How is length implemented in Java Arrays?

Question

I was wondering about the implementation of length of a Java Array. I know that using arrayName.length gives us the number of elements in the array, but was wondering if this is a method / function or it is just a data member of Array?

I guess it must be a data member as we do not use parenthesis() when invoking it. But if it is a data member how/when is the value of this length assigned/computed?

Answer 1

According to the Java Language Specification (specifically §10.7 Array Members) it is a field:

• The public final field length, which contains the number of components of the array (length may be positive or zero).

Internally the value is probably stored somewhere in the object header, but that is an implementation detail and depends on the concrete JVM implementation.

The HotSpot VM (the one in the popular Oracle (formerly Sun) JRE/JDK) stores the size in the object-header:

[...] arrays have a third header field, for the array size.