How to mock a final class with mockito

Question

I have a final class, something like this:

public final class RainOnTrees{

   public void startRain(){

        // some code here
   }
}

I

Answer 1

Actually there is one way, which I use for spying. It would work for you only if two preconditions are satisfied:

1. You use some kind of DI to inject an instance of final class
2. Final class implements an interface

Please recall Item 16 from Effective Java. You may create a wrapper (not final) and forward all call to the instance of final class:

public final class RainOnTrees implement IRainOnTrees {
    @Override public void startRain() { // some code here }
}

public class RainOnTreesWrapper implement IRainOnTrees {
    private IRainOnTrees delegate;
    public RainOnTreesWrapper(IRainOnTrees delegate) {this.delegate = delegate;}
    @Override public void startRain() { delegate.startRain(); }
}

Now not only can you mock your final class but also spy on it:

public class Seasons{
    RainOnTrees rain;
    public Seasons(IRainOnTrees rain) { this.rain = rain; };
    public void findSeasonAndRain(){
        rain.startRain();
   }
}

IRainOnTrees rain = spy(new RainOnTreesWrapper(new RainOnTrees()) // or mock(IRainOnTrees.class)
doNothing().when(rain).startRain();
new Seasons(rain).findSeasonAndRain();