Java Reference Variables: Stack & Heap Secrets

Key Vocabulary

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Recap & Contrast

Primitives vs. Reference Variables

Yesterday, you learned about primitives (like int, double, boolean). Primitives are simple values stored directly in Stack memory.

Today, we meet Reference Variables. Instead of holding raw data, a reference variable stores a memory address (pointer) that tells Java where to find the actual object in Heap memory.

The Memory Division

Stack Memory

Stores primitive variables
Stores references (addresses)
Super fast lookup

Heap Memory

Stores all objects (e.g., Strings, Dogs, Arrays)
Dynamic sizing
Objects persist longer

Interactive Guide

Step through the visualizer lines 1-4 to earn memory interaction points.

0/4 Views

MemoryVisualizer.java

Interactive Execution

1int age = 17;

2Dog d1 = new Dog("Rex");

3Dog d2 = d1;

4d1 = new Dog("Buddy");

Stack (Variables) Immediate Values

Heap (Objects) Dereferenced Data

Step 0 of 4

Click 'Next' to start executing our code line-by-line and watch the physical memory structures construct!

How-To Guide

Creating Your Own Reference Variables

To create reference variables of a custom type, we first define a Class (the blueprint), then instantiate it using the new keyword.

1 The Blueprint (Custom Class)

public class Dog {
    String name; // instance variable

    // Constructor
    public Dog(String dName) {
        name = dName;
    }
}

2 Instantiation Anatomy

Dog d1 = new Dog("Rex");

Dog: The class type (defines the reference type).

d1: Name of the reference variable stored on the Stack.

new: The magic operator that dynamically allocates memory on the Heap.

Dog("Rex"): Calls the constructor to initialize the actual data of the object on the Heap.

The "NullPointer" Pitfall!

If you declare a reference variable but don't allocate an object to it (e.g., Dog myDog;), its default value is null. Attempting to use a null reference triggers a NullPointerException crash!

Most Common Class 0/3 Sandbox Runs

Java Strings & Methods

Strings in Java are objects, meaning variables initialized to strings are actually references pointing to character arrays/sequences stored on the heap.

Your Target String:

Run Methods Interactive

.substring(start, end)

Unrun

Extracts a portion. Crucial: Includes start index, EXCLUDES end index!

Start:

End:

.split(regex)

Unrun

Splits the String around matches of the delimiter, returning an array of Strings.

Delimiter: (e.g., Space " " or Dash "-")

Interactive String Sandbox

HEAP STORAGE SIMULATION

String Indexed Memory Cells

Evaluated Java Code:

String str = "AP Computer Science";

Live Console Output:

Use the controls on the left to invoke a string operation and witness the JVM response!

Assessment Challenge

Reference Types & String Mastery Quiz

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