Question on using return value and not.

[Jeffrey Tian]

In K&B book, there's a question like below:
Given that CharSequence is an interface implemented by both the String and StringBuilder classes, and then given the following method:

public <S extends CharSequence> S foo(S s) {
// INSERT HERE
}

Which of th following can be inserted at //INSERT HERE to compile and run without error?

A : return s;
B : return (Object) s;
C : return s.toString();
D : return new StringBuilder(s);
E : return (S) new StringBuilder(s);
F : return null;

Answer : A and F.

I'm not sure whether D is correct, so I wrote the code below:

public class Test {
    public static void main(String argv[]) 
    { 
        new Test().foo("test");
        System.out.println(new Test().foo("test"));
    }
    
    public <S extends CharSequence> S foo(S s) 
    {
        return (S) new StringBuilder(s);
    }
}

Line 4 runs method foo(), no problem; Line 5 prints foo()'s return value, an exception is thrown:
Exception in thread "main" java.lang.ClassCastException: java.lang.StringBuilder cannot be cast to java.lang.String
at Test.main(Test.java:5)

I think both Line 4 and 5 should throw exceptions. Could someone explain to me why there's no exception at Line 4?
Thanks.

[Harsha Smith]

The option "D" is wrong because you are hard-coding the return type where it could be String or StringBuilder based on the argument passed.

Line 4 and Line 5 compile and don't throw any exceptions because they are instances of CharSequence

[naveen yadav]

Hi harsha ,

i have tried this code it does throw exception at line 5 .