IterableAndIterator
Class should not implement both `Iterable` and `Iterator`

Category
JDK
Severity
WARNING
Maturity

The problem

An Iterator is a state-ful instance that enables you to check whether it has more elements (via hasNext()) and moves to the next one if any (via next()), while an Iterable is a representation of literally iterable elements. An Iterable can generate multiple valid Iterators, though.

Suppression

Suppress false positives by adding an @SuppressWarnings("IterableAndIterator") annotation to the enclosing element.


Positive examples

IterableAndIteratorPositiveCases.java

/*
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * 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.google.errorprone.bugpatterns.testdata;

import java.util.Iterator;

/**
 * @author jsjeon@google.com (Jinseong Jeon)
 */
public class IterableAndIteratorPositiveCases {

  /**
   * Test Node
   */
  public static class MyNode {
    String tag;
    MyNode next;
  }

  /**
   * Test List that implements only Iterator
   */
  public static class MyList1 implements Iterator<MyNode> {
    private MyNode head;

    public MyList1() {
      head = null;
    }

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

    @Override
    public MyNode next() {
      if (hasNext()) {
        MyNode ret = head;
        head = head.next;
        return ret;
      }
      return null;
    }

    @Override
    public void remove() {
      throw new UnsupportedOperationException("remove is not supported");
    }

    public void add(MyNode node) {
      if (!hasNext()) {
        head.next = node;
      }
      head = node;
    }
  }

  /**
   * Test List that implements only Iterable
   */
  public static class MyList2 implements Iterable<MyNode> {

    @Override
    public Iterator<MyNode> iterator() {
      MyList1 l = new MyList1();
      // code to populate the list goes here
      return l;
    }
  }
}

Negative examples

IterableAndIteratorNegativeCases.java

/*
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * 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.google.errorprone.bugpatterns.testdata;

import java.util.Iterator;

/**
 * @author jsjeon@google.com (Jinseong Jeon)
 */
public class IterableAndIteratorNegativeCases {

  /**
   * Test Node
   */
  public static class MyNode {
    String tag;
    MyNode next;
  }

  /**
   * Test List that implements both Iterator and Iterable
   */
  // BUG: Diagnostic contains: both
  public static class MyBadList implements Iterator<MyNode>, Iterable<MyNode> {
    private MyNode head;

    public MyBadList() {
      head = null;
    }

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

    @Override
    public MyNode next() {
      if (hasNext()) {
        MyNode ret = head;
        head = head.next;
        return ret;
      }
      return null;
    }

    @Override
    public void remove() {
      throw new UnsupportedOperationException("remove is not supported");
    }

    public void add(MyNode node) {
      if (!hasNext()) {
        head.next = node;
      }
      head = node;
    }

    @Override
    public Iterator<MyNode> iterator() {
      return this;
    }
  }

  /**
   * Test List that extends the above bad implementation
   * Diagnostic should bypass this
   */
  public static class MyBadListInherited extends MyBadList {
    public MyBadListInherited() {}
  }

  /**
   * Test List that implements only Iterator
   */
  public static class MyGoodList implements Iterator<MyNode> {
    private MyNode head;

    public MyGoodList() {
      head = null;
    }

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

    @Override
    public MyNode next() {
      if (hasNext()) {
        MyNode ret = head;
        head = head.next;
        return ret;
      }
      return null;
    }

    @Override
    public void remove() {
      throw new UnsupportedOperationException("remove is not supported");
    }

    public void add(MyNode node) {
      if (!hasNext()) {
        head.next = node;
      }
      head = node;
    }
  }

  /**
   * Test List that implicitly implements both interfaces
   */
  // BUG: Diagnostic contains: both
  public static class MyImplicitlyBadList extends MyGoodList implements Iterable<MyNode> {

    public MyImplicitlyBadList() {}

    @Override
    public Iterator<MyNode> iterator() {
      return this;
    }
  }
}