Stack and Heap Diagram — 3. References

Conceptual Advantage

Mapping

PL	NM
...	(all aspects of 1 - Objects and 2 - Stack Frames)
reference value	arrow from a variable rectangle (anywhere, i.e., in the stack or the heap) to an object rounded rectangle
null value	a ground symbol (i.e., the absence of an arrow)

Draws Attention To

References.

Use When

Introducing the concept of references (pointers), and aliasing (multiple references pointing to the same object).

Cost

Straightforward; just draw an arrow.

Details

We use this notional machine in instruction as well as assessments (e.g., clicker questions, mastery checks, exams). For instruction, the instructor draws the representation. For assessment, the student draws the representation.

The name of this notional machine is inspired by two of the three regions that make up the memory of the runtime environment of imperative programming languages:

• Stack: where stack frames are allocated that hold local variables and parameters of functions/procedures/methods
• Heap: where dynamically allocated memory (e.g., objects and arrays in Java) is located

The third area, “globals”, could be added to the diagram, but it is less important when using an object-oriented style where static members are less prevalent.

• Globals: where global variables (e.g., static fields in Java) are located

While we primarily use this notional machine for Java, it could easily be adapted for other imperative object-oriented programming languages, such as C# or Python.

Notation

Objects, Stack Frames, and References

Arrays

Origins of Notation

The notation for this notional machine is based on how the BlueJ IDE represents objects, with a few differences:

• BlueJ only shows objects, we also show stack frames (as rectangles)
• Unlike BlueJ, we show references as arrows
• Unlike BlueJ, we refrain from associating names with objects, to avoid the ObjectsMustBeNamed misconception.
• We represent fields (and variables in general) as rectangles, but we place the type of the variable above the rectangle, the name of the variable on the left, and the value inside, and we do not show access modifiers (private, protected, public).
• We draw a bold rectangle outline for final variables.

Stack & Heap Diagrams in Informa Clicker

The Informa Clicker tool, a software-based classroom response system, provides a question type with an integrated editor for stack and heap diagrams.

Constructor Calls

Local Variables

Method Calls

Web-Based Stack & Heap Diagram Editor

Davide Ciulla developed a prototype of a web based stack and heap diagram editor: