Making java method arguments as final

Question

What difference that final makes between the code below. Is there any advantage in declaring the arguments as final.

public String changeTimezone( Timestamp stamp, Timezone fTz, Timezone toTz){  
    return ....
}

public String changeTimezone(final Timestamp stamp, final Timezone fTz, 
        final Timezone toTz){
    return ....
}

Answer 1

For the body of this method the final keyword will prevent the argument references to be accidentally reassigned giving a compile error on those cases (most IDEs will complain straight away). Some may argue that using final in general whenever possible will speed things up but that's not the case in recent JVMs.

Answer 2

Its just a construct in Java to help you define a contract and stick to it. A similar discussion here : http://c2.com/cgi/wiki?JavaFinalConsideredEvil

BTW - (as the twiki says), marking args as final is generally redundant if you are following good programming principles and hance done reassign / redefine the incoming argument reference.

In the worst case, if you do redefine the args reference, its not going to affect the actual value passed to the function - since only a reference was passed.

Answer 3

The final keyword when used for parameters/variables in Java marks the reference as final. In case of passing an object to another method, the system creates a copy of the reference variable and passes it to the method. By marking the new references final, you protect them from reassignment. It's considered sometimes a good coding practice.

Answer 4

As a formal method parameter is a local variable, you can access them from inner anonymous classes only if they are declared as final.

This saves you from declaring another local final variable in the method body:

 void m(final int param) {
        new Thread(new Runnable() {
            public void run() {
                System.err.println(param);
            }
        }).start();
    }

Answer 5

Extract from The final word on the final keyword

Final Parameters

The following sample declares final parameters:

public void doSomething(final int i, final int j)
{
  // cannot change the value of i or j here...
  // any change would be visible only inside the method...
}

final is used here to ensure the two indexes i and j won't accidentally be reset by the method. It's a handy way to protect against an insidious bug that erroneously changes the value of your parameters. Generally speaking, short methods are a better way to protect from this class of errors, but final parameters can be a useful addition to your coding style.

Note that final parameters are not considered part of the method signature, and are ignored by the compiler when resolving method calls. Parameters can be declared final (or not) with no influence on how the method is overriden.

Answer 6

The final keyword prevents you from assigning a new value to the parameter. I would like to explain this with a simple example

Suppose we have a method

method1(){

Date dateOfBirth =new Date("1/1/2009");

method2(dateOfBirth);

method3(dateOfBirth); }

public mehod2(Date dateOfBirth) {
....
....
....
}

public mehod2(Date dateOfBirth) {
....
....
....
}

In the above case if the "dateOfBirth" is assigned new value in method2 than this would result in the wrong output from method3. As the value that is being passed to method3 is not what it was before being passed to method2. So to avoid this final keyword is used for parameters.

And this is also one of the Java Coding Best Practices.