为什么String.length（）是一个方法？

Question

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.

Answer 1

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?

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

You should always use accessor methods in public classes rather than public fields, regardless of whether they are final or not (see Item 14 in Effective Java).

When you allow a field to be accessed directly (ie is public) you lose the benefit of encapsulation, which means you can't change the representation without changing the API (you break peoples code if you do) and you can't perform any action when the field is accessed.

Effective Java provides a really good rule of thumb:

If a class is accessible outside its package, provide accessor methods, to preserve the flexibility to change the class's internal representation. If a public class exposes its data fields, all hope of changing its representation is lost, as client code can be distributed far and wide.

Basically, it is done this way because it is good design practice to do so. It leaves room to change the implementation of String at a later stage without breaking code for everyone.

Answer 6

String is using encapsulation to hide its internal details from you. An immutable object is still free to have mutable internal values as long as its externally visible state doesn't change. Length could be lazily computed. I encourage you to take a look as String's source code.

Answer 7

Checking the source code of String in Open JDK it's only a getter.

But as @SteveKuo points out this could differ dependent on the implementation.

Answer 8

In most current jvm implementations a Substring references the char array of the original String for content and it needs start and length fields to define their own content, so the length() method is used as a getter. However this is not the only possible way to implement String.

In a different possible implementation each String could have its own char array and since char arrays already have a length field with the correct length it would be redundant to have one for the String object, since String.length() is a method we don't have to do that and can just reference the internal array.length .

These are two possible implementations of String, both with their own good and bad parts and they can replace each other because the length() method hides where the length is stored (internal array or in own field).