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Recursion and the Method Call Stack

In Chapter 6, the stack data structure was introduced in the context of understanding how Java performs method calls. We discussed both the method call stack (also known as the program execution stack) and activation records. In this section, we will use these concepts to demonstrate how the program execution stack handles recursive method calls.

Let us begin by returning to the Fibonacci examplespecifically, calling method fibonacci with the value 3, as in Fig. 15.7. To more easily demonstrate in what order the activation records for the method calls are placed on the stack, we have lettered the method calls, as shown in Fig. 15.8.

Figure 15.8. Method calls made within the call fibonacci ( 3 ).

When the first method call (A) is made, an activation record is pushed onto the program execution stack which contains the value of the local variable number (3, in this case). The program execution stack, including the activation record for method call A, is illustrated in part a of Fig. 15.9. [Note: In an actual computer, the program execution stack and its activation records would be more complex than in Fig. 15.9, containing such information as where the method call is to return to when it has completed execution. We use a simplified stack to demonstrate how the program execution stack works.]

Figure 15.9. Method calls on the program execution stack.

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Within method call A, method calls B and E are made. The original method call has not yet completed, so its activation record remains on the stack. The first method call to be made from within A is method call B, so the activation record for method call B is pushed onto the stack on top of the activation record for method call A. Method call B must execute and complete before method call E is made. Within method call B, method calls C and D will be made. Method call C is made first, and its activation record is pushed onto the stack (part b of Fig. 15.9). Method call B has not yet finished, and its activation record is still on the method call stack. When method call C executes, it does not make any further method calls, but simply returns the value 1. When this method returns, its activation record is popped off the top of the stack. The method call at the top of the stack is now B, which continues to execute by performing method call D. The activation record for method call D is pushed onto the stack (part c of Fig. 15.9). Method call D completes without making any more method calls, and returns the value 0. The activation record for this method call is then popped off the stack. Now, both method calls made from within method call B have returned. Method call B continues to execute, returning the value 1. Method call B completes and its activation record is popped off the stack. At this point, the activation record for method call A is at the top of the stack and the method continues its execution. This method makes method call E, whose activation record is now pushed onto the stack (part d of Fig. 15.9). Method call E completes and returns the value 1. The activation record for this method call is popped off the stack, and once again method call A continues to execute. At this point, method call A will not be making any other method calls, and can finish its execution, returning the value 2 to A's caller (Fib(3) = 2). A's activation record is popped off the stack. Note that the current method executing is always the method whose activation record is at the top of the stack, and that the activation record for that method contains the values of the method's local variables.

Introduction to Computers, the Internet and the World Wide Web

Introduction to Java Applications

Introduction to Classes and Objects

Control Statements: Part I

Control Statements: Part 2

Methods: A Deeper Look

Arrays

Classes and Objects: A Deeper Look

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

GUI Components: Part 1

Graphics and Java 2D™

Exception Handling

Files and Streams

Recursion

Searching and Sorting

Data Structures

Generics

Collections

Introduction to Java Applets

Multimedia: Applets and Applications

GUI Components: Part 2

Multithreading

Networking

Accessing Databases with JDBC

Servlets

JavaServer Pages (JSP)

Formatted Output

Strings, Characters and Regular Expressions

Appendix A. Operator Precedence Chart

Appendix B. ASCII Character Set

Appendix C. Keywords and Reserved Words

Appendix D. Primitive Types

Appendix E. (On CD) Number Systems

Appendix F. (On CD) Unicode®

Appendix G. Using the Java API Documentation

Appendix H. (On CD) Creating Documentation with javadoc

Appendix I. (On CD) Bit Manipulation

Appendix J. (On CD) ATM Case Study Code

Appendix K. (On CD) Labeled break and continue Statements

Appendix L. (On CD) UML 2: Additional Diagram Types

Appendix M. (On CD) Design Patterns

Appendix N. Using the Debugger

Inside Back Cover





Java(c) How to Program
Java How to Program (6th Edition) (How to Program (Deitel))
ISBN: 0131483986
EAN: 2147483647
Year: 2003
Pages: 615
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