Java Shell - JShell Tutorial

In this tutorial, we will learn how to use JShell which is introduced in Java 9.

The Java Shell tool (JShell) is an interactive tool for learning the Java programming language and prototyping Java code. JShell is a Read-Evaluate-Print Loop (REPL), which evaluates declarations, statements, and expressions as they are entered and immediately shows the results.

JShell is an amazing tool, we simply need to run JShell from the command line and use it.

Why Use JShell?

Using JShell, you can enter program elements one at a time, immediately see the result, and make adjustments as needed.

Java program development typically involves the following process:
  • Write a complete program.
  • Compile it and fix any errors.
  • Run the program.
  • Figure out what is wrong with it.
  • Edit it.
  • Repeat the process.
JShell helps you try out code and easily explore options as you develop your program. You can test individual statements, try out different variations of a method, and experiment with unfamiliar APIs within the JShell session. 

JShell doesn’t replace an IDE. As you develop your program, paste code into JShell to try it out, and then paste working code from JShell into your program editor or IDE.

Requirements

Install JDK 9+
Note: Before using JShell, make sure that you have installed JDK 9+ in your system. 

Launch JShell

To start or launch JShell, enter the jshell command on the command line:

rameshfadatare@Rameshs-MacBook-Air rameshfadatare % jshell

|  Welcome to JShell -- Version 17.0.2

|  For an introduction type: /help intro

When run, this command displays basic information about the installed JShell program. A jshell prompt then appears, which waits for your input.

JShell Intro

The JShell command /help, with a parameter intro, gives you basic guidelines on how you use the tool.

jshell> /help intro

|  

|                                   intro

|                                   =====

|  

|  The jshell tool allows you to execute Java code, getting immediate results.

|  You can enter a Java definition (variable, method, class, etc), like:  int x = 8

|  or a Java expression, like:  x + x

|  or a Java statement or import.

|  These little chunks of Java code are called 'snippets'.

|  

|  There are also the jshell tool commands that allow you to understand and

|  control what you are doing, like:  /list

|  

|  For a list of commands: /help

JShell Expressions

Let's use JShell to perform addition, subtraction, multiplication, division, and modulo operations:

jshell> 2 + 5
$2 ==> 7

jshell> 5 - 2
$3 ==> 3

jshell> 10 * 5
$4 ==> 50

jshell> 10 / 2
$5 ==> 5

jshell> 9 % 3
$6 ==> 0

jshell> 9 % 2
$7 ==> 1
Note that the $2, $3, $4, $5, $6, and $7 are variables to store the result of the expressions.

Let's look at some simple expressions with multiple operators.
jshell> 10 + 20 + 5
$8 ==> 35

jshell> 20 - 10 * 2
$9 ==> 0

jshell> 20 * 2 - 10
$10 ==> 30

jshell> 15 * 2 / 30
$11 ==> 1

jshell> 10 + 20 + 30 * 2 - 10 / 50
$12 ==> 90

JShell Statements

Java hello world program using JShell:
jshell> System.out.println("hello world!");
hello world!
Concatenate two string literals:
jshell> "java" + "guides"
$13 ==> "javaguides"
Check two String content equal using the equals() method:
jshell> "java".equals("java")
$14 ==> true
Using substring() method:
jshell> "java guides".substring(0, 4)
$15 ==> "java"


Java Control Statements using JShell

if Statement

jshell> int x = 10;    

   ...> int y = 12;    

   ...> if(x+y > 20) {    

   ...> System.out.println("x + y is greater than 20");    

   ...> }    

x ==> 10

y ==> 12

x + y is greater than 20


One more if statement example:

jshell> int x, y;

   ...>         x = 10;

   ...>         y = 20;

   ...>         if (x < y) {

   ...>             System.out.println("x is less than y");

   ...>         }

   ...>         x = x * 2;

   ...>         if (x == y) {

   ...>             System.out.println("x now equal to y");

   ...>         }

   ...>         x = x * 2;

   ...>         if (x > y) {

   ...>             System.out.println("x now greater than y");

   ...>         }

   ...>         // this won't display anything

   ...>         if (x == y)

   ...>             System.out.println("you won't see this");

x ==> 0

y ==> 0

x ==> 10

y ==> 20

x is less than y

x ==> 20

x now equal to y

x ==> 40

x now greater than y

if-else Statement

jshell> int x = 10;  

   ...> int y = 12;  

   ...> if(x+y < 10) {  

   ...> System.out.println("x + y is less than      10");  

   ...> }   else {  

   ...> System.out.println("x + y is greater than 20");  

   ...> }  

x ==> 10

y ==> 12

x + y is greater than 20

if-else-if Ladder

jshell> int month = 4; // April

   ...>         String season;

   ...>         if (month == 12 || month == 1 || month == 2)

   ...>             season = "Winter";

   ...>         else if (month == 3 || month == 4 || month == 5)

   ...>             season = "Spring";

   ...>         else if (month == 6 || month == 7 || month == 8)

   ...>             season = "Summer";

   ...>         else if (month == 9 || month == 10 || month == 11)

   ...>             season = "Autumn";

   ...>         else

   ...>             season = "Bogus Month";

   ...>         System.out.println("April is in the " + season + ".");

month ==> 4

season ==> null

April is in the Spring.

Nested if-statement

jshell> int num = 50;

   ...>         if (num < 100) {

   ...>             System.out.println("number is less than 100");

   ...>             if (num == 50) {

   ...>                 System.out.println("number equal to 50");

   ...>                 if (num > 40) {

   ...>                     System.out.println("number is greater than 40");

   ...>                 }

   ...>             }

   ...>         }

num ==> 50

number is less than 100

number equal to 50

number is greater than 40


for Loop

jshell> //Code of Java for loop  

   ...>     for(int i=1;i<=10;i++){  

   ...>         System.out.println(i);  

   ...>     }  

1

2

3

4

5

6

7

8

9

10

Nested for Loop

jshell> //loop of i  

   ...> for(int i=1;i<=3;i++){  

   ...> //loop of j  

   ...> for(int j=1;j<=3;j++){  

   ...>         System.out.println(i+" "+j);  

   ...> }//end of i  

   ...> }//end of j  

1 1

1 2

1 3

2 1

2 2

2 3

3 1

3 2

3 3

for-each Loop

jshell> //Declaring an array  

   ...>     int arr[]={12,23,44,56,78};  

   ...>     //Printing array using for-each loop  

   ...>     for(int i:arr){  

   ...>         System.out.println(i);  

   ...>     }  

arr ==> int[5] { 12, 23, 44, 56, 78 }

12

23

44

56

78

while Loop

jshell>     int i=1;  

   ...>     while(i<=10){  

   ...>         System.out.println(i);  

   ...>     i++;  

   ...>     } 

i ==> 1

1

2

3

4

5

6

7

8

9

10

do-while Loop

jshell> int i=1;    

   ...>     do{    

   ...>         System.out.println(i);    

   ...>     i++;    

   ...>     }while(i<=10);    

i ==> 1

1

2

3

4

5

6

7

8

9

10

Declare a Java Method

The Java Shell allows to declare a method without writing any enclosing class and invoke the method later. 

Example 1: Write a method printNumbers(int n) that prints all successive integers from 1 to n.

jshell> void printNumbers(int n) {

   ...> for(int i=0; i< n; i++) {

   ...> System.out.println(i);

   ...> }

   ...> }

   ...> 

|  created method printNumbers(int)


Let's invoke the method directly without creating a new instance of a class:

jshell> printNumbers(10);

0

1

2

3

4

5

6

7

8

9


Example 2: Write a method printSquaresOfNumbers(int n) that prints the squares of all successive integers from 1 to n.

jshell> void printSquaresOfNumbers(int n) {

   ...> for(int i=0; i< n; i++) {

   ...> System.out.println(i*i);

   ...> }

   ...> }

   ...> 

|  created method printSquaresOfNumbers(int)


jshell> printSquaresOfNumbers(10);

0

1

4

9

16

25

36

49

64

81

Declare a Java Class

Similarly to method declaration, you can declare a class by typing its code just like you write Java code in an editor, and the shell can detect when the class completes. 

For example:

jshell> public class Student {

   ...>     private String name = "Ramesh";

   ...>     private String college = "ABC";

   ...> 

   ...>     public Student() {

   ...>         super();

   ...>     }

   ...> 

   ...>     public Student(String name, String college) {

   ...>         super();

   ...>         this.name = name;

   ...>         this.college = college;

   ...>     }

   ...> 

   ...>     public String getName() {

   ...>         return name;

   ...>     }

   ...> 

   ...>     public void setName(String name) {

   ...>         this.name = name;

   ...>     }

   ...> 

   ...>     public String getCollege() {

   ...>         return college;

   ...>     }

   ...> 

   ...>     public void setCollege(String college) {

   ...>         this.college = college;

   ...>     }

   ...> }

|  created class Student


Let's test above class:

jshell> Student student = new Student();

student ==> Student@2e5d6d97


jshell> System.out.println(student.getName());

Ramesh


jshell> System.out.println(student.getCollege());

ABC

Declare a Java Interface

Similar to the class declaration, you can declare an interface and the shell can detect when the declaration completes. 

For example:

jshell> public interface Vehicle {

   ...>  String getBrand();

   ...> 

   ...>  String speedUp();

   ...> 

   ...>  String slowDown();

   ...> 

   ...>  default String turnAlarmOn() {

   ...>   return "Turning the vehice alarm on.";

   ...>  }

   ...> 

   ...>  default String turnAlarmOff() {

   ...>   return "Turning the vehicle alarm off.";

   ...>  }

   ...> 

   ...>  static int getHorsePower(int rpm, int torque) {

   ...>   return (rpm * torque) / 5252;

   ...>  }

   ...> }

|  created interface Vehicle


Simple Array Example using JShell

jshell> // declares an array of integers

   ...>         int[] anArray;

   ...>  

   ...>         // allocates memory for 10 integers

   ...>         anArray = new int[10];

   ...>             

   ...>         // initialize first element

   ...>         anArray[0] = 100;

   ...>         // initialize second element

   ...>         anArray[1] = 200;

   ...>         // and so forth

   ...>         anArray[2] = 300;

   ...>         anArray[3] = 400;

   ...>         anArray[4] = 500;

   ...>         anArray[5] = 600;

   ...>         anArray[6] = 700;

   ...>         anArray[7] = 800;

   ...>         anArray[8] = 900;

   ...>         anArray[9] = 1000;

   ...>  

   ...>         for (int i = 0; i < anArray.length; i++) {

   ...>                 System.out.println("Element at index " + i + ": " +anArray[i]);

   ...>         }

anArray ==> null

anArray ==> int[10] { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }

$65 ==> 100

$66 ==> 200

$67 ==> 300

$68 ==> 400

$69 ==> 500

$70 ==> 600

$71 ==> 700

$72 ==> 800

$73 ==> 900

$74 ==> 1000

Element at index 0: 100

Element at index 1: 200

Element at index 2: 300

Element at index 3: 400

Element at index 4: 500

Element at index 5: 600

Element at index 6: 700

Element at index 7: 800

Element at index 8: 900

Element at index 9: 1000

Two Dimensional Array Example using JShell

jshell> String[][] names = {

   ...>             {"Mr. ","Mrs. ","Ms. "},

   ...>             {"Smith","Jones"}};

   ...>  

   ...>         // Mr. Smith

   ...>         System.out.println(names[0][0] +

   ...>                            names[1][0]);

   ...>         // Ms. Jones

   ...>         System.out.println(names[0][2] + 

   ...>                            names[1][1]);

names ==> String[2][] { String[3] { "Mr. ", "Mrs. ", "Ms. " ... [2] { "Smith", "Jones" } }

Mr. Smith

Ms. Jones

ArrayList Example using JShell

jshell>     // Creating an ArrayList of String using

   ...>      List<String> fruits = new ArrayList<>();

   ...>         // Adding new elements to the ArrayList

   ...>      fruits.add("Banana");

   ...>      fruits.add("Apple");

   ...>      fruits.add("mango");

   ...>      fruits.add("orange");

   ...>         System.out.println(fruits);

fruits ==> []

$80 ==> true

$81 ==> true

$82 ==> true

$83 ==> true

[Banana, Apple, mango, orange]


One more example:

jshell> // Creating an ArrayList of String using

   ...>      List<String> fruits = new ArrayList<>();

   ...>         // Adding new elements to the ArrayList

   ...>      fruits.add("Banana");

   ...>      fruits.add("Apple");

   ...>      fruits.add("mango");

   ...>      fruits.add("orange");

   ...>      fruits.add("Watermelon");

   ...>      fruits.add("Strawberry");

   ...> 

   ...>         System.out.println("=== Iterate using Java 8 forEach and lambda ===");

   ...>         fruits.forEach(fruit -> {

   ...>             System.out.println(fruit);

   ...>         });

fruits ==> []

$86 ==> true

$87 ==> true

$88 ==> true

$89 ==> true

$90 ==> true

$91 ==> true

=== Iterate using Java 8 forEach and lambda ===

Banana

Apple

mango

orange

Watermelon

Strawberry

View All the Code Snippets

USe /list command to view all the code snippets that types in JShell:

jshell> /list


   3 : public class Student {

           private String name = "Ramesh";

           private String college = "ABC";

       

           public Student() {

               super();

           }

       

           public Student(String name, String college) {

               super();

               this.name = name;

               this.college = college;

           }

       

           public String getName() {

               return name;

           }

       

           public void setName(String name) {

               this.name = name;

           }

       

           public String getCollege() {

               return college;

           }

       

           public void setCollege(String college) {

               this.college = college;

           }

       }

   4 : public interface Vehicle {

        String getBrand();

       

        String speedUp();

       

        String slowDown();

       

        default String turnAlarmOn() {

         return "Turning the vehice alarm on.";

        }

       

        default String turnAlarmOff() {

         return "Turning the vehicle alarm off.";

        }

       

        static int getHorsePower(int rpm, int torque) {

         return (rpm * torque) / 5252;

        }

       }


Drop a Snippet

Use /drop command to drop a snippet: Drop a interface

jshell> /drop Vehicle

|  dropped interface Vehicle


Drop a class snippet:

jshell> /drop Student

|  dropped class Student


Complete example:

jshell> public interface Vehicle {

   ...>  String getBrand();

   ...> 

   ...>  String speedUp();

   ...> 

   ...>  String slowDown();

   ...> 

   ...>  default String turnAlarmOn() {

   ...>   return "Turning the vehice alarm on.";

   ...>  }

   ...> 

   ...>  default String turnAlarmOff() {

   ...>   return "Turning the vehicle alarm off.";

   ...>  }

   ...> 

   ...>  static int getHorsePower(int rpm, int torque) {

   ...>   return (rpm * torque) / 5252;

   ...>  }

   ...> }

|  created interface Vehicle


jshell> /drop Vehicle

|  dropped interface Vehicle


jshell> public class Student {

   ...>     private String name = "Ramesh";

   ...>     private String college = "ABC";

   ...> 

   ...>     public Student() {

   ...>         super();

   ...>     }

   ...> 

   ...>     public Student(String name, String college) {

   ...>         super();

   ...>         this.name = name;

   ...>         this.college = college;

   ...>     }

   ...> 

   ...>     public String getName() {

   ...>         return name;

   ...>     }

   ...> 

   ...>     public void setName(String name) {

   ...>         this.name = name;

   ...>     }

   ...> 

   ...>     public String getCollege() {

   ...>         return college;

   ...>     }

   ...> 

   ...>     public void setCollege(String college) {

   ...>         this.college = college;

   ...>     }

   ...> }

|  created class Student


jshell> /drop Student

|  dropped class Student

Exit JShell

Type /exit to end the session and quit the shell:

jshell> /exit

|  Goodbye


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