program, you execute the class.
• But first the source file must be 'compiled." This can be done in Linux with a command line statement like:
  $ javac ProgName.java
  The javac compiler will create a file named ProgName.class
• The class file is then executed with:
  $ java ProgName
• Execution starts with the first instruction in the 'main' method, and ends with the 'main' method closing curly brace.
• (go to:) Calling a class from within another class:
• (go to:) Program Statements:
• Variables in a program:
  "Instance variables" are defined within a class, but
  "local variables" are defined within a method.
  Local variables must be initialized before they can be used.
  Not so for instance variables.

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