h01: HFJ 3,4: Primitives, References, Instance Variables, Methods

When a homework assignment is due in lab (i.e. discussion section), it is, technically, due at the start of your assigned section time. However, as a courtesy, we’ll allow you to turn it in during any lab section (so if you forget to bring it, but you can get it to us by the start of the last lab section of the day, that’s fine.)

Reading Assignment:

• Read (especially pages 59-62) and online reading notes.
• Read and online reading notes.
  • If you don’t have your book yet, buy it! But in the meantime you can read this either at the UCSB library, or online.
  • Visit the course wiki at http://foo.cs.ucsb.edu/56wiki and you’ll find the link to the online textbook.
• As you read, also consult the online reading notes. To find them, go to the online version of this homework, at /hwk/h01/ and click the links.
• Then, do the problems below.

1. (10 pts) Please fill in the information at the top of this homework sheet, including your name and umail address. Put the time your discussion section starts (2pm, 4pm or 5pm) in the space indicated (the one you are registered for—even if you usually attend a different one.) If the other two items apply, please fill them in as well. Please do this every single time you submit homework for this class.
2. Based on your reading in :

   1. (5 pts) In Java, a variable can store a primitive or a reference. Briefly: What’s the difference?

   2. (5 pts) If I write 3.4, is that of type double, or float?

   3. (5 pts) Declare x as a double and assign it the value 3.4 (as a double)

   4. (5 pts) Declare y as a float and assign it the value 3.4 (as a float)

3. (5 pts) In C++, the name of a plain old array of Student objects is not an object, but is rather a pointer to a Student (i.e. it is of type Student *. What about in Java—is an array an object, yes or no?

4. Variables that represent a primitive type (e.g. boolean x; or int y;) and variables containing object references (String w; or Student z;) have this in common—they are both composed of bits in memory.

   But, as explained in , they differ in what the bits ‘‘actually’’ represent. You won’t get this one by just guessing—you really have to read the book.

   1. (5 pts) What do the bits that represent int y; represent?

      Assume that y is assigned the value 13

   2. (5 pts) What do the bits that represent String w; represent?

      Assume that w is assigned the value "foo".

5. Based on your reading in , p. 59-62 and p. 84:

   1. (10 pts) Suppose I have a class called Student.
      How do I declare and allocate space for a plain old Java array called students that can hold 5 references to Student objects?

   2. (10 pts)

      Java for loops look pretty much just like C++ for loops (see HFJ page 10 if you really need to check.) Given that, assuming there is a default constructor Student() that you can call to create a new Student object, write a for loop that initializes all of the elements of the array students (from the previous problem) to be instances of the Student> class.

6. Consider these questions about memory—answers are in

   1. (5 pts) Does the amount of memory taken up by an object reference differ for different kinds of objects (say String vs. ArrayList<String>?)

   2. (5 pts) Does the amount of memory taken up by the object itself differ for different kinds of objects (assuming the same JVM)?

   3. (5 pts) Can the amount of memory taking up for an object reference for a object particular type (say String) differ from one JVM to another?

7. discusses the difference between the == operator and the .equals method.

   1. (10 pts) Under what circumstances should you use the == operator to compare two variables?

   2. (10 pts) Under what circumstances should you use the .equals method to compare two variables?