Java-Calculator more than one calculation

Question

I have written a code for a calculator but it is not working 100% reliably,yet. Every time I do a calculation, for example: "1+1=2", and I want to do another calculation I have to close the applet and start it again. How can I make it go back at the start.

Here is the code:

package beispiele;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class Kalkulator extends Applet {
    String arg1= "", arg2="";
    int ergebnis;
    Button zahl[] =new Button[10];
    Button funktion[] = new Button[4];
    Button ausfuehren;
    Panel zahlPanel,funktionPanel,ergebnisPanel;
    TextField ergebnisFeld;
    int operationArgument;
    char operation;
    public void init () {
    operationArgument= 1; operation =' ';
    setLayout(new BorderLayout());
    zahlPanel = new Panel();
    zahlPanel.setLayout(new GridLayout (4,3));
    for (int i=9; i>=0; i--) {
        zahl[i] = new Button(String.valueOf(i));
        zahl[i].addActionListener(new ButtonZahlen());
        zahlPanel.add(zahl[i]);
    }
    zahlPanel.add(new Button("."));   //leere Taste
    ausfuehren = new Button("=");
    ausfuehren.addActionListener(new ButtonAusfuehren()); //zu dem Listener
    zahlPanel.add(ausfuehren);
    add("Center",zahlPanel);
    funktionPanel = new Panel();
    funktionPanel.setLayout(new GridLayout(4,1));
    funktion[0] = new Button("+");
    funktion[0].addActionListener(new ButtonOperation());
    funktionPanel.add(funktion[0]);
    funktion[1] = new Button("-");
    funktion[1].addActionListener(new ButtonOperation());
    funktionPanel.add(funktion[1]);
    funktion[2] = new Button("*");
    funktion[2].addActionListener (new ButtonOperation());
    funktionPanel.add(funktion[2]);
    funktion[3] = new Button("/");
    funktion[3].addActionListener (new ButtonOperation());
    funktionPanel.add(funktion[3]);

    //funktionPanel.add(new Button("*"));
    //funktionPanel.add(new Button("/"));
    add("East",funktionPanel);
    ergebnisPanel = new Panel();
    ergebnisFeld = new TextField("0",5);
    ergebnisPanel.add(ergebnisFeld);
    add("North",ergebnisPanel);
}
class ButtonZahlen implements ActionListener{
    public void actionPerformed(ActionEvent e) {
        switch (operationArgument)   {
        case 1 :  {
            arg1+=e.getActionCommand();
            ergebnisFeld.setText(arg1);
            break;
        }
        case 2 :   {
            arg2 +=e.getActionCommand();
            ergebnisFeld.setText(arg2);
            break;
        }
        default: { }

        }
        }
    }
class ButtonAusfuehren implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        if(operation =='+')
            if (arg1.equals("")) {
                ergebnis += new Integer(arg2);
            } else {
                ergebnis = new Integer(arg1) + new Integer(arg2);
            }

        else if(operation =='-') {
            if(arg1.equals("")) {
                ergebnis -= new Integer(arg2);
            } else {
                ergebnis = new Integer(arg1) - new Integer(arg2);
            }               
        }
        else if(operation =='*') {
            if(arg1.equals("")) {
                ergebnis *= new Integer(arg2);
            } else {
                ergebnis = new Integer(arg1) * new Integer(arg2);
            }
        }
        else if(operation =='/') {
            if(arg1.equals("")) {
                ergebnis *= new Integer(arg2);
            } else {
                ergebnis = new Integer(arg1) / new Integer(arg2);
            }
        }
    }
}
class ButtonOperation implements ActionListener{
    public void actionPerformed(ActionEvent e) {
        if(e.getActionCommand().equals("+")) {
            operation = '+'; operationArgument = 2;
            }
        else if(e.getActionCommand().equals("-")) {
                operation = '-'; operationArgument = 2;
                }
        else if(e.getActionCommand().equals("*")) {
                    operation = '*' ; operationArgument =2;
                    }
        else if(e.getActionCommand().equals("/")) {
                        operation = '/' ; operationArgument =2;
                    }

            }
        }
public void paint(Graphics g){   }


}

Answer 1

You are having problems here because you are not careful with your if and else statements and the logic of the program. Firstly, in the actionPerformed method of ButtonOperation, you have an if(e.getActionCommand().equals("-")) inside the if(e.getActionCommand().equals("+")). That doesn't make sense, does it? The commands you are testing for with those ifs are mutually exclusive, so an else if for the - operation is what you want. Then in the ButtonAusfuehren actionPerformed() method: again, think about what you want to be doing. You don't need the last else - you can always set the result field (ergebnisFeld) to the result. Also, the operations themselves wouldn't have worked as the second time an operation was done, one of the arguments (arg1 or arg2) would have been an empty string. If you take this into account, you get something like this, which, with the other changes, makes your calculator work:

        if (operation == '+')
            if (arg1.equals("")) {
                ergebnis += new Integer(arg2);
            } else {
                ergebnis = new Integer(arg1) + new Integer(arg2);
            }

        else if (operation == '-') {
            if (arg1.equals("")) {
                ergebnis -= new Integer(arg2);
            } else {
                ergebnis = new Integer(arg1) - new Integer(arg2);
            }
        }