Lab 5

OOP with Collections, Iterators, and Iterables

Lab spec: https://inst.eecs.berkeley.edu/~cs61b/sp22/materials/lab/lab5/index.html

Welcome to lab 5! I won't be covering Object Oriented Programming or the hierarchical structure of the Java Standard Library in this guide. Instead, I'll be focusing on Iterators and the Iterable interface.

If you're wondering where lab 4 went, there was no lab 4 assignment! The section was converted to project office hours.

Iterables and Iterators

A lot of students get confused between Iterables and Iterators when first seeing them -- I know I was confused too. Let me use an analogy:

Here we have a vase and a hammer. From real world experience, we know that vases are quite fragile -- a property which I will call Breakable. Vases aren't the only Breakable objects, and plates, glass cups, and eggs could all be considered Breakable.

On the other hand, a hammer is an object that breaks Breakable objects. We could call such an object a Breaker. Breakers could also include sledgehammers or wrecking balls.

We observe the relationship between these two types as: Breaker objects break Breakable objects. Similarly, Iterator objects iterate over Iterable objects.

TableFilter

Before you can complete this section, it's crucially important that you understand how to read and understand skeleton code. The lab spec will help you greatly in figuring out what each method does -- it's your job to figure out which methods are relevant to whatever you're doing.

For the following filters that you're asked to complete, you'll find that they are all extremely similar. I would recommend starting out with the ColumnMatchFilter, as it will teach you the most. At this point, I recommend you go and try to figure out the answer by yourself. I will provide hints in increasing level of specificity, but you will learn a lot more if you figure it out yourself. If you get stuck, feel free to come back and look at a tip or two before moving on.

I have no idea what to do

Let's work backwards. We need to fill in the keep() method so that it keeps only the rows that match our criteria. In doing so, we need to:

1. Get the values of interest from the Table(where should this happen?)

2. Somehow get those values into keep()

3. Make comparisons to determine whether or not the TableRow should be kept

What exactly does keep() do, anyway?

Many students have a misconception that the keep() method will apply to the entire table and try to use for or while loops. Stop! You don't need to use any loops while doing this assignment. Take note of the return type of keep(): boolean. keep() decides, for a given TableRow, if it should be kept or filtered out. How it gets that particular TableRow is of no concern and is abstracted away.

How do I get values of interest into keep()?

Consider using instance variables :)

How do I get the next TableRow / I'm getting the TableRow correctly but can't seem to access it twice?

Look at TableFilter.java -- is there a method that returns a TableRow, and more specifically the next TableRow that we want? Using this method over a next() call will also ensure that we don't advance the iterator when we don't want to.

JoinIterator

Many students are confused about what exactly this JoinIterator is doing. The JoinIterator actually consists of two iterators, one for each table, and is constructing the joined table. You can see an example below.

The JoinIterator iterates through table B for each item in table A.

For extra programming help, take a look at the JoinIterator constructor.

Table.java

private JoinIterator(Table t1, Table t2) {
    _table2 = t2;
    _tableIter1 = t1.iterator();
    _tableIter2 = t2.iterator();
    _currRow1 = _tableIter1.next();
}

Why did the skeleton code include these specific variables as instance variables? Why was _table2 included, but not _table1? Is it possibly, to construct a new iterator when necessary? This then begs the question, when do we need to create a new iterator? As a final question, how do we know when we are done iterating through both tables?

Answering these questions will put you well on the path to completing this assignment. While doing so, remember that hasNext() is concerned only with whether there is a next row to iterate over, or not.