/* * 251.Flatten 2D Vector * 2016-6-20 by Mingyang *------- boolean hasNext() * Returns true if the iteration has more elements.  * (In other words, returns true if next() would return an element rather than throwing an exception.) *-------E next() *Returns the next element in the iteration. *Returns: *the next element in the iteration *Throws: *NoSuchElementException - if the iteration has no more elements *关键问题是掌握好Iterator的用法 */class Vector2D {      private Iterator
     
      
       > row = null;    private Iterator
       
         col = null;     public Vector2D(List
        
         
          > vec2d) {          row = vec2d.iterator();          if(row.hasNext())              col = row.next().iterator();      }      public int next() {          int lastValue = col.next();          return lastValue;      }      public boolean hasNext() {          if(col == null) {              return false;          }          if(col.hasNext()) {              return true;          } else {              while(row.hasNext()) {                  col = row.next().iterator();                  if(col.hasNext())                      return true;              }              return false;          }      }