Bank.java

/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package og11;

/**
 *
 * @author Marlon
 */
public class Bank {
    
    public Konto testKonto;
    
    
}

Konto.java

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package og11;

/**
 *
 * @author Marlon
 */
public class Konto {
    
    boolean versuche = true;
    private double kontostand = 100;
    
    double Abheben(double abhbetrag) {
        

        if(kontostand < 0) {
            
            if(versuche == true) {
                
            
           double überzogen = 0;
           versuche = false;
            return überzogen;
            }
            if(versuche == false) {
                 double überzogen = -1;
            versuche = false;
            return überzogen;
                 
            }
            
        }

        
        
        kontostand = kontostand - abhbetrag;
        return abhbetrag;
        
        
    }
    
}

OG11.java

package og11;

import java.util.Scanner;


public class OG11 {

 
    public static void main(String[] args) {
        
        boolean go = true;
        Bank Volksbank = new Bank();
        Bank Sparkasse = new Bank();
        Konto konto1 = new Konto();
  while(go == true) {
        
        double abhebebetrag;
       double ergbetrag;
       
        Scanner scan1 = new Scanner(System.in);
        abhebebetrag = scan1.nextDouble();
        
        ergbetrag = konto1.Abheben(abhebebetrag);
        
        System.out.println(ergbetrag);
        
  }
    }
    
}