Script Java

8.1 Abstrakte Methoden und Klassen
}
public void changeBase(NumberSystem n) {
int value=getNumber();
numberSystem=n;
setNumber(value);
}
public class UserFrame extends Frame
implements ActionListener, ItemListener{
private NumberSystem base[] = new NumberSystem[5];
private NumberField op1, op2, result;
private Choice choice;
private void place(Component comp,int x,int y,int width,int height) {
comp.setBounds(x, y, width, height);
add(comp);
}
public UserFrame() {
Button button;
setTitle("Calculator");
setLayout(null);
setSize(220,360);
setResizable(false);
base[0] = new DecNumber();
base[1] = new BinNumber();
base[2] = new OctNumber();
base[3] = new HexNumber();
base[4] = new RomanNumber();
place(choice=new Choice(),70,60,100,20);
choice.addItemListener(this);
choice.addItem("Decimal");
choice.addItem("Binary");
choice.addItem("Octal");
choice.addItem("Hex");
choice.addItem("Roman");
choice.select(0);
place(op1=new NumberField(base[0]),60,180,100,20);
place(op2=new NumberField(base[0]),60,205,100,20);
place(button=new Button("+"),60,230,20,20);
button.addActionListener(this);
place(button=new Button("-"),85,230,20,20);
button.addActionListener(this);
place(button=new Button("*"),110,230,20,20);
button.addActionListener(this);
place(button=new Button("/"),135,230,20,20);
button.addActionListener(this);
place(new Label("result:"),60,260,100,20);
place(result=new NumberField(base[0]),60,280,100,20);
op1.setBackground(Color.white);
op2.setBackground(Color.white);
result.setBackground(Color.white);
result.setEditable(false);
setVisible(true);
}
public void actionPerformed(ActionEvent event){
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