如何使用 Java 中的迭代器从通用数组列表中打印元素

Question

I'm working in a school project, where I want to implement the Iterator design pattern. I want to use generic arrays.

Container.java

public interface Container {
     Iterator getIterator();
}

Iterator.java

public interface Iterator <T> {
     boolean hasNext();
     T next();
}

TransactionRepository.java

public class TransactionRepository<T> implements Container {
    public TransactionRepository(){
        userTransactions = new ArrayList<>();
    }
    public List<T> userTransactions;

    @Override
    public Iterator <T> getIterator() {
        return new UserTransactions();
    }

    private T t;

    public void add(T t) {
        this.t = t;
    }

    public T get() {
        return t;
    }

    private class UserTransactions implements Iterator <T> {

        int index;

        @Override
        public boolean hasNext() {
            return index < userTransactions.size();
        }

        @Override
        public T next() {
            if(this.hasNext())
                return userTransactions.get(index);
            return null;
        }
    }
}

In my other class, I add the elements to the list by first creating the TransactionRepository object like this: TransactionRepository<String> companyName = new TransactionRepository<String>(); . Then I add elements to the array with the add method companyName.add("CompanyName"); . After that I want to print the array using Iterator, but It just won't print the elements. I have tried multiple variations, but none of them worked.

Iterator <String> stringIterator = companyName.getIterator();

        while (stringIterator.hasNext()) {
            System.out.println("Name : " + companyName.get());
        }

Answer 1

1. With the current implementation List<T> userTransactions is never updated.

In this case userTransactions.size() in hasNext() method will always return 0 so the result of method will be false .

2. Moreover, you should use stringIterator.next() instead of companyName.get() . Since you implement your own iterator you don't want to use get() method at all.

3. There is also a need to update index counter variable after calling next() method.

    @Override public T next() { if (this.hasNext()) return userTransactions.get(index++); return null; }

4. Change modifier on userTransactions to private final as it should be referenced just with iterator.

Code with proposed improvements:

public class TransactionRepository<T> implements Container {
    public TransactionRepository() {
        userTransactions = new ArrayList<>();
    }

    public List<T> userTransactions;

    @Override
    public Iterator<T> getIterator() {
        return new UserTransactions();
    }

    public void add(T t) {
        userTransactions.add(t);
    }

    private class UserTransactions implements Iterator<T> {

        int index;

        @Override
        public boolean hasNext() {
            return index < userTransactions.size();
        }

        @Override
        public T next() {
            if (this.hasNext()) {
                return userTransactions.get(index++);
            }
            return null;
        }
    }
}

Answer 2

It seems that you are never adding elements to your userTransactions List on the add method

Answer 3

You add() method doesnt add anything to your list, it's just like a setter of the attribute t, you should use it to add elements to the list instead

public void add(T t) {
    userTransactions.add(t);
}

There is also another problem, the index , your next() method gets the index element while you didnt initialise your index variable, i recommand you to do it in this way:

int index = 0 ;
...
public T next() {
   if(this.hasNext())
          int temp = index;
          index++;
          return userTransactions.get(temp);
        return null;
    }