Using the JShell in Java
problem
You wish to try out some Java coding without spinning up a whole IDE. Java 9 introduced the jshell, a command-line tool that helps developers write and execute code quickly and easily. E.g. there’s a small bit of logic you wish to try out and see the result.
SOLUTION
Using the jshell tool. In the terminal, type the following:
jshell
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We can see the REPL shown above, waiting for some input.
Create a variable
int x = 1
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No need for a semicolon in this case.
Basic arithmetic operations
It always shows us the result as well.
Let’s create an array
int[] numbers = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100};
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List the declared variables:
/vars
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Run a for loop
A simple for loop to print each number from the array squared.
for(int num: numbers) {
System.out.println(num * num);
}
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Create a method
A simple method that accepts a number and returns its square value.
int sqr(int x){
return x * x;
}
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Let’s use it
sqr(10)
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Now, let’s plug it in the for loop example above
for(int num: numbers){
System.out.println(sqr(num));
}
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Let’s create a “hello world” style method
void hey(){
System.out.println(“Hello from jshell”);
}
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LIst of the methods declared
/methods
LIst all the commands typed
/history
To reset the state e.g. all the declared variables, methods, history:
/reset
Rerunning the last executed command
/!
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To see more commands try
/help
To see more information for each command option, type help and the command.
/help save
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We can save the context in a local file, e.g.
/save /tmp/jshell.txt
Show contents in a different terminal window or tab
less /tmp/jshell.txt
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Stopping jshell
/exit
conclusion
In this post we saw how to use the jshell command line tool. It’s like a super calculator. We can create variables, methods, etc.
References:
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Ellion
Professional IT consultant, writer, programmer enthusiast interested in all sorts of coding.
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