This assignment has an FAQ page.

Before You Begin #

As usual, pull the skeleton code.

Learning Goals #

In this lab, we’ll wrap up the Java-focused portion of the class.

First, we will expand upon our knowledge of interfaces from Lab 6 by looking at some existing Java interfaces that allow us to implement useful behaviors for our data structures and other types.

We’ll also consider what happens when an error occurs, like a NullPointerException, and what we can do to stop them from crashing the entire program.

Interfaces as Behaviors #

In Lab 6, we discussed ADTs, a description of a data structure’s behavior; and how we can implement ADTs in Java using interfaces. We actually don’t need to limit ourselves to expressing data structure behavior - as long as we have a list of things we want to do, we can make an interface.

We’ll take a look at two really common things that we want to do with our classes:

  • Making them able to be compared (useful for things that go in certain kinds of data structures)
  • Making them able to be iterated over (useful for the data structures themselves)

Comparison #

Read Chapter 4.3 from Max Function through Comparables to help motivate the problem we’re solving and the tools we’ll use along the way.

Remember casting is a bit of special syntax where you can tell the compiler that a specific expression has a specific compile-time type. If the maxDog method below returns an object of static type Dog, the code normally wouldn’t compile as Poodle is a subtype of Dog. Casting tells Java to treat the Dog as if it were a Poodle for the purposes of compilation because it’s possible that the Dog returned from maxDog could be a Poodle.

Poodle largerPoodle = (Poodle) maxDog(frank, frankJr);

While we haven’t explicitly learned about sorting yet, the idea of sorting should be intuitive enough. You have a list of things, and you want to put it in sorted order. While this makes sense immediately for things like ints, which we can compare with primitive operations like < and ==, this becomes less clear for general objects.

So, what does “sorted order” for general objects? To sort, we must first say that < means something, or that we can meaningfully compare two objects. For example, to sort Strings, we could say that the “smaller” one is the one that would come first in the dictionary (“alphabet” “<” “zebra”).

In Java, how do we say that a particular type can be compared? This is exactly what the Comparable<T> interface describes. When a type implements Comparable<T>, we say that it is “able to be compared” to objects of type T. Usually, T is the same type (that is, you’ll usually see class MyClass implements Comparable<MyClass>), but it doesn’t have to be.

The only method required by Comparable<T> is compareTo(T o) which takes another object of the type T and returns an int whose value represents whether this or o should come first.

In order to sort a list in Java, most sorting algorithms will call compareTo and make pairwise comparisons to determine which object should come first, repeatedly, and swap elements until the entire list is sorted. (The hard part of sorting, then, is to determine which compareTo ‘questions’ are necessary to ask!)

Here are a few key details from compareTo, slightly adapted:

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer if this object is less than, equal to, or greater than the specified object, respectively.

There are other requirements that typically happen naturally with a “reasonable” implementation, but are still important to specify:

The implementor must also ensure that the relation is transitive: (x.compareTo(y) > 0 && y.compareTo(z) > 0) implies x.compareTo(z) > 0.

It is strongly recommended, but not strictly required that x.compareTo(y) == 0 is equivalent to x.equals(y). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is “Note: this class has a natural ordering that is inconsistent with equals.”

Why do we care about making things comparable?
This means that we can implement data structures that require ordering or comparison (like sorted lists, and in the future, search trees). We would say “this collection can only contain types that are Comparable to themselves”.
What if we want to compare things that don’t implement Comparable, or want to compare things in a different way?
The compareTo method defines an object’s “natural order”. However, a type may not have a “natural order”, or we may want to order it in a different way (for example - ordering people by their height, name, or age). We can instead use the Comparator<T> interface to impose our own ordering on objects. We can get a Comparator either by directly implementing the interface, or by using Java’s higher-order functions (out-of-scope).

Exercise: Comparing Users #

In User.java, we’ve provided an example of how a website might model a user.

Make User implement the Comparable interface. Use parameterization (ie. <>) with Comparable to ensure that User can only be used to compare against other Users.

The natural ordering for User is to compare by ID number. If their ID numbers are the same, then compare them on their username.

After implementing this, you should be able to sort Users. The example below is also in the main method of your User class. Feel free to run it as a sanity check.

public static void main(String[] args) {
    User[] users = {
        new User(2, "Erik", ""),
        new User(4, "Vanessa", ""),
        new User(5, "Natalia", ""),
        new User(1, "Alex", ""),
        new User(1, "Circle", "")
    for (User user : users) {
User{id=1, name=Alex, email=}
User{id=1, name=Circle, email=}
User{id=2, name=Erik, email=}
User{id=4, name=Vanessa, email=}
User{id=5, name=Natalia, email=}

Note that here we use Arrays.sort because users is an array; if it was a Java Collection like ArrayList, we would use Collections.sort.

Iteration #

In CS 61BL, we’re going to encounter a variety of different data structures, or ways to organize data. We’ve implemented linked lists like SLList and DLList, and a couple different sets. Starting next Friday, we’ll start to see more complicated data structures such as trees, hash tables, heaps, and graphs.

A common operation on a data structure is to process every item it contains. But often, the code we need to write to setup and iterate through a data structure differs depending on the data structure’s implementation.

For an array, you might iterate over it like this:

int[] array = ...
for (int i = 0; i < array.length; i += 1) {
    // Do something with array[i]

For SLList, the pattern significantly differs from above.

SLList list = ...
for (IntNode p = list.sentinel.next; p != null; p = p.next) {
    int item = p.item;

Evidently, we need to write two very different codes in order to do the same high-level thing. It would be nice if we can write one piece of code that we can reuse for different things that we can iterate over. In other words, we wish to abstract away the internal implementation of data structures from the operations on them.

Furthermore, if we use a different data structure, a for loop like the one above may not make sense. For example, what would it mean to access the kth item of a set, where the order of items is not defined? We need a more abstract notion of processing every item in a data structure, something that allows us to check every item regardless of how they’re organized.

To do that, we’re going to define the idea of a data structure being iterable.

Iterable #

The interface that lets us say that something can be iterated over is called Iterable<T>:

public interface Iterable<T> {
    Iterator<T> iterator();

The generic parameter T indicates the type of the elements that we visit while iterating. For example, for the SLList that contains only ints, we would write:

public class SLList implements Iterable<Integer>

Similarly, a generic list would implement public class MyList<T> implements Iterable<T>.

Since a Java Collection is a group of objects, it makes sense that we would like to iterate over those objects. Therefore, Collection<T> is a sub-interface of Iterable<T>.

Let’s now consider the return type, Iterator<T>.

Iterator #

Remember how everything is an object in Java? If Iterable is the “thing that can be iterated over”, then Iterator is “where we are during iteration”.

As an analogy, think of a walking trail, with specific waypoints along the trail (like a list). The walking trail is Iterable, because we can visit each waypoint. Imagine that we have a person walking along the trail. The person and where they are on the trail is an Iterator. We can have multiple people on the trail, just like we can have multiple iterators.

Since the “state” that Iterator needs to keep track of will likely be different for each class, it’s an interface as well:

package java.util;
public interface Iterator<T> {
    boolean hasNext();
    T next();
    // Some methods not shown
hasNext is a boolean method that says whether there are any more remaining items in the iterable to return. In other words, returns true if next() would return an element rather than throwing an exception. In our analogy, this returns true if the walker is not at the end of the trail.
next successively returns items in the iterable one by one. The first call to next returns a value, the second call to next returns another value, and so on. If you’re iterating over a set – a data structure that does not necessarily have an order – we don’t necessarily guarantee that next returns items in any specific order. However, what we do guarantee is that it returns each item in the iterable exactly once. If we were iterating over a data structure that does have an ordering, like a list, then we would also like to guarantee that next returns items in the right order. In our analogy, this causes the walker to “visit” a waypoint on the trail.

Every call to next() is typically preceded by a call to hasNext(), thus ensuring that the Iterator does indeed have a next value to return. If there are no more elements to remaining, it is common practice to throw a NoSuchElementException.

Why design two separate interfaces, one for iterator and one for iterable? Why not just have the iterable do both?
The idea is similar to Comparable and Comparator. We can provide a ‘default’ iterator, but also allow for other iterators. For example, we could implement an iterator that skips every other element, visits each element, twice, or skips elements that return false for some condition.

Enhanced for Loop #

You may have been using the idea of a data structure being iterable already! When Java executes an enhanced for loop (the one using a colon), it does a bit of work to convert it into iterators and iterables. The following code represents the enhanced for loop you have most likely already seen and then a translated version which reveals what is happening behind the hood using an iterator.

List<Integer> friends = new ArrayList<>();
for (int x : friends) {
List<Integer> friends = new ArrayList<>();
Iterator<Integer> seer = friends.iterator();
while (seer.hasNext()) {
    int x = seer.next();

SLListIterator #

Here’s an example of implementing Iterable for SLList:

public class SLList implements Iterable<Integer> {
    /* The first item (if it exists) is at sentinel.next. */
    private IntListNode sentinel;
    private int size;

    // Constructor and other methods...

    public Iterator<Integer> iterator() {
        return new SLListIterator();

    // We can define "inner classes" that have access to the outer class's
    // variables. Since this isn't a static class, it's tied to a particular
    // instance of SLList and can access its instance variables.
    private class SLListIterator implements Iterator<Integer> {

        // For example, here we access the outer class's sentinel node.
        private IntListNode curr = sentinel.next;

        public Integer next() {
            // Check if we're out of items here
            if (!hasNext()) {
                throw new NoSuchElementException();
            int toReturn = curr.item;
            curr = curr.next;
            return toReturn;

        public boolean hasNext() {
            return curr != sentinel;

The code maintains an important invariant: prior to any call to next, curr contains the index of the next value in the list to return.

We can then use our SLList class in an enhanced for loop.

SLList friends = SLList.of(5, 23, 42);
Iterator<Integer> seer = friends.iterator();
while (seer.hasNext()) {
    int x = seer.next();
SLList friends = SLList.of(5, 23, 42);
for (int x : friends) {

Designing Iterators #

Often, when writing our own iterators, we’ll follow a similar pattern of doing most of the work in next.

  1. We save the item to output with int toReturn = curr.item;.
  2. Move the current state to the next item with curr = curr.next.
  3. Return the item we saved earlier.

An important feature of the code is that hasNext doesn’t change any state. It only examines existing state by comparing the progress of the iteration to the number of list elements. hasNext can then be called any number of times in a row and nothing should change, or it could be called not at all and the iteration should still work as long as there are elements left to be returned.

Discussion: Iterator Invariants #

Consider the following SLListIterator, slightly different from those we just encountered.

private class SLListIterator implements Iterator<Item> {
    private IntListNode curr = sentinel;

    public Integer next() {
        curr = curr.next;
        return curr.item;

    public boolean hasNext() {
        return curr.next != sentinel;

Now, discuss the following questions with your partner:

  1. What’s the invariant relation that’s true between calls to next?
  2. In general, most experienced programmers prefer the organization introduced first over this organization. What might explain this preference? Think about both writing the iterator, and debugging it while it’s in use.

Finally, let’s consider some questions about the order in which methods may be called on an Iterator:

What if someone calls next when hasNext returns false?
This violates the iterator contract so the behavior for next is undefined. Crashing the program is acceptable. However, a common convention is to throw a NoSuchElementException.
Will hasNext always be called before next?
Not necessarily. This is sometimes the case when someone using the iterator knows exactly how many elements are in the sequence. For this reason, we can’t depend on the user calling hasNext when implementing next, and don’t typically change any state in hasNext.

Exercise: AListIterator #

As mentioned before, it is standard practice to use a separate iterator object (and therefore a separate, typically nested class) as the actual Iterator. This separates the Iterator from the underlying data structure or iterable.

Modify the provided AList (array-backed list) class so that it implements Iterable<Item>. Then, add a nested AListIterator class which implements Iterator<Item>. Note that if you submit to the autograder before you implement this, your code likely will say that there are compilation errors coming from the autograder tests (you will see errors like “error: cannot find symbol” for calls to a.iterator or similar). Once you have properly completed this, the errors should go away. Likewise, if you want to test locally, you’ll need to uncomment the test method in AListTest.java, and make sure it doesn’t have compilation errors.

Note that AList itself does not implement Iterator. This is why we need a separate, nested, private class to be the iterator. Typically, this class is nested inside the data structure class itself so that it can access the internals of the object that instantiated the instance of the nested class. See SLList above for an example. Make sure that you’ve completed the following checklist.

  1. Does your AList object know anything about its AListIterator’s state? Information about iteration (index of the next item to return) should be confined to Iterator alone.
  2. Are multiple Iterators for the same AList object independent of each other? There can be multiple Iterators for a single AList object, and one iterator’s operation should not affect the state of another.
  3. Does hasNext alter the state of your Iterator? It should not change state.
  4. If there are no more elements left in the Iterator and the user tries to call next(), throw a NoSuchElementException with the line throw new NoSuchElementException();

After you have modified your AList class, write some test code to see if Java’s enhanced for loop works as expected on your AList.

Concurrent Modification #

For our lab, we regarded our data structure to be “frozen,” while the Iterator was at work. In other words, we assumed that while we were operating on the iterators, the data structure would remain as is. However, this is not generally true in the real world.

ArrayList<BankAccount> accounts = ...;
Iterator<BankAccount> it = accounts.iterator();
while (it.hasNext()) {
    // Remove the next account!
    checkValidity(it.next()); // Wait, what?

If all clients of the data structure were to only read, there would be no problem. However, if any were to modify the data structure while others are reading, this could break the fundamental invariant that next returns the next item if hasNext returns true!

To handle such situations, many Java iterators throw ConcurrentModificationException if they detect that the data structure has been externally modified during the iterator’s lifetime. This is called a “fail-fast” behavior.

Your main takeaway from this section should be that modifying data structures while iterating over them is dangerous!

Error-Handling #

Above, we mentioned NoSuchElementException and ConcurrentModificationException. We’ve also (probably) seen NullPointerException and ArrayIndexOutOfBoundsException, among others before. These are errors, but why does Java stop the entire program when it hits an error, and is there any way to avoid it?

So far in this course, we have not dealt much with error-handling. You were allowed to assume that the arguments given to methods were formatted or structured appropriately. However, this is not always the case due to program bugs and incorrect user input. Here are a few examples of this:

  1. Bugs in your programs might create inconsistent structures or erroneous method calls (e.g. division by zero, indexing out of bounds, dereferencing a null pointer).
  2. Users (or the outside world in general) cannot be trusted to give valid input (e.g. non-numeric input where a number is required or search failures where a command or keyword was misspelled).

We assume in the following discussion that we can detect the occurrence of an error and at least print an error message about it.

A big challenge in dealing with an error is to provide information about it at the right level of detail. For instance, consider the error of running off the end of an array or list. If the contents of a list are inconsistent with the number of elements supposedly contained in the list, you might end up trying to “reference through a null pointer” or “index out of bounds”. What should happen in this case?

Discussion: Error Handling #

The programmer may wish to pass information about the error back to the caller method with the hope that the caller can provide more appropriate and useful information about the root cause of the error and perhaps be able to deal with the error. However, this may be difficult.

Here are three approaches to error handling:

  • Don’t try to pass back any information to the caller at all. Just print some kind of error message (hopefully a useful one?) and stop the entire program.
  • Detect the error and set some global error indicator (like a public static variable in Java) to indicate its cause.
  • Detect the error and directly “return” the error information. This typically is handled with a particular return type that indicates a possible error, or by passing in a mutable argument that can be set to indicate the error.

Different languages geared towards solving different types of problems take different approaches to error handling. Some newer languages, such as Go, and Rust, for example, support a design similar to the third option.

Which seems most reasonable? Discuss with your partner, and defend your answer. If none, justify why all the options are bad.

Exceptions #

There is a fourth option for handling errors, called an exception. Provided by Java and other modern programming languages, including C++ and Python, an exception signals that an error of some sort has occurred. Java allows both the signaling of an error and selective handling of the error. Methods called between the signaling method and the handling method need not be aware of the possibility of the error.

An exception is thrown by the code that detects the exceptional situation, and it is caught by the code that handles the problem, if any.

Read Chapter 6.2 of the online textbook to learn more about exceptions.

To manually throw an exception, we use the throw keyword, along with the exception instance we’re throwing:

throw new RuntimeException("yeet");  // Ideally you'll write better error messages...

If we want to do anything with the exception, such as gracefully continuing the program, or printing a better error message, we’ll need to catch it with a try catch block.

try {
    // code that might throw an exception
} catch (IOException e) {  // We have to catch a specific type of exception
    // Let's handle the exception somehow.

Different exceptions will have different constructors. We can also define our own exception classes, but this is out of scope.

An extension to the try catch block construct that often comes in handy is the finally block. A finally block comes after the last catch block and is used to do any cleanup that might be necessary, such as releasing resources the try block was using. This is very common when working with input-output like opening files on your computer.

Scanner scanner = new Scanner(System.in);
int k;
try {
    k = scanner.nextInt();
} catch (NoSuchElementException e) {
    // Ran out of input
} catch (InputMismatchException e) {
    // Token isn't an integer
} finally {
    // finally will be executed as long as JVM does not exit early

This use of the finally block so common that the Java language developers introduced the try-with-resources block. It allows you to declare resources being used as part of the try block, and automatically release those resources after the block finishes executing. The code below is equivalent to the snippet above, but it doesn’t use the finally block.

int k;
try (Scanner scanner = new Scanner(System.in)) {
    k = scanner.nextInt();
} catch (NoSuchElementException e) {
    // ran out of input
} catch (InputMismatchException e) {
    // token isn't an integer

Even though we’ve presented exceptions last, this is solely because Java uses them as its error-handling mechanism. This shouldn’t be interpreted as “exceptions are the best method of error-handling”.

Exceptions, similar to the other methods, of error-handling, have benefits and drawbacks. What are some of these benefits and drawbacks?

Deliverables #

Here’s a quick recap of the tasks you’ll need to do to complete this lab:

  • Make the User class implement Comparable.
  • Make AList implement Iterable, as well as adding your iterator class to the AList.java file.

Additionally, you’ll need to work with exceptions in Gitlet, so understanding those will be helpful as well.

Be sure to submit to Gradescope and add your partner if you have one!