Java 'do while' loop not working

I have this code:

import java.util.*; 

public class Vormpjes {

    public static void main(String[] args){

        String vorm;
        String nogmaals;
        double diameter;
        double basis;
        double hoogte;

        Scanner keyboard = new Scanner(System.in);

        do {
            System.out.println("Kies uit de 'cirkel', 'driehoek' of 'vierhoek':");

            vorm = keyboard.nextLine();

            if (vorm.equals("cirkel")){
                System.out.println("Geef een diameter op (in cm)");
                diameter = keyboard.nextDouble();

                cirkel C1 = new cirkel();
                C1.setDiam(diameter);
                System.out.println("De diameter = " + C1.getDiam() + " cm");
                System.out.println("De straal = " + C1.getRadius() + " cm");

                System.out.println("De oppervlakte van de cirkel = " + C1.berekenOpp() + " cm2");
                System.out.println("");
            }

            if (vorm.equals("driehoek")){
                System.out.println("Geef de basis van de driehoek (in cm)");
                basis = keyboard.nextDouble();

                driehoek D1 = new driehoek();
                D1.setBasis(basis);

                System.out.println("Geef de hoogte van de driehoek (in cm)");
                hoogte = keyboard.nextDouble();
                D1.setHoogte(hoogte);
                System.out.println("De oppervlakte = " + D1.berekenOppDriehoek() + " cm2");
                System.out.println("");
            }

            System.out.println("Typ 'J' in als u het programma nogmaals wilt uitvoeren, druk anders op een andere wilekeurige toets.");
            nogmaals = keyboard.nextLine();
        }

        while (nogmaals.equalsIgnoreCase("J"));
    }
}

And the class 'cirkel' (that's how you spell circle in Dutch haha)

public class cirkel extends Vormpjes{

    public double diam;

    public double r;

    //constructor
    public cirkel(){

    }

    public double getDiam(){
        return this.diam;
    }


    public void setDiam(double diam){
        this.diam = diam;
    }

    public double getRadius(){
        r = 0.5 * getDiam();
        return this.r;
    }


    public double berekenOpp(){
        return Math.PI * r * r; 
    }

    public double berekenOmtrek(){
        return 2 * Math.PI * r;
    }

}

When the program asks the user if he wants to run the program again or not, I'm unable to type anything after while condition.

Why is it not working? I don't get errors or anything...

Answer

Each time you call nextDouble() the user will enter a double and press enter. This will put a double and a newline in the buffer. The double is consumed by nextDouble(), but the newline character lingers around until you call nextLine().

In your case this becomes a problem, because when you ask the user for a J (or something else) the "old" newline character is consumed before the user manages to enter anything.

In other words, simply change

diameter = keyboard.nextDouble();

to

diameter = keyboard.nextDouble();
keyboard.nextLine();

to consume the newline character that was entered after the double.