leetcode-go/medium/284.peeking-iterator.java

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
	private Iterator<Integer> iter;
	private Integer peek;

	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    this.iter = iterator;
	}

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

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

	@Override
	public boolean hasNext() {
		return peek != null || this.iter.hasNext();
	}
}