二传手与抽象类

Question

I'm not sure how to call a setter method that's in an abstract class from another class. Here's what I have:

public abstract class Media {
private String loanedTo;
    public String getLoanedTo() {
        return loanedTo;
    }
    public void setLoanedTo(String loanedTo) {
        this.loanedTo = loanedTo;
    }
    private Date dueOn;
    public Date getDueOn() {
        return dueOn;
    }
    public void setDueOn(Date dueOn) {
        this.dueOn = dueOn;
    }
}

public class Library {
public void loan(Loanable item, String name){
        Calendar calcDueDate = Calendar.getInstance();
        calcDueDate.add(Calendar.DAY_OF_MONTH, item.getDaysToLoan());
        dueOn = calcDueDate.getTime();
        loanedTo = name;
    }
}

How can I set the dueOn and loanedTo variables from Library that doesn't extend Media which can't be instantiated? I couldn't find an answer when I searched. Just beginning to learn this stuff.

Answer 1

well, you have to make a concrete class like

public class Book extends Media {
}

If you never have a concrete subclass of an abstract class, then your abstract class can never be used.

Answer 2

There may be some misconceptions in play here.

1. First, your "Abstract" class really is not abstract (there are no unimplemented method signatures).
2. Your Library Class will, one way or another, need to reference an object of a class that extends Media

Check back to the problem you are asked to solve and make sure you are not intended to implement Library as a class that extends Media.

Answer 3

If you tried to instantiate the abstract Media class, you would fail (because you can't instantiate abstract classes).

If you had a concrete class, say Book , and had it extends Media , then you could invoke book.setLoanedTo("Bob") and book.setDueOn(someJavaDate) .

Answer 4

Try something like this:

 public abstract class Media {
            private String loanedTo;
            public String getLoanedTo() {
                return loanedTo;
            }
            public void setLoanedTo(String loanedTo) {
                this.loanedTo = loanedTo;
            }
            private Date dueOn;
            public Date getDueOn() {
                return dueOn;
            }
            public void setDueOn(Date dueOn) {
                this.dueOn = dueOn;
            }
        }
        public class ConcreteMedia extends Media {
            private String loanedTo;
            public String getLoanedTo() {
                return loanedTo;
            }
            public void setLoanedTo(String loanedTo) {
                this.loanedTo = loanedTo;
            }
            private Date dueOn;
            public Date getDueOn() {
                return dueOn;
            }
            public void setDueOn(Date dueOn) {
                this.dueOn = dueOn;
            }
        }

        public class Library {
            public void loan(Loanable item, String name){
                Calendar calcDueDate = GregorianCalendar.getInstance();
                calcDueDate.add(Calendar.DAY_OF_MONTH, item.getDaysToLoan());
                Media concrete = new ConcreteMedia();
                dueOn = concrete.setDueOn(calcDueDate.getTime());
                loanedTo = concrete.SetLoanedTo(name);
            }
        }

Answer 5

I think your Library.loan method's Item param needs to have the methods you've implemented in abstract Media. Create an interface Loanable with the method declarations. You can have Media implement the interface and remain abstract if you want.

public interface Loanable {
   String getLoanedTo();
   void setLoanedTo(String loanedTo);
   Date getDueOn();
   void setDueOn();
}


public abstract class Media implements Loanable {
   private String loanedTo;
   public String getLoanedTo() {
      return loanedTo;
   }
   public void setLoanedTo(String loanedTo) {
      this.loanedTo = loanedTo;
   }
   private Date dueOn;
   public Date getDueOn() {
      return dueOn;
   }
   public void setDueOn(Date dueOn) {
      this.dueOn = dueOn;
   }
}


public class Library {
   public void loan(Loanable item, String name){
      Calendar calcDueDate = Calendar.getInstance();
      calcDueDate.add(Calendar.DAY_OF_MONTH, item.getDaysToLoan());
      dueOn = calcDueDate.getTime();
      loanedTo = name;
      item.setDueOn(dueOn);
      item.setLoanedTo(loanedTo);
}