284_PeekingIterator284.java

/**
 * Given an Iterator class interface with methods: next() and hasNext(), design
 * and implement a PeekingIterator that support the peek() operation -- it
 * essentially peek() at the element that will be returned by the next call to
 * next().
 * 
 * Example:
 * Assume that the iterator is initialized to the beginning of the list: [1,2,3].
 * 
 * Call next() gets you 1, the first element in the list.
 * Now you call peek() and it returns 2, the next element. Calling next() after
 * that still return 2. 
 * You call next() the final time and it returns 3, the last element. 
 * Calling hasNext() after that should return false.
 * 
 * Follow up: How would you extend your design to be generic and work with all
 * types, not just integer?
 */


public class PeekingIterator284 {
    // Java Iterator interface reference:
    // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
    class PeekingIterator implements Iterator<Integer> {
        private Integer next;
        private Iterator<Integer> iterator;
        
        public PeekingIterator(Iterator<Integer> iterator) {
            // initialize any member here.
            this.iterator = iterator;
            if (this.iterator.hasNext()) {
                this.next = this.iterator.next();
            }
        }

          // Returns the next element in the iteration without advancing the iterator.
        public Integer peek() {
              return next;
        }

        // hasNext() and next() should behave the same as in the Iterator interface.
        // Override them if needed.
        @Override
        public Integer next() {
            Integer returned = next;
            if (iterator.hasNext()) {
                next = iterator.next();
            } else {
                next = null;
            }
            return returned;
        }

        @Override
        public boolean hasNext() {
            return next != null;
        }
    }

}