hasPrevious() 方法不起作用

Question

I don't understand why my hasPrevious iterator is not functioning, it looks like this:

public void previousSong(){
    Iterator<Song> iterator = songs.iterator();
if(iterator.hasPrevious()) {
        System.out.println(iterator.previous().toString());
}
}

I have another function using hasNext() and it works, but this one will not.

public void nextSong(){
    Iterator<Song> iterator = songs.iterator();
if(iterator.hasNext()) {
        System.out.println(iterator.next().toString());
}
}

I am not sure if this method is no longer in existence or not, but when I look into it, Java websites speak of it. I don't understand why it is not functioning.

Answer 1

The hasPrevious and previous methods are only defined for ListIterators ( http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ListIterator.html ), a sub-interface of Iterator. As you're only using an Iterator the only defined methods are hasNext() , next and remove - see here: http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Iterator.html .

Answer 2

Only the ListIterator can walk backwards. The normal Iterator class can't.

So if you use ListIterator instead of Iterator it will work :)

Answer 3

It should be like this:

       public void previousSong(){
        ListIterator<Song> it = songs.listIterator();
    while(it.hasNext()) {
            System.out.println(it.next().toString());
    }

   // and now you can use hasprevious()

      while(it.hasPrevious()) {
            System.out.println(it.previous().toString());
    }
}

Answer 4

// Create an array list

ArrayList list = new ArrayList();   

  // add elements to the array list
      list.add("C++");
      list.add("Java");
      list.add("Php");
      list.add("Python");
      list.add("Ruby");
      list.add("Perl");

     //Create the object of ListIterator through listIterator() method
      ListIterator object=list.listIterator();
     // Modify the list element 
      while(object.hasNext()) {
          String language=(String) object.next() ;
          object.set(language+"+");
      }
     // Show element in pervious order
      while(object.hasPrevious()) {
          System.out.println(object.previous());
      }

It should work by first implementing in forward direction then in backward direction.

Answer 5

don't use Iterator in this Interface hasPrevious() function is not defined use ListIterator

    ListIterator lt =al.listIterator(al.size());
    while(lt.hasPrevious())
    {
        System.out.println(lt.previous());
    }

this will take into the last location of the collection object