How to close a scanner without closing the underlying System.in?
If I close one scanner object and make a new one and try to read some more input, I'm getting the NoSuchElementException exception.
My code works fine but it gives me a warning if I don't close the scanner. However, if I close it to get rid of the warning, I also close System.in ... How do I avoid this?
Also, are there any consequences of not closing the scanner?
EDIT: Here's my code:
This is the NameAddressExists() method:
public void NameAddressExists() {
System.out.println("Enter name");
Scanner sc = new Scanner(System.in);
String n = sc.next();
System.out.println("Enter address");
String a = sc.next();
int flag = 0;
for(int i = 0; i < count; i++) {
if(array[i].name .equals(n) && array[i].address .equals(a)) {
System.out.println("True");
flag = 1;
}
}
if(flag != 1) {
new Agency(n, a);
}
sc.close();
}
This is the PanNumberExists() method:
public boolean PanNumberExists() {
Scanner s = new Scanner(System.in);
String n = "";
System.out.println("Enter the 5 digits");
try {
n = s.nextLine();
}catch(Exception e) {
System.out.println(e);
}finally {
s.close();
}
if(n .equals(this.PAN.subSequence(4,9))) {
return true;
}
else {
return false;
}
}
These methods are called from the following main() method:
public static void main(String args[]) {
Agency obj1 = new Agency("XYZ", "ABCD");
Agency obj2 = new Agency("XYZ", "ABCDEFG", "+91083226852521", "ab 1234567", "abcd12345ab");
// Agency obj3 = new Agency("XYZ", "TSRK", "36", "ab 1234567", "abcd12345ab");
obj1.NameAddressExists();
System.out.println(obj2.PanNumberExists());
}
As you can see, I first call the NameAddressExists() method, in which I open, use and close a Scanner named 'sc'. This works fine and gives me the correct output. Next, I call the PanNumberExists() method, in which I open another Scanner named 's' and try to use it to get some input from the user. This is where I receive the NoSuchElementException exception. If I leave the Scanner 'sc' open in my NameAddressExists() method, then I don't get this error.
Scanner scan = new Scanner(new FilterInputStream(System.in) {
@Override
public void close() throws IOException {
// do nothing here !
}
});
Or , Just ignore close() by implementing custom decorator.
public class UnClosableDecorator extends InputStream {
private final InputStream inputStream;
public UnClosableDecorator(InputStream inputStream) {
this.inputStream = inputStream;
}
@Override
public int read() throws IOException {
return inputStream.read();
}
@Override
public int read(byte[] b) throws IOException {
return inputStream.read(b);
}
@Override
public int read(byte[] b, int off, int len) throws IOException {
return inputStream.read(b, off, len);
}
@Override
public long skip(long n) throws IOException {
return inputStream.skip(n);
}
@Override
public int available() throws IOException {
return inputStream.available();
}
@Override
public synchronized void mark(int readlimit) {
inputStream.mark(readlimit);
}
@Override
public synchronized void reset() throws IOException {
inputStream.reset();
}
@Override
public boolean markSupported() {
return inputStream.markSupported();
}
@Override
public void close() throws IOException {
//do nothing
}
}
And while using in main() ,
public static void main(String[] args) throws Exception {
System.setIn(new UnClosableDecorator(System.in));
}
You can use the Decorator pattern and create custom InputStream that can't be closed, then pass it to the Scanner
import java.io.IOException;
import java.io.InputStream;
public class PreventClosingInputStream extends InputStream {
private InputStream inputStream;
public PreventClosingInputStream(InputStream inputStream) {
this.inputStream = inputStream;
}
@Override
public int read() throws IOException {
return inputStream.read();
}
@Override
public void close() throws IOException {
// Don't call super.close();
}
}
Then, in your code:
PreventClosingInputStream in = new PreventClosingInputStream(System.in);
Scanner s = new Scanner(in);
// ...
s.close(); // This will never close System.in as there is underlying PreventClosingInputStream with empty close() method
Using try-with-resources:
try (PreventClosingInputStream in = new PreventClosingInputStream(System.in);
Scanner s = new Scanner(in);) {
// ...
// resources will be automatically closed except of System.in
}
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