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An introduction to JShell for Java Programmers

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JShell for Java Programmers

An introduction to JShell for Java Programmers

Notes

  • Clear Screen
System.out.print("\033[H\033[2J");

Course Details

Introduction to JShell

  • Java REPL (Read Evaluate Print Loop) Shell
  • Comes by default with JDK 9
  • Quickly debug or test or investigate an API or try a new Library
  • Quick Learning with immediate feedback. No need to write a PSVM or a Unit test.

Launching JShell

  • Install JDK 9
  • Open Command Prompt and type in jshell
  • Other option is to cd to the bin folder where JDK 9 is installed and type in jshell
  • /exit

JShell Basics - Variables and Expressions

  • int i=10;
  • int i=10; int j=10; //multiple statements
  • System.out.println(i);
  • i = i + j
   1 : int x = 10;
   2 : x
   4 : x = x + 10;
   5 : System.out.println(x);
   6 : int i = 10;
   7 :  int j = 20;
   8 : i = i + 10;
   9 :  j = j + 10;

JShell Basics - A few tips

  • Tip : Semicolon is not mandatory unless you want to seperate statements on a single line.
  • Tip : Comments are supported //
  • Quick Tip - Nothing is saved by default!
  • Quick Tip - Verbose mode!
  • jshell -v

JShell Commands - list, drop and history

  • numerical identifier
  • /drop
  • /history

JShell Basics - Multiple Lines

  • i =
    • i + j
  • Multi line comments

JShell Tip : Implicit Variables and Feedback options

  • When expression returns a value
  • 10 + 10
  • System.out.println($1)
  • /set feedback verbose
  • silent | normal | concise

JShell Basics - Methods

int cube(int n) {
    return n * n * n; 
}

void printTwice(String str) {
	System.out.println(str);
	System.out.println(str);
}

JShell Basics - Imports

  • Default imports
  • /imports
  • importing a new class
    • 2 options - import statement or auto import
    • import java.sql.Timestamp
    • new Timestamp => Shift + Tab i
  • Creating a new variable using imported class
    • new Timestamp(); => Shift + Tab v
    • new Timestamp(1000L);=> Shift + Tab v
    • Timestamp temp = new Timestamp(1000L);

JShell Tip : Forward Referencing

  • methods
  • constants
  • variables
  • Cannot be used in variable initialization

JShell Basics - Class

  • Basic Class
  • Editing a class using external editor
  • /edit
  • /edit Course
  • Course course1 = new Course()
  • course1.setName("Microservices with Spring Boot");
  • Cannot access private variables in a class - course1.name

JShell Basics - Auto completion

  • Class in a package
  • Class members
  • Parameters
  • Overloaded methods
  • Documentation of a Class

More JShell Tips : Exceptions, Commands and Help

  • All commands start with / - slash or forward slash
    • /help, /?, / followed by tab
  • JShell Commands - var, methods and types
  • Exceptions

Saving and Reloading JShell Sessions

  • /save file.jsh
  • /open file.jsh

JShell - Setting Custom Start Options

  • Getting a command to be executed at start of JShell
/reset
System.out.print("\033[H\033[2J");
/list
/save start.jsh
/set start start.jsh
/reset
/list
/list -all
/set start start.jsh DEFAULT
/reset
/list
/list -all
/history

JShell - Playing around with an External Library

  • /env -class-path commons-lang3-3.6.jar
  • import org.apache.commons.lang3.*
  • StringUtils.trim("1234 ");

More JShell Tips

  • Shortcuts to commands and options
    • /l
    • /h
    • /l -a
  • Navigation
    • Ctrl + a
    • Ctrl + e
    • Ctrl + k
  • Search Snippets
    • Ctrl + r
    • Ctrl + s
  • For the Lazy Guys
/set start -retain DEFAULT PRINTING

Thank You

Code Examples from Course

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An introduction to JShell for Java Programmers