What is the benefit to assign the parameter to another variable

Question

I am reading a tutorial form this site.http://tutorials.jenkov.com/java-unit-testing/matchers.html The author I believe is very experienced. I saw the code like this. I also saw someone else always like to assign the parameter of a method to a variable then use it inside the method. This one here is this line. protected Object theExpected = expected;

Can anyone please tell me, what is the benefit of this coding style? Is this trying to avoid the object status being changed or something?

What if the parameter is not an Object but a primitive variable.

And what if it is a immutable Object like String. Thank you.

public static Matcher matches(final Object expected){

    return new BaseMatcher() {

        protected Object theExpected = expected;

        public boolean matches(Object o) {
            return theExpected.equals(o);
        }

        public void describeTo(Description description) {
            description.appendText(theExpected.toString());
        }
    };
}

Here is the update

I just did another test, to see if this parameter still accessible after we got the object.

package myTest;


public class ParameterAssignTest {

    public static void main(String[] args) {
        MyInterface myClass = GetMyClass("Here we go");
        System.out.println(myClass.getValue());
        System.out.println(myClass.getParameter());
    }

    public static MyInterface GetMyClass(final String myString){

        return new MyInterface() {

            protected String stringInside  = myString;

            @Override
            public String getValue() {
                return stringInside;
            }

            @Override
            public String getParameter() {
                return myString;
            }
        };
    }
}

Output:

Here we go
Here we go

So does this mean that even we do assign this parameter to the a local variable it still works?

Answer 1

I do not believe assigning to theExpected achieves anything.

As expected is final it can be accessed within the anonymous class. If it were used directly in describeTo the object would not be GC'd and the reference would remain valid when the local scope in which expected was declared was left.

Possibly the author of the post you link to believes this explicit style is more readable.

Answer 2

It doesn't matter if theExpected is primitive or Object (though in this example it's an Object), and whether it's mutable or not.

The matches method returns an instance of an anonymous class that extends BaseMatcher (and implements the Matcher interface, assuming that's an interface).

Once it returns the instance, the local variable - expected - that was passed to it is out of scope, but the theExpected member, containing the same value as that local variable, stays within the instance, and can be used by the methods of that instance.

Answer 3

If you want/need to use a local variable in an inner class (in this case, an anonymous local class), the variable must be declared as final regardless if it's primitive or a reference type. This is better explained here: Why Java inner classes require "final" outer instance variables?. Quoting the best explanation IMO:

The reason the language insists on that is that it cheats in order to provide your inner class functions access to the local variables they crave. The runtime makes a copy of the local execution context (and etc. as appropriate), and thus it insists that you make everything final so it can keep things honest.

If it didn't do that, then code that changed the value of a local variable after your object was constructed but before the inner class function runs might be confusing and weird.

In this case, seems like the author want to keep a copy of the parameter sent to the method in the reference of the anonymous class generated for further evaluation. Once the method finishes its execution, the parameter Object expected isn't available anymore, so if you want/need to keep it then you must assign it into a field of your class.