Generics return type

[Harikrishna Gorrepati]

Hi, This is from K&B mock exam.

public class Question56_Generics_Ask {

    public <S extends CharSequence> S fooC(S s) {
	return s.toString();//Type mismatch: cannot convert from String to S.Why? 
    }

    public <S extends CharSequence> S fooC1(S s) {
	return (S) s.toString();//Compile Warning :Type safety: Unchecked cast from String to S..Why? 
    }
    
    public <S extends CharSequence> S fooD(S s) {
	return new StringBuilder(s);//Type mismatch: cannot convert from StringBuilder to S..Why ??
    }

    public <S extends CharSequence> S fooE(S s) {
	return (S) new StringBuilder(s);//Compile Warning..Type safety: Unchecked cast from StringBuilder to S..Why ??
    }

}

Given answer is although String and StringBuffer are both subtypes of CharSequence, they are not necessarily
the same subtype as whatever is passed in (S)..I am not clear. Please let me know

[Jeanne Boyarsky]

1) S extends CharSequence. It could be any class that extends CharSequence and not necessarily String. What happens if I call fooC(StringBuffer)? Java needs to return a StringBuffer, but the method has a String.

2) What happens if you try to do return (StringBuilder) s.toString(); without generics. Why doesn't it work?

3) Same reason as #1.

4) Same reason as #2.

[Paul Clapham]

Yes, the given answer is correct.

[Malte Wannerskog]

I have a question about this particular question aswell...

the line

 return (S) new StringBuilder(s);

compiles and runs just fine without any compiler warnings for me. Am I missing something?

Same goes for

return (S) s.toString();

No compiler warnings there aswell. Even though this was not an option in my question.

Thanks for your help

[Raghavan Muthu]

Welcome to JavaRanch, Malte Wannerskog !

[Malte Wannerskog]

Raghavan Muthu wrote:Welcome to JavaRanch, Malte Wannerskog !

Thank you, been reading posts here for a while but thought i would be a member now since Im studying for the SCJP exam

Any ideas on my question?

[Malte Wannerskog]

Malte Wannerskog wrote:I have a question about this particular question aswell...

the line

 return (S) new StringBuilder(s);

compiles and runs just fine without any compiler warnings for me. Am I missing something?

Same goes for

return (S) s.toString();

No compiler warnings there aswell. Even though this was not an option in my question.

Thanks for your help

Ah nevermind, I realized that the compiler error didnt occur until you actually try to use the method .
For example with

String s = new Q56().fooE("ss");

[michael schroeder]

Hmm, now I think I am missing something:

public class Test{
   public  <S extends CharSequence> S foo(S s) {
        // return (S) new StringBuilder();  // throws an Exception if uncommented
        return (S)s.toString();
    }

    public static void main(String[] args) {
       System.out.println(new Test().foo("s"));
    }
}

This program compiles with warnings, but runs fine. This is part of the mock questions of the SCJP Java 6 Study Guide for 310-065 (Quiz #56). The given answer to the question is, that both return statements will not compile or they will not run without errors. But the compiler thinks different about that.