Java

Although we can configure jupyter to run Java interactively, Java is not meant to be run in that way. Doing so in jupyter requires additional packages to be installed. Instead, we are going to use jupyter to create files and run commands as though we were running them on our Terminal.

In this first example, we are creating a file called Hello.java.

%%file Hello.java

public class Hello {

    public static void main(String [] args) {
        System.out.println("Hello, world!");
    }

}

Overwriting Hello.java

Java programs must be compiled before we can run them. Compilers turn java code into low level machine code. Compilers also check to make sure our programs are syntatically correct. Compiling a java program creates a class file. Before compiling, let’s remove all previously compiled class files and look at what’s left.

%%bash 

rm *.class
ls Hello.*

rm: *.class: No such file or directory

Hello.java

Now let’s compile our program. After compiling, if we use ls again to see the contents of our directory, a class file now exists.

%%bash

javac Hello.java
ls Hello.*

Hello.class

Hello.java

Now let’s run our program using the java command.

%%bash

java Hello

Hello, world!

As an aside, note that we could use a similar technique with Python in jupyter. It’s usually more convenient to run Python interactively, but it would work in the same way.

%%file hello.py

def main():
    print("Hello, World!")
    
if __name__ == "__main__":
    main()

Overwriting hello.py

%%bash

python3 hello.py

Hello, World!

Note that this technique (of creating files and running commands as though we are on the Terminal) doesn’t work well when input is required, unfortunately. The following example is best run outside of jupyter (that is, in the terminal directly).

def main ():
  fahr = input ("Enter the temperature in F: " )
  cel = (float(fahr) - 32) * 5.0/9.0
  print ("The temperature in C is:" , cel)

if __name__ == "__main__":
    main()

---------------------------------------------------------------------------
StdinNotImplementedError                  Traceback (most recent call last)
Input In [7], in <cell line: 6>()
      4   print ("The temperature in C is:" , cel)
      6 if __name__ == "__main__":
----> 7     main()

Input In [7], in main()
      1 def main ():
----> 2   fahr = input ("Enter the temperature in F: " )
      3   cel = (float(fahr) - 32) * 5.0/9.0
      4   print ("The temperature in C is:" , cel)

File ~/cs-courses/cs134-f22/staff/_env/lib/python3.9/site-packages/ipykernel/kernelbase.py:1174, in Kernel.raw_input(self, prompt)
   1167 """Forward raw_input to frontends
   1168 
   1169 Raises
   1170 ------
   1171 StdinNotImplementedError if active frontend doesn't support stdin.
   1172 """
   1173 if not self._allow_stdin:
-> 1174     raise StdinNotImplementedError(
   1175         "raw_input was called, but this frontend does not support input requests."
   1176     )
   1177 return self._input_request(
   1178     str(prompt),
   1179     self._parent_ident["shell"],
   1180     self.get_parent("shell"),
   1181     password=False,
   1182 )

StdinNotImplementedError: raw_input was called, but this frontend does not support input requests.

%%file TempConv.java

import java.util.Scanner;

public class TempConv {
    public static void main (String args[]) {
        Double fahr;
        Double cel;
        Scanner in;

        in = new Scanner (System.in);
        System.out.print("Enter the temperature in F: ");
        fahr = in.nextDouble ();

        cel = ( fahr - 32) * 5.0/9.0;
        System.out.println("The temperature in C is: " +cel);
    }
}

Overwriting TempConv.java

%%bash

javac TempConv.java

ls TempConv.*

TempConv.class  TempConv.java