Understanding the toString Method in Java: Why Are the Outputs Different?

Explore how Java's `toString` method behaves differently for reference types and primitive types, and uncover the encoding used for arrays.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Implementation of toString method in Java

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the toString Method in Java: Why Are the Outputs Different?

Java’s toString method is an essential part of the language that dictates how objects are represented as strings. However, when it comes to arrays, especially with different types like reference types and primitive types, the output can be surprising. In this post, we'll explore the behavior of the toString method, clarify the differences between these outputs, and explain the underlying encoding used for array types.

The Problem: Unexpected Output in Array Printing

When printing an empty array of Boolean (a reference type), you may encounter a result that looks like this:

[[See Video to Reveal this Text or Code Snippet]]

Conversely, printing an empty array of primitive boolean types produces a different output:

[[See Video to Reveal this Text or Code Snippet]]

These outputs lead to some questions:

Why do they differ based on the type?

What exactly does the output mean?

How does Java encode these types in the output of the toString method?

Let's dive into the details!

Analyzing the toString Method

The default toString() method implementation in Java class looks like this:

[[See Video to Reveal this Text or Code Snippet]]

The name of the class representing the array

The hash code of the array instance in hexadecimal format

What Each Part Represents

Class Name: Indicates the type of the array.

Hash Code: A unique identifier for the specific array instance.

The Encoding of Element Types in Arrays

Java has a specific internal form for the names of array classes. The Java documentation explains how these names are structured:

Primitive Types: Each primitive type has a unique letter for its representation:

boolean = Z

byte = B

char = C

int = I

long = J

short = S

float = F

double = D

Reference Types: For class or interface types, the representation is prefixed by an L and suffixed by a semicolon, e.g., Lclassname;.

Example Breakdown

Given this information, let's break down our previous outputs:

Output for Reference Type Boolean Array:

[L indicates it's an array of a reference type (the L signifies that Boolean is a class/type).

@ 3764951d is the hexadecimal hash code of the array instance.

Output for Primitive Type boolean Array:

[Z@ 4b1210ee:

[Z indicates it's an array of the primitive boolean type. The Z directly corresponds to boolean as identified in the encoding table.

@ 4b1210ee is the hexadecimal hash code of the array instance.

Output for an Empty int Array:

[I@ 4d7e1886:

[I indicates it's an array of primitive int.

@ 4d7e1886 is again the hash code.

Conclusion: The Takeaway

Understanding how Java's toString method works with arrays reveals much about the language's handling of reference and primitive types. The different outputs are not a bug; rather, they are intrinsic qualities of how Java implements type representation as strings.

Next time you print an array in Java, you can confidently interpret the output knowing the logic behind it and the encoding used for different data types.

If you have further questions about Java's toString method or specific scenarios, feel free to ask in the comments!