Closing Reader/Stream in Java

Question

In my code I have to read user input from console:

class Demo {
    //...some code
    public String readUserInput() throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String userInput = reader.readLine();
        reader.close();
        return userInput;
    }
}

On first time when I use method readUserInput() on Demo object everything is OK. But when I create another Demo object and call method - it throws Exception with the message

"Stream closed"

Can anybody tell me, why I have Exception on different not equal objects? Thank you for your attention.

Answer 1

The problem is here:

new InputStreamReader(System.in)

When you close your BufferedReader, it closes the InputStreamReader, which in turn closes System.in.

This is all (kind of) explained in the contract of Closeable's .close(), which says:

Closes this stream and releases any system resources associated with it.

As a result, the second time you try and read from your BufferedReader, this ultimately results in data being read from System.in, but it is unavailable. Hence your error.

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More generally, you handle your resources poorly. Please read about, and learn to use, the try-with-resources statement.