Java error "Value of local variable is not used"

You get that error because you don't have the main method that is used to start the java program:

public class main {

public static void main(String[] args) {
   try {
       Runtime rt = Runtime.getRuntime() ;
       Process p = rt.exec("calc.exe") ; // here, eclipse is WARINING(so you can ignore it) you that that the variable p is never used(it's just a warning)
   } catch(Exception exc) {
       /*handle exception*/
       // never do this, always put at least a System.out.println("some error here" + e); so you don't ignore a potential exception
   }
}

I believe what you have is not an error but a warning; eclipse (and other IDEs/compilers) will tell you that, although you assigned a value to the variable p, you did not use it anywhere. It tells you this because this is sometimes an error; mostly when you assign a value to a variable, you later use that variable in some way.

You can eliminate the error by changing that particular statement to just

rt.exec("calc.exe")

since you are not required to assign a value from the call to exec.

There is no such thing as a stupid qiestion(only misplaced ones, in the worst case).

The "Editor does not contain a main type" refers to the fact that you have not defined a main method. All java programs require a main method, as such:

public static void main(String [] args){
    <code>
}

This is where you must place your code.

The "Value not used" is just a warning; it tells you that your variable p only exists within the try-block. You can declare your p-variable before the try - that way, you can use it outside the try-scope(the scope of a variable refers to where it exists, in this case, only inside the try-block).

If you want to use your p, this is what you're after:

public class Main {
    public static void main(String[] args) {
        Process p;
        try {
            Runtime rt = Runtime.getRuntime();
            p = rt.exec("calc.exe");
        } catch(Exception exc) {/*handle exception*/}
    }
}

[EDIT]: Note that it is part of the java coding convention to use Capital letters for the first letter of a class, e.g. Main.java(not main.java)

The use of the variable is not in issue here. That error appears because JVM needs a method with the signature to know where to start execution.

public static void main( String args[] ){ //TODO: Stuff here } 

Introduce a method with this signature in your class, and it shall clear that error. Alternatively, you may embed your code in a static block as below - but this method is not to be recommended.

static {
    // TODO: Your code here
}

Error "The value of local variable p is not used" due to the fact that nowhere in the code, you do not use the variable p.

To remove the error - it is necessary to remove the variable "p".

To run the calculator, you must use the code:

public class MainClass {

    public static void main(String args[]) throws IOException {

        Runtime.getRuntime().exec("cmd /c calc.exe");

    }

}

This and all other comments translated by Google Translate

you are not using main how can you compile please use main method. Ans second one is use p local variable in your method.other wise declare p variable starting of method.