About Question enthuware.ocajp.i.v8.2.925 :

[Aayushma]

The question asks which of the following code "compiles without error"?

import java.util.*;
public class TestClass {
public static void main(String[] args) throws Exception {
ArrayList<Double> al = new ArrayList<>();

//INSERT CODE HERE
}
}
Then why option d) Double d = al.get(al.length); is correct since it compiles fine.

[admin]

Are you sure it compiles fine? Did you read the explanation given with this option?

[JuergGogo]

al.add(111); is not a valid option.
The add() is expecting Double, so autoboxing has to take place. Since al.add(111d); is working fine.

I don't understand:
< autoboxing will call new Double(), its constructor accepts double and String. Why al.add("111") doesn't work then?
< why the int 111 is't implicitely casted to double, then autoboxed to Double?

Thank you.

[admin]

Because the rules of boxing are clearly defined in JLS ( https://docs.oracle.com/javase/specs/jl ... #jls-5.1.7 ) and it states that if you have an int value then it can only be autoboxed into an Integer. Converting an int to double and then boxing that double into a Double is not permitted in a single step.

[Rinkesh]

Can you give me an example of how the indexOf Method works?

[Rinkesh]

And one more thing a1.contains("string") will print Boolean value false right??

[admin]

The code given in the question itself illustrates the use of indexOf. Try adding a few elements in al and then see what it prints for al.indexOf(1.0);

>And one more thing a1.contains("string") will print Boolean value false right?
What happened when you tried it out?

[Rinkesh]

Code: Select all

import java.util.*;
public class TestClass {
    public static void main(String[] args) throws Exception {
        ArrayList<Double> al = new ArrayList<>();
      al.add(123.0); al.add(114.0); al.add(138.0); al.add(153.0); al.add(168.0);
        System.out.println(al.indexOf(1.0)); //Getting -1 here no matter how many elements I add
        System.out.println(al.contains("string")); //This returns false as expected
    }
}

[admin]

Well, the purpose of indexOf is to find out if the passed object exists in the list. If so, it returns the index at which it exists otherwise it returns -1.
So why do you expect it to return anything other than -1 if your list doesn't contain 1.0?

[Rinkesh]

Okay got it.Thanks!