Lecture Code #

Code from this lecture available at

https://github.com/Berkeley-CS61B/lectureCode-fa22/tree/main/lec11_inheritance4.

Exceptions #

Most likely you have encountered an exception in your code such as a NullPointerException or an IndexOutOfBoundsException. Now we will learn about how we can “throw” exceptions ourselves. Here is an example of an exception that we throw:

throw new RuntimeException("For no reason.");

This is useful to ensure reasonable functioning of our code, even when facing unexpected behavior.

Iterators and Iterables #

These two words are very closely related, but have two different meanings that are often easy to confuse. The first thing to know is that these are both Java interfaces, with different methods that need to be implemented. Here is a simplified interface for Iterator:

public interface Iterator<T> {
  boolean hasNext();
  T next();
}

Here is a simplified interface for Iterable:

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

Notice that in order for an object (for example an ArrayList or LinkedList) to be iterable, it must include a method that returns an iterator. The iterator is the object that actively steps through an iterable object. Keep this relationship and distinction in mind as you work with these two interfaces.

toString #

The toString() method returns a string representation of objects.

This is most helpful when we are debugging, as it allows us to much more easily understand the current state of our Objects.

== vs .equals #

We have two concepts of equality in Java- “==” and the “.equals()” method. The key difference is that when using ==, we are checking if two objects have the same address in memory (that they point to the same instance or object). On the other hand, .equals() is a method that can be overridden by a class and can be used to define some custom way of determining equality. This permits the class to utilize the additional knowledge it has about itself to more accurately answer questions of equality.

For example, say we wanted to check if two stones are equal:

public class Stone{
  int weight;
  public Stone(int weight){
    this.weight = weight;
  }
}
Stone s = new Stone(100);
Stone r = new Stone(100);

If we want to consider s and r equal because they have the same weight. If we do check equality using ==, these Stones would not be considered equal because they do not have the same memory address.

On the other hand, if you override the equals method of Stone as follows

public boolean equals(Object o){
  return this.weight == ((Stone) o).weight
}

We would have that the stones would be considered equal because they have the same weight.

Exercises #

Factual #

1. What are some examples of exceptions you’ve used directly before? Hint, anything in your test files?
2. What methods are required for a class that is Iterable?
3. The check-in exercises, linked here

Conceptual #

1. Why do we want to override the .equals method?
2. What are shortcomings of the implementation provided above for the .equals() method for the Stone class?

Procedural #

1. Implement a .toString() method for your Project 1 Deque implementations.
2. Implement an updated version of .equals() for the Stone class which addresses some of the weaknesses you noted above.

Metacognative #

1. In lecture, you built the ArraySet iterator with the professor. Modify the lecture class to include an iterator that takes in a user value when initially defined (when the constructor is called). Your iterator should only provide items which are “greater than” the user defined value. Hint, do we have to ensure our generic type can be compared?

2. Spring 2018 Midterm 2 Question 7