Can I redefine String#length?

Question

I'd like to re-implement a method of a Java class. For example, for "hi".length() to return 4. (How) Can I do that?

I know using SomeClass.metaClass I can get a reference to an existing method and define new (or overriding) method, but I can't seem to be able to do that for existing Java methods.

Answer 1

Seems like it could be possible by abusing String metaClass. But the attempt I've done so far in groovy console didn't led to the expected result :

def oldLength = String.metaClass.length
String.metaClass.length = { ->
    return oldLength+10;
}

println "hi".length()

outputs the sad 2

I think you could take a look at Proxy MetaClass or Delegating metaClass.

Answer 2

If you did redefine it, it would only work in Groovy code. Groovy can't change the way Java code executes.

In Groovy, "hi".length() is roughly equivalent to this Java:

stringMetaClass.invokeMethod("hi","length");

Because Groovy doesn't actually call length directly, metaClass tricks work in Groovy code. But Java doesn't know about MetaClasses, so there is no way to make this work.

Answer 3

Using Groovy, you can replace any method (even those of final classes) with your own implementation. Method replacement in Groovy uses the meta-object protocol, not inheritance.

Here's the example you requested, i.e. how to make String.length() always return 4

// Redefine the method
String.metaClass.invokeMethod = { name, args ->

    def metaMethod = delegate.metaClass.getMetaMethod(name, args)
    def result = metaMethod.invoke(delegate, args)

    name == 'length' ? 4 : result
}

// Test it
assert "i_do_not_have_4_chars".length() == 4