What Does \'[Ljava.lang.Object;\' Mean in Java Array Type Encoding?

Understanding the Object Array Type Encoding

When retrieving an object from a function call and printing its string representation, you may encounter a puzzling notation like "[Ljava.lang.Object;". This enigmatic expression represents the class type of the object, but how do we decipher its cryptic format?

The class type encoding scheme is elegantly detailed in Class.getName(): for reference types (excluding arrays), the binary name is returned. Primitive types and void are assigned their Java keywords. However, for arrays, their encoding takes a unique form.

Array Encoding: Breaking Down [ Ljava.lang.Object; ]

"Ljava.lang.Object;" is the notation for the Java class representing an array of Objects, also known as Object[].class. The naming pattern follows the following rules:

• The name of the element type is stated first.
• For arrays, the element type is preceded by one or more '[' characters, indicating the nesting depth.

Understanding the Encoding Notation

The format is as follows:

Element Type Encoding
boolean Z
byte B
char C
double D
float F
int I
long J
short S
class or interface Lclassname;

Example Decodings

For illustration, consider these examples:

• [[[I@xxxxx: This represents an array with three nesting levels, containing integers as elements.
• [[Ljava.lang.String;@xxxxx: This denotes an array with two nesting levels, containing arrays of Strings.
• [Z@xxxxx: This represents an array with boolean elements.

Why Arrays Override toString()

Contrary to expectation, the toString() method for arrays deviates from the Object class's implementation. It returns a string in the aforementioned format because arrays do not override this method. Thus, the method inherited from Object, specified as follows, is invoked:

String.class returns a string in the format: getClass().getName() '@' Integer.toHexString(hashCode())

Alternative Array String Representation

The java.util.Arrays class offers more user-friendly string representations for arrays:

• Arrays.toString(nums): For primitive arrays and Object[], a comma-separated list of elements is displayed.
• Arrays.deepToString(table): For multidimensional arrays, a nested representation is provided.

Additional Useful Resources

• Arrays.equals and Arrays.deepEquals: These methods compare arrays for element-wise equality.
• Java Arrays.equals() returns false for two-dimensional arrays
• Effective Java 2nd Edition, Item 53: Prefer interfaces to reflection

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