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CS156

HFJ3: Chapter 3—

The big picture

A good deal of what you already know from C/C++ programming will carry over directly into Java.

But one new thing that you must understand to be an effective Java programer is the difference between primitive vs. reference variables.

This chapter also talks about Java memory management, which is one of the specific learning objectives in the official USCB course description for CMPSC56, so it is one of the things that I must put on the exams in this course in order to make the University happy. And since they pay my salary, and these are times of budget cuts, I definitely want to keep them happy. So if you want a good grade in this course, you would be well advised to find and read all the places in this chapter that the authors talk about “garbage-collection” and “the heap”.

By Section

Declaring a variable

As you progress in Computer Science, you’ll learn more about the nature of programming languages. In the upper division of the UCSB BS CS degree, there are two courses: CMPSC160 and CMPSC162 that cover how to write a compiler/interpreter and how to design a programming language. One of the things we want to do in CMPSC 156 is start to plant the seeds that will blossom into full understanding in these later courses.

So, we’ll talk about some things in this course that may seem obvious—like the idea that a variable has a name and a type. Why is this important, when it’s so bleeping obvious? Because, as it turns out, “name” and “type” are just the first two things on a much longer list of “stuff a variable has”.

So, I love that this page is highlighting that variables have a name and a type—because this lays the foundation for a much longer list of things that a variable has (e.g. scope, lifetime, size, address, storage class, just to name a few). The book also highlights the important concept of type-safety, which is something we’ll come to appreciate about Java.

If you really want to learn this stuff deeply, do your own hunting around on the web and see if you can find some explanations of these other aspects of variables, and perhaps even some that I left out, along with explanations of each.

You might post some of this stuff on the #general channel on the Slack and get a discussion started.

Also see if you can find an explanation of type-safety and include a similar explanation of that as well.

As you get into how to write the source code for a compiler/interpreter for a given language, you have to begin to think about all of those aspects of variables explicitly, where before you may have taken them for granted.

And, as you get into the more fundamental science of designing new programming languages, this becomes even more important.

The fact that the authors make reference to these important principles of Computer Science is part of why I say that the Head First books are not “for dummies” books, but rather “for very intelligent people that want to learn deeply” books.

This section also mentions primitive vs. object references again—so I hope you getting the message that this is a big deal.

“I’d like a double mocha, no, make it an int.”

We see here the eight primitive types. I’m not a bit one for memorizing things (because you usually just forget them later), but there is short list of things worth memorizing if you are going to be working in Java, and that list includes the eight primitive types.

This page illustrates them very nicely, grouping them into three easy groups for convenience. (Note: four integer types, two floating point types, and two non-numeric types.)

(As an aside, for any programming language X, where X could be Java, JavaScript, Python, or any number of other languages that distinguish between primitive types and objects—memorizing the list of primitive types is a good thing to do when first learning the language).

One other detail buried on this page is “how do you make a float literal, as opposed to a double literal?”. See if you can find that. Maybe I should make that a homework question.

Write the following declarations/assignments. Be sure to pay attention to the types of the literals on the right hand side.

  • If I write 3.4, is that of type double, or float?
  • Declare x as a double and assign it the value 3.4 (as a double)
  • Declare y as a float and assign it the value 3.4 (as a float)

You really don’t want to spill that…

Here’s more of the stuff you have to think about when designing or learning a new programming language—what happens when you write an assignment statement, and the right and left sides have different types?

Something like the “Sharpen Your Pencil” exercise on p. 52 would make a good exam question. Keep in mind that all of these would appear in a single method, one right after the other.

Back away from that keyword!

Most programming languages have reserved words. It is interesting that Java has const and goto as reserved words even though they have no meaning in Java.

Its also interesting that variables in Java can start with the $, and the $ can be a symbol inside a variable as well.

This table reserved

A handy table of the reserved words in Java.

Controlling your Dog object

This page gets to the question “what is a reference”?

Another way to think of a reference is as a pointer, but one that you:

  • Can’t print the literal value of
  • Can’t do math on
  • You can only dereference it.

And the way you dereference it is just to use the dot operator.

In C/C++, we have an explicit dereference operator (‘’). The dot operator is used for fields in a struct, data members of an object, or member functions of an object (instance of a class). And, if you need to use the ‘’ and the ‘.’ together, we use the -> operator.

But in Java, the ONLY thing you can do with a reference is dereference it, so there IS not dereference operator—the deference is always implied every time you use the reference variable. So to access things inside an object (instance variables, or member functions, which we call methods in Java), we always use the dot operator (.).

That’s why you’ll never see a (*) used as a dereference operator in Java, and you’ll never see -> used as an operator either.

This page also talks about the heap, and the fact that ALL OBJECTS LIVE ON THE HEAP. The reference variables that point to the objects might live on the stack, but the objects themselves always live on the heap.

This page also establishes an important distinction between what is stored inside a primitive variable and what is stored inside an object reference—and that makes a good exam question:

A variable representing a primitive type (e.g. boolean x; or int y;) and a variable containing an object reference (String w; Student z;) have this in common—they are both composed of bits in memory.

But they differ in what the bits represent:

  • What do the bits that represent int y; represent?
  • What do the bits that represent String w; represent?

An object reference is just another variable value

There’s a lot of great stuff in this section. Read it all.

And, there are two things I want to particularly highlight on this page:

(1) The three parts of Dog myDog = new Dog();

Make sure you understand what each part of that means.

(2) The footnote at the bottom of the first column, which contains a joke about “the phase of the moon”. Clearly, the authors are making a funny here—the number of bits in a reference variables has nothing to do with the phase of the moon. What is the larger point that the authors are trying to make with this joke? (The rest of the text on pages 54–55, and the “there are no dumb questions” section on p. 56 may help you figure this out if the point is still lost on you.)

Make sure you read the ‘There are no dumb questions’ part of this page—it makes an important point about memory management in Java.

  • Does the amount of memory taken up by an object reference differ for different kinds of objects (say String vs. ArrayList<String>?)
  • Does the amount of memory taken up by the object itself differ for different kinds of objects?
  • Can the amount of memory taking up for an object reference for a object particular type (say String) differ from one JVM to another?

Java Exposed

Read the interview with the object reference. Although this is corny, it introduces the idea of “final” and the idea of “null”, which are both important concepts dealing with object references.

Life on the garbage-collectible heap

This is an important concept to understand.

Question 4 on this Sample Midterm Exam from Summer 2018 can be used to test whether you have the concept or not.

Life and death on the heap

Here’s another sample exam question of the same type: Question 1 on This Sample Final Exam from Winter 2018

An array is like a tray of cups

A key idea here is that an array is always an object, even if it is an array of primitives.

Arrays are objects too

A key idea here is that when you have an array of object references, creating the array and creating the objects that the elements of the array point to are two separate steps.

Make an array of Dogs

Be sure that you understand that you can’t really create an array of Dogs all at once in Java; you can only create an array of Dog references; instantiating each Dog has to be done separately.

Control your Dog (with a reference variable)

Pay attention to the syntax differences between how this is done in Java vs C++.

What happens if the Dog is in a Dog array?

Pay attention to the syntax differences between how this is done in Java vs C++.

A Dog example

Pay attention to the syntax differences between how this is done in Java vs C++.

Bullet Points

Be sure to go over the “Bullet Points” at the end of each chapter; they are a useful summary of the main points.