[Java] Overview of Java

Java edition (environment)

• JavaSE（Java Platform, StandardEdition）

• Java SE provides a basic execution / development environment for running Java applications

• General-purpose library for string / number, date manipulation, input / output processing, etc.

• Basically only this is used

• JavaEE（Java Platform, Enterprise Edition）

• JavaEE is a library for developing server applications that run on this Java SE.

• Functions mainly designed for corporate system development

• JavaME（Java Platform, Micro Edition）

• For embedding in resource-limited environments (mobile phones, home appliances, etc.)

• Integrated into Java SE in the recent IoT trend

Java history

• After the editions were organized according to the environment in Java2 (version 1.2), the JDK came to be called J2SE (Java2 Platform, Standard Edition).
• In Ver6, the name J2SE will be changed to Java SE.
• Java 8 and 9 add modern features such as lambda expressions and modules
• Oracle JDK is paid for with Java 11 (OpenJDK is free)
• LTS is not provided for OpenJDK (Long Term Support)

environment

• JDK（Java Development Kit）

• JDK is the software required to develop / execute Java applications.

• Development kit with Java virtual machine, compiler, class library, debugger

• OracleJDK, OpenJDK (open source), AdoptOpenJDK (IBM, Microsoft, etc.)

• ** Integrated Development Environment (IDE) **

• Software with various functions to support coding

• Code editor, debugger, project management function, etc.

• Execute from compilation to execution at once with the touch of a button

• Eclipse, NetBeans, IntelliJ IDEA, etc.

The big picture of Java code

Class

• Java program basics
• Classes are ** chunks that perform specific functions within the app **
• String Class: String
• Math Class: Math function
• FileReader / FileWriter: Reading and writing text files
• Create an app by combining these classes
• In Java, make files and classes correspond
• ex: Hello class is defined in a file called Hello.java

Method

• Elements included under class {...} are members (fields, methods, constructors, etc.)
• Methods are ** members to represent the "functions" of the class, and ** represent the processing to be performed by the app **
• In Java, when you start the app, first find and execute the main method: ** Entry point **

package

• ** A container for classifying classes **
• package com.example.demo.trySpring; Under HelloController class etc.

Name resolution

• Class is identified by ** package + class name **: ** Fully Qualified Class Name (FQCN) **
• FQCN：package com.example.demo.trySpring.HelloController

Documentation comments

• Start with 2 asterisks in comments that can be used later to generate documents (* 1 for multi-line comments)
• Describe a description of the class and its members

/***Reads text from a character input stream,...  //Class overview
*
*@see FileReader //Related item
*@author MarkReinhold //author
*@since1.1 //Introduced version
*/

/***Create a buffering character-input stream from…  //Constructor overview
*@param in A Reader //argument
*@param sz Input-buffer size //argument
*@exception //exception
*@retutn //Return value
*/

Debugging basics

• ** Breakpoint setting **
• Ability to pause a running program
• Click the line number from the editor
• Check the status of the program at the breakpoint
• ** Debug execution **
• You can execute code sentence by statement from breakpoints: ** Step execution **
• Step in: Run one line
• Step over: Execute a method call on the way to the next line
• Step return: Execute until returning to the current method caller
• ** Execution resume / end **
• Resume to return to normal execution, end button to end