> For the complete documentation index, see [llms.txt](https://cs61b-2.gitbook.io/cs61b-textbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cs61b-2.gitbook.io/cs61b-textbook/12.-inheritance-iv-iterators-object-methods/12.6-exercises.md).

# 12.6 Exercises

### Factual

1. What methods are required for a class that is Iterable?
2. Which of the following is true about the `java.util.Se`t and the `java.util.List` interfaces?
   * [ ] If we add `String[][]` objects to a `Set` and a `List`, the size of the set will always be less than or equal to the size of the list.
   * [ ] The `java.util.ArrayList` class is an implementation of the `java.util.List` interface.
   * [ ] The `Set` and `List` interfaces extend the `Iterator` interface.
   * [ ] The `Set` and `List` interfaces extend the `Iterable` interface.
3. Suppose we have a class that implements `Iterator`. What methods must it override in order to compile?

<details>

<summary>Problem 1</summary>

An `Iterable` is required to have the `iterator()` method, which returns an `Iterator`.

</details>

<details>

<summary>Problem 2</summary>

* **If we add `String[][]` objects to a `Set` and a `List`, the size of the set will always be less than or equal to the size of the list.** Sets only have unique items, while lists can have duplicates, so if we add the same elements to both the list will always have at least as many elements as the set.
* **The `java.util.ArrayList` class is an implementation of the `java.util.List` interface.** One implementation of the `List` interface in Java is the ArrayList class.
* **The `Set` and `List` interfaces extend the `Iterable` interface.** Sets and Lists in Java can be used in enhanced for loops, which means that they are `Iterable`.

</details>

<details>

<summary>Problem 3</summary>

An `Iterator` must override `hasNext()`, which returns a boolean indicating whether there are more elements in the `Iterator`, and `next()`, which returns the next item.

</details>

### Conceptual

1. Why do we want to override the `.equals` method?

<details>

<summary>Problem 1</summary>

The `.equals()` method inherited from `Object` only checks if two items have the same memory address. This is undesireable behavior for many user-written classes in Java.

</details>

## Metacognitive

1. In lecture, you built the `ArraySetIterator` class. Modify the lecture class to take in a `Comparator<T>` and an item of generic type `T` called `ref` in the constructor. This new iterator should only return items greater than `T`. For reference, the code for `ArraySetIterator` is included below.

```java
private class ArraySetIterator implements Iterator<T> {
	private int pos;
	
	public ArraySetIterator() { 
		pos = 0; 
	}
	
	public boolean hasNext() { 
		return pos < size; 
	}
	
	public T next() {
	 	T returnItem = items[pos];
	 	pos += 1;
	 	return returnItem;
	}
}
```

2. Problem 7 from the Spring 2018 Midterm 2

<details>

<summary>Problem 1</summary>

```java
public class ArraySetGreaterIterator implements Iterator<T> {
    private int pos;
    private T ref;
        
    private Comparator<T> comp;

    public ArraySetGreaterIterator(T ref, Comparator<T> comp) {
        this.ref = ref;
        this.comp = comp;
    }

    @Override
    public boolean hasNext() {
        return pos < size;
    }

    @Override
    public T next() {
        T returnItem = items[pos];
        while (comp.compare(returnItem, ref) <= 0) {
            pos += 1;
            returnItem = items[pos];
        }
        return returnItem;
    }
}
```

</details>

<details>

<summary>Problem 2</summary>

[Solutions](https://drive.google.com/file/d/1LIyFXwHYCWXNqIgKTsTyKiOYnB79_ykk/view) and [walkthrough](https://www.youtube.com/watch?v=nMZn4EV0gGw) are linked here and on the course website.

</details>


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