Unreachable Statement

[Benjamin Thvedt]

I am having a little trouble figuring this one out. Basically, I am reading Head First Java and I am trying to write a method that
gets some user input, but if the user input doesn't match one of the Strings in an ArrayList, then to return "invalid". I am just a beginner, so the code closely resembles the code given in chapter 5 of the book for getting user input.

public class UserInput {

    public String getUserInput(String prompt,ArrayList<String> args) {

        String input = null;
        System.out.print(prompt + " ");

        try {
            BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
            input = is.readLine();
            if (input.length() == 0) return null;

        }  catch (IOException e) {
            System.out.println("IOException : " + e);

        }  // end try/catch

        if (args.contains(input)) {
            return input;

        } else {
             return "invalid";  // <---- unreachable statement???

        } // end if

        return "invalid"; // return invalid if args doesn't contain the user input.  If I get rid of this line, it works

    } // end method

} // end class

basically, this won't compile and it says that the first "return "invalid"" line is unreachable.. I'ts a problem I can get around, but I was just wondering why the compiler thinks this is unreachable. I mean, it's just a simple if/else, right? If I get rid of the second return invalid line, it complies fine though. Not a big problem, just trying to understand how the method is compiling. Shouldn't it not care that there are two lines that say return "invalid"? If that makes any sense.

[Seetharaman Venkatasamy]

the compiler is smart enough to recognize one of the return statement in if-else must be execute! so the statements after else block can not be reached thus the compiler throw an error!

use decompiler for below code. you can get some light!

String print(String input)
{
    String s = "";
    if(input.equals(s))
    {
        return "true";
    }
    else
    {
	
    }
    return "false";
}

[Sathi Krishna]

You have to remove the else block or you have to comment the last return statement.(line#26)

[H Jetly]

This is because the code will terminate in the if or the else statement, it will never go beyond the else block. Hence unreachable.


For eg if you change the else with an else if (something) you can write the last return "invalid"

public class UserInput {

    public String getUserInput(String prompt,ArrayList<String> args) {

        String input = null;
        System.out.print(prompt + " ");

        try {
            BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
            input = is.readLine();
            if (input.length() == 0) return null;
            

        }  catch (IOException e) {
            System.out.println("IOException : " + e);

        }  // end try/catch

        
        if (args.contains(input)) {
            return input;

        } else if (true){
             return "invalid";  // <---- unreachable statement???

        } // end if
        
        return "invalid";

      // return invalid if args doesn't contain the user input.  If I get rid of this line, it works

        
    } // end method
  
} // end class

[Rob Spoor]

Benjamin, what compiler are you using? Because with Oracle's JDK 1.6.0_26 on my system, it's the second return "invalid";" statement that's causing the error, not the first.

[Seetharaman Venkatasamy]

Rob Spoor wrote:Benjamin, what compiler are you using? Because with Oracle's JDK 1.6.0_26 on my system, it's the second return "invalid";" statement that's causing the error, not the first.

Indeed! for me too .

[Benjamin Thvedt]

I'm using the JDK 1.7.0. Just got it! Anyway, this just had me scratching my head is all. What's weird is I just tried it again, and it compiles fine now...same code and everything! Really strange. (Ofc I added the approperate import statements, but I think other than that it is the same code word for word..) I have no idea.