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Java Code Execution Visualizer

Interactive, Thonny-style Java execution tutor. Step through code to see Call Stack frames, Heap allocations, object references, and recursion trees in real-time.

Quick Answer & Summary

A Java execution visualizer simulates the Java Virtual Machine (JVM) step-by-step. It provides a real-time graphical breakdown of the Call Stack (methods and local variables) and Heap Memory (arrays and objects), visually proving how pointers, recursion unwinding, and pass-by-value work in memory.

Java Runtime Memory Layout
JVM Memory = Call Stack (LIFO Frames + Primitives) + Heap Space (Dynamic Objects & Arrays) + Metaspace

Stack memory holds short-lived method invocation frames and primitive values; Heap memory holds all dynamic class instances and arrays managed by Garbage Collection.

Primitives vs References & Aliasing: Primitives (int, double) are stored directly inside the Stack Frame. References (arrays, objects) store a memory address pointing to the Heap.
Java Source Code
Line 1
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public class Main {
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 public static void main(String[] args) {
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 // 1. Primitive Copy
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 int x = 10;
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 int y = x;
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 y = 99;
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 System.out.println("x = " + x + ", y = " + y);
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 // 2. Reference Aliasing
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 int[] arr1 = new int[3];
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 arr1[0] = 10;
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 arr1[1] = 20;
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 arr1[2] = 30;
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 int[] arr2 = arr1; // Copies memory address, not array!
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 arr2[0] = 99; // Mutates the shared Heap array
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 System.out.println("arr1[0] = " + arr1[0]);
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 }
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}
Step 1 of 1

Ready to execute.

Program Standard Output (stdout)
No output printed yet.
Stack is currently empty.
Heap memory is currently empty.

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Documentation & Guide

How to Use Java Code Execution Visualizer

1

Choose a Pedagogical Preset or Write Custom Java

Select from 8 curated educational examples (Aliasing, Factorial Recursion, Pass-by-Value, 2D Arrays, Linked Lists) or click Edit Code to paste your own Java class.

2

Step Through Code Line-by-Line

Use Step Forward, Previous, or Auto Play to observe statement-by-statement execution with synchronized line highlighting.

3

Inspect Stack Frames & Heap Space

Watch local primitives allocate inside stack activation frames and reference variables point to dynamically allocated heap objects (@0x101) with pointer badges.

Practical Examples & Conversions

Input
int[] arr1 = new int[3]; arr1[0] = 10; int[] arr2 = arr1; arr2[0] = 99;
Output
arr1[0] outputs 99 because arr1 and arr2 point to identical Heap address @0x101.

Frequently Asked Questions (PAA)

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Authoritative Standards & Citations

Calculations and algorithms on this page are implemented and verified in strict accordance with the following official technical specifications:

IETF (Internet Engineering Task Force)

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