Stream closed and not reopened - Java

问题

I have an easy 'homework' to do, but I have found a little problem with the closure of the input stream. To put it simply, I have to make a contact 'list' application in Java, just to use the polymorphism in the correct way. So I have a class Contact and a subclass Private (contact). In both class there is a modify method to change the value of the variables.

public void modify() throws IOException {
    System.out.println("Previously name: " + name);
    System.out.println("Insert new name");
    try(InputStreamReader ir = new InputStreamReader(System.in);    
    BufferedReader in = new BufferedReader(ir) ) {  
        name= in.readLine();
        System.out.println("You've changed the name to: "+ name);                       
    System.out.println("Previously surname: " + surname);
    System.out.println("Insert new surname");
        surname= in.readLine();
        System.out.println("You've changed the surname to: "+ surname);                         
    System.out.println("Previously e-mail: " + email);
    System.out.println("Insert new e-mail");
        email = in.readLine();
        System.out.println("You've changed the e-mail to: "+ email);    }                   
}

This is the Contact method that doesn't create problem

@Override 
public void modify() throws IOException {
    super.modifica();
    System.out.println("Numero di cellulare precedente: " + cell);
    System.out.println("Inserire nuovo numero");
    try (InputStreamReader ir = new InputStreamReader(System.in);   
    BufferedReader in = new BufferedReader(ir)) {
        cell = in.readLine();
        System.out.println("Hai cambiato il numero in: "+ cell);                        
    System.out.println("Contatto skype precedente: " + skype);
    System.out.println("Inserire nuovo contatto");
        skype = in.readLine();
        System.out.println("Hai cambiato il contatto in: "+ skype);                         
}   
}

Instead, this is the override of the method in Private. In the main, I create a Private object and I call the modify method. I can insert name, surname and e-mail without problem, then the method throws an IO exception because the stream is closed. I can't understand why I have this kind of problem. I think that the stream is closed by try with resources in the first code, but then it is opened in the second code by another try with resources. Probably something in my idea is wrong.

回答1:

Your problem is indeed due to the try-with-resource statement that closes new InputStreamReader(System.in) which behind the scene closes also the underlying input stream that is System.in (in is a public static field of System) such that in your modify method System.in is already closed and then cannot be read anymore this is why you get this exception.

回答2:

You could still use try-with-resources if you wrap System.in with a CloseShieldInputStream.

I would also recommend using a Scanner instead of the InputStreamReader and BufferedReader, because of it's simplicity:

import java.util.Scanner;
import org.apache.commons.io.input.CloseShieldInputStream;

public class Contact {

    protected String name;
    protected String surname;
    protected String email;

    public void modify() throws IOException {
        System.out.println("Previously name: " + name);
        System.out.println("Insert new name");
        try (Scanner sc = new Scanner(new CloseShieldInputStream(System.in))) {
            name = sc.nextLine();
            System.out.println("You've changed the name to: " + name);
            System.out.println("Previously surname: " + surname);
            System.out.println("Insert new surname");
            surname = sc.nextLine();
            System.out.println("You've changed the surname to: " + surname);
            System.out.println("Previously e-mail: " + email);
            System.out.println("Insert new e-mail");
            email = sc.nextLine();
            System.out.println("You've changed the e-mail to: " + email);
        }
    }
}

public class Private extends Contact {

    private String cell;
    private String skype;

    @Override
    public void modify() throws IOException {
        super.modify();
        System.out.println("Numero di cellulare precedente: " + cell);
        System.out.println("Inserire nuovo numero");
        try (Scanner sc = new Scanner(new CloseShieldInputStream(System.in))) {
            cell = sc.nextLine();
            System.out.println("Hai cambiato il numero in: " + cell);
            System.out.println("Contatto skype precedente: " + skype);
            System.out.println("Inserire nuovo contatto");
            skype = sc.nextLine();
            System.out.println("Hai cambiato il contatto in: " + skype);
        }
    }
}

See also: Closing BufferedReader and System.in

来源：https://stackoverflow.com/questions/38807544/stream-closed-and-not-reopened-java