Why are arrays Objects, but can not be used as a base class?

Question

The Java language specification specifies that

In the Java programming language arrays are objects (§4.3.1), are dynamically created, and may be assigned to variables of type Object (§4.3.2). All methods of class Object may be invoked on an array.

So, considering arrays are objects — why did the Java designers make the decision not to allow inherit and override from it, for example, toString() or equals()?

The current syntax wouldn't allow creating anonymous classes with an array as the base class, but I don't think that was the reason for their decision.

Answer 1

Java was a compromise between non-object languages and very slow languages of that time where everything was an object (think about Smalltalk).

Even in more recent languages, having a fast structure at the language level for arrays (and usually maps) is considered a strategic goal. Most people wouldn't like the weight of an inheritable object for arrays, and certainly nobody wanted this before JVM advances like JIT.

That's why arrays, while being objects, weren't designed as class instances ("An object is a class instance or an array"). There would be little benefit in having the ability to override a method on an array, and certainly not a great-enough one to counterbalance the need to check for the right method to apply (and in my opinion not a great-one enough to counterbalance the increased difficulty in code reading, similar to what happens when you override operators).

Answer 2

I came across UNDER THE HOOD - Objects and arrays which explains almost anything you need to know about how JVM handles arrays. In JVM, arrays are handled with special bytecodes, not like other objects we are familiar with.

In the JVM instruction set, all objects are instantiated and accessed with the same set of opcodes, except for arrays. In Java, arrays are full-fledged objects, and, like any other object in a Java program, are created dynamically. Array references can be used anywhere a reference to type Object is called for, and any method of Object can be invoked on an array. Yet, in the Java virtual machine, arrays are handled with special bytecodes.

As with any other object, arrays cannot be declared as local variables; only array references can. Array objects themselves always contain either an array of primitive types or an array of object references. If you declare an array of objects, you get an array of object references. The objects themselves must be explicitly created with new and assigned to the elements of the array.

Arrays are dynamically created objects, and they serve as a container that hold a (constant) number of objects of the same type. It looks like arrays are not like any other object, and that's why they are treated differently.