Junit test loop condition with user input - user-input

Question

I have a function that would keep looping until it got a correct username input. I try to write the unit test of it, but cannot figure out a proper way. The test always failed at the scan.nextLine(). Can anyone help?
Function:
User currentUser = null;
String name = null;
do {
System.out.print("Please enter your user name: ");
name = scan.nextLine();
currentUser = User.userExist(name); //if user exist return user, else return null
if (currentUser == null)
logging.log("<LOG> Non-existent user "+name+", Please enter again!");
}while(currentUser == null);

Answer 1

Unit tests (assume your using jUnit) are supposed to run without any human input. Your best bet is to simulate the user input by creating a mock object of the scan class and have your tests set the mock object during setup.
EG something like this (pseudo code):
// test class
#Before() {
MockScan mockscan = new MockScan();
classToTest.setScan(mockScan);
}
// mock class - should implement same interface as original
// scan object or extend it and override its methods
class MockScan implements ScanInterface {
public String nextLine() {
return "John Doe";
}
}
If the object you need to mock is complicated or provided by the system there are a few frameworks out there to help - search google.
HTH

Answer 2

Try like that to work scan.nextLine();
User currentUser = null;
String name = null;
Scanner scan = null
while(currentUser == null)
System.out.print("Please enter your user name: ");
scan = new Scanner(System.in); //to ask user input
name = scan.nextLine();
currentUser = User.userExist(name); //if user exist return user, else return null
if (currentUser == null)
logging.log("<LOG> Non-existent user "+name+", Please enter again!");
else
scan.close();
}