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GS Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework.

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/*
 * Copyright 2014 Goldman Sachs.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.gs.collections.impl.lazy.iterator;

import java.util.Iterator;
import java.util.NoSuchElementException;

import com.gs.collections.api.set.MutableSet;
import com.gs.collections.impl.set.mutable.UnifiedSet;

public final class DistinctIterator
        implements Iterator
{
    private static final Object NULL = new Object();
    private final Iterator iterator;
    private final MutableSet seenSoFar = UnifiedSet.newSet();
    private Object next = NULL;

    public DistinctIterator(Iterable iterable)
    {
        this(iterable.iterator());
    }

    public DistinctIterator(Iterator iterator)
    {
        this.iterator = iterator;
    }

    public void remove()
    {
        throw new UnsupportedOperationException("Cannot remove from a distinct iterator");
    }

    public boolean hasNext()
    {
        if (this.next != NULL)
        {
            return true;
        }
        while (this.iterator.hasNext())
        {
            T temp = this.iterator.next();
            if (this.seenSoFar.add(temp))
            {
                this.next = temp;
                return true;
            }
        }
        return false;
    }

    public T next()
    {
        if (this.next != NULL || this.hasNext())
        {
            Object temp = this.next;
            this.next = NULL;
            return (T) temp;
        }
        throw new NoSuchElementException();
    }
}