You can do an enhanced for loop (for java 5 and higher) for iteration on array's elements:

String[] elements = {"a", "a", "a", "a"};   
for (String s: elements) {           
    //Do your stuff here
    System.out.println(s); 
}

Found the answer

public class App {
    public static void main(String[] args) throws Exception {
        Car car = new Car();
        Bicycle bicycle = new Bicycle();
        Van van = new Van();

        Object[] racers = {car, bicycle, van};  

        for(Object x : racers) {
            System.out.println(x.getClass()); 

            ((Vehicle) x).go();  // this is the only change I made
        }
    }
}

Since atleast Java 1.5.0 (Java 5) the code can be cleaned up a bit. Arrays and anything that implements Iterator (e.g. Collections) can be looped as such:

public static boolean inArray(int[] array, int check) {
   for (int o : array){
      if (o == check) {
         return true;
      }
   }
   return false;
}

In Java 8 you can also do something like:

// import java.util.stream.IntStream;

public static boolean inArray(int[] array, int check) {
   return IntStream.of(array).anyMatch(val -> val == check);
}

Although converting to a stream for this is probably overkill.