Why is String.length() a method?

If a String object is immutable (and thus obviously cannot change its length), why is length() a method, as opposed to simply being public final int length such as there is in an array?

Is it simply a getter method, or does it make some sort of calculation?

Just trying to see the logic behind this.

Java is a standard, not just an implementation. Different vendors can license and implement Java differently, as long as they adhere to the standard. By making the standard call for a field, that limits the implementation quite severely, for no good reason.

Also a method is much more flexible in terms of the future of a class. It is almost never done, except in some very early Java classes, to expose a final constant as a field that can have a different value with each instance of the class, rather than as a method.

The length() method well predates the CharSequence interface, probably from its first version. Look how well that worked out. Years later, without any loss of backwards compatibility, the CharSequence interface was introduced and fit in nicely. This would not have been possible with a field.

So let's really inverse the question (which is what you should do when you design a class intended to remain unchanged for decades): What does a field gain here, why not simply make it a method?

Perhaps a .length() method was considered more consistent with the corresponding method for a StringBuffer, which would obviously need more than a final member variable.

The String class was probably one of the very first classes defined for Java, ever. It's possible (and this is just speculation) that the implementation used a .length() method before final member variables even existed. It wouldn't take very long before the use of the method was well-embedded into the body of Java code existing at the time.

Perhaps because length() comes from the CharSequence interface. A method is a more sensible abstraction than a variable if its going to have multiple implementations.

This is a fundamental tenet of encapsulation.

Part of encapsulation is that the class should hide its implementation from its interface (in the "design by contract" sense of an interface, not in the Java keyword sense).

What you want is the String's length -- you shouldn't care if this is cached, calculated, delegates to some other field, etc. If the JDK people want to change the implementation down the road, they should be able to do so without you having to recompile.