1. CS 280 Data Structures Professor John Peterson

2. Phase 2 We’re moving on from array-based to link- based structures. You already have experience with links but we’ll be doing some new things and exploring some advanced Java stuff as we beat linked structures to death …

3. Aliasing What is aliasing? The result of shared structure in your program. Many, many program bugs involve aliasing – we need to understand this thoroughly!

4. A Quiz class Link { public T data; public Link link = null; } a = new Link(); b = a; a.data = 1; b.data = 2; System.out.print(a.data);

5. Object Identity The big deal here is that we HAVE to understand the basic idea behind OO programming: identity. Java objects are divided into two classes: Primitive types (values only) Normal objects (objects with identity) What are the differences?

6. Types of Objects Primitive Compare values with == Never need to say “new” (no constructor) Fixed vocabulary Passed by value A “boxed type” is available – implicit coercion x = y copies value No internal structure Default depends on type Ordinary == compares identity, not values Always need to say “new” to make a value Can add user-defined classes Passed by reference x = y creates alias Contains instance vars Default is null

7. Representing Identity In the debugger, we see “Object numbers” to convey the idea of identity. In class, we deal with boxes and arrows. A box is an ordinary object A value (like 1.2 or null) is a primitive object – written as an ordinary value An object is represented by a arrow leading to a box Instance vars are components in a box Variables are names with either values or pointers

8. Example 5 null 7 ab Link Data

9. Example 5 null 7 ab Link Data Link a = new Link () a.data = b Link b = new Link () b.data = 7 a.link = b