Question

大家好我有一个图书馆管理程序

我有一个名为borrowHolding（holdingId）的方法抛出BalanceLow，InvalidHolding 我从模型接口调用那个调用facade类，它将方法调用传递给库类但是从这里方法定义在接口borrower中声明，另一个接口成员扩展这个borrower接口然后AbstractMember实现borrowHoling方法可能小编码片段清除：

所有导入都是正确的，每个类和接口都以不同的密度分隔。

  public interface LMSModel {
    public void borrowHolding(int holdingId)
                throws InsufficientCreditException, MultipleBorrowingException;
    }


    public class LMSFacade implements LMSModel {
    private Library lib = new Library();
        private Member member;
    public void borrowHolding(int holdingId)
                throws InsufficientCreditException, MultipleBorrowingException {
            // TODO Auto-generated method stub
            lib.borrowHolding(holdingId);

        }
    }


    public class Library {
    public Member member;
    public void borrowHolding(int holdingId) throws InsufficientCreditException, MultipleBorrowingException {

            member.borrowHolding(holdingId);
        }
    }


        public interface Member extends Borrower{
    public abstract Holding[] getCurrentHoldings(); 
        public abstract double getMaxCredit();  
        public abstract void resetCredit();
        public abstract String getFullName();
        public abstract String getMemberId();

    }


    public abstract class AbstractMember implements Member, Borrower {
        public void borrowHolding(int holdingId)
                    throws InsufficientCreditException, MultipleBorrowingException {

            // TODO Auto-generated method stub

                System.out.println("Hello");
    //      Map<Integer, Holding> LibHoldingMap = libCollection.holdingMap;     
    //      holding = (Holding)LibHoldingMap.get((Integer)holdingId);
    //      
    //      if (holding.isOnLoan()) {
    //          System.out.println("Can not be issued Currently on Load");
    //      } else {
    //          System.out.println("Can be issued");
    //      }
        }
    }

    public interface Borrower {
        void borrowHolding(int holdingId)throws InsufficientCreditException, MultipleBorrowingException; 
    }

当我调用borrowHolding时我得到nullPointerException我不知道该方法无法调用不知道为什么。

Answer 1

member属性永远不会在您的Library课程中初始化，因此每次尝试使用时都会获得NullPointerException。

由于member是接口Member的实例，因此无法直接构造它。为此，您应该提供一个实现Member接口的类。我发布了两个如何解决这个问题的例子：

使用实现接口的类：

public class MemberImpl implements Member {
    public void borrowHolding(int holdingId)
        throws InsufficientCreditException, MultipleBorrowingException {
        //some code here...
    }
}

public class Library {
    public Member member = new MemberImpl();
    //...
}

实例化实现接口的匿名类：

public class Library {
    public Member member = new Member() {
        public void borrowHolding(int holdingId)
            throws InsufficientCreditException, MultipleBorrowingException {
            //some code here...
        }
    };
    //...
}

可替换地，̶因为你必须加入一个实现到̶̶b̶o̶r̶r̶o̶w̶H̶o̶l̶d̶i̶n̶g̶̶方法内̶̶M̶e̶m̶b̶e̶r̶̶接口，̶这意味着你想声明一个类而不是一个接口

public class Member implements Borrower {
    //...
}

否则，̶如果̶̶M̶e̶m̶b̶e̶r̶̶确实是一个接口，̶记住接口̶*̶*不实现任何方法*̶*̶，̶他们只是限定合同中实施者类必须遵循。 ̶

编辑：

与接口类似，抽象类无法直接实例化，您将需要一个扩展抽象类的类。解决方案类似于提供的解决方案：

使用直接扩展抽象AbstractMember类的类：

public ConcretMember extends AbstractMember {
}

public class Library {
    public Member member = new ConcretMember();
    //...
}

实例化扩展抽象AbstractMember类的匿名类：

public class Library {
    public Member member = new AbstractMember() {
        @Override
        public void borrowHolding(int holdingId)
            throws InsufficientCreditException, MultipleBorrowingException {
            //some code here...
        }
    };
    //...
}

无法从界面调用该方法

1 个答案: