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Thread: Small problem regarding inheritance of classes

  1. #1
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    Default Small problem regarding inheritance of classes

    Hey, just moved onto inheritance in college and was working through our first exercise sheet when I got this problem. For the exercise, we have to write a class encapsulating the notion of an employee, then make a Tradesman class by inheritance from Employee and finally, a class Staff to encapsulate the notion of a group of employees.

    I'm having trouble printing out a Tradesman, for example in the following code, when I print the personnel my output should be:

    Mike, Sales
    Fred, Engineering, Welder

    but instead it only prints:

    Mike, Sales
    Fred, Engineering



                    Staff personnel = new Staff();
    		Employee e1 = new Employee("Mike","Sales");
    		Employee e2 = new Tradesman("Fred","Engineering","Welder");
    		personnel.hire(e1); personnel.hire(e2); 
    		personnel.put();
    Can anyone help? I think the problem could be that I'm not storing a Tradesman correctly into the array in the hire method in class Staff. Any guidance in the right direction will help. Thanks in advance.
    Here's the code for the classes so far:


    class Employee {
    	private String name, department;
     
    	Employee(String n, String d) {
    		name = n; department = d;
    	}
     
    	void putEmp() {
    		System.out.print(name + ", " + department);
    	}
     
    	boolean equals(Employee e) {
    		return name==e.name &&
    			department==e.department;
    	}
    }
     
    class Tradesman extends Employee {
    	private String trade;
     
    	Tradesman(String n, String d, String t) {
    		super(n, d); trade = t;	
    	}
     
    	void putTrade() {
    		putEmp();
    		System.out.print(", " + trade);
    	}
     
    }
     
    class Staff {
    	private final int numStaff = 100;
    	private Employee[] staff = new Employee[numStaff];
    	private int count = 0; // num. of staff
     
    	void hire(Employee e) {
    		staff[count] = e;
    		count++;
    	}
     
    	void put() {
    		for(int i=0; i<count; i++) {
    			staff[i].putEmp();
    			System.out.println();
    		}
    	}
    }
     
    class EmployeeTest {
    	public static void main(String[] args) {
    		Staff personnel = new Staff();
    		Employee e1 = new Employee("Mike","Sales");
    		Employee e2 = new Tradesman(
    						"Fred","Engineering","Welder");
    		personnel.hire(e1); personnel.hire(e2); 
    		personnel.put();
    	}
    }


  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: Small problem regarding inheritance of classes

    You seem to expect putTrade() to be called. But where do you call that?
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    Default Re: Small problem regarding inheritance of classes

    But how would I use it? For instance in put(), if i use putTrade() instead of putEmp() I will get an error any time I try to print an employee without a trade as they have different parameters
    Last edited by Stockholm Syndrome; October 7th, 2011 at 02:37 PM.

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    Default Re: Small problem regarding inheritance of classes

    I believe what you want to do is override the putEmp() method.
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    Default Re: Small problem regarding inheritance of classes

    Quote Originally Posted by KevinWorkman View Post
    I believe what you want to do is override the putEmp() method.
    How do I do that?

  6. #6
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    Default Re: Small problem regarding inheritance of classes

    Quote Originally Posted by Stockholm Syndrome View Post
    How do I do that?
    First hit for googling "java overriding": Overriding and Hiding Methods (The Java™ Tutorials > Learning the Java Language > Interfaces and Inheritance)
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  7. #7

    Default Re: Small problem regarding inheritance of classes

    Quote Originally Posted by KevinWorkman View Post
    And after you have finished learning about overridding...

    Consider using toString instead of putEmp().

    If you set up the toString() correctly in the superclass, it allows for easy continuation through subclasses. You can do this with any method, of course, but using the toString() method just feels right.

    Example:
    class Manager
    {
         public String name, department;
         ...
         public String toString() // overriding the toString() method of the superclass (which is Object)
         {
              return this.name + ", " + this.department;
         }
    }
     
    class SpecialManager extends Manager
    {
         public String title;
         ...
         public String toString() // overriding the toString() method of the superclass (which is Employee)
         {
              //since SpecialManager is a subclass of Manager and Manager.toString() is not private
              //we can use our superclass's toString() method
              //we also want to add what our special title is
              return super.toString() + ", " + this.title;
         }
    }
     
    public static void main(String[] args)
    {
         Manager fred = new Manager("Fred","Programming");
         SpecialManager jimmy = new SpecialManager("Jimmy","Programming","MBA");
     
         System.out.println(fred.toString());
         System.out.println(jimmy.toString());
     
         /*******************
         Output
         Fred, Programming
         Jimmy, Programming, MBA
         *******************/
    }
    Kenneth Walter
    Software Developer
    http://kennywalter.com

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    Default Re: Small problem regarding inheritance of classes

    Thanks for the help I solved it by making a new print method in class Employee and then overriding it in the subclass Tradesman.

  9. #9
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    Default Re: Small problem regarding inheritance of classes

    Quote Originally Posted by Stockholm Syndrome View Post
    Thanks for the help I solved it by making a new print method in class Employee and then overriding it in the subclass Tradesman.
    I'm confused about why you needed a new print method. Wouldn't overriding putEmp() have done the trick?
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    Default Re: Small problem regarding inheritance of classes

    Yea sorry I phrased that wrong.. I rewrote my code and my print method that I overrode was no longer called putEmp(). Basically the same though. Thanks again for the help

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