尝试捕获for循环

Question

I have met something that I cannot explain to myself today. It was a small task on an online resource to write a program that will be run on some tests. Can you please explain me the difference between two methods and why one of them fails on some tests (I don't have them).

The task is to write a static method that opens a connection and then tries 3 times to do some abstract stuff with this connection by calling its method. The problem is that any method you use can throw an exception (open connection and connection method). You must try to do the stuff exactly 3 times (if all attempts failed - throw an exception) and another condition is that every opened connection must be closed.

The connection class called RobotConnection and it implements AutoCloseable . This class has method void moveRobotTo(int x, int y) (it is the "do stuff method" from the task). You can get instance of RobotConnection only by calling RobotConnectionManager.getConnection() . Methods of these classes can throw RobotConnectionException extends RuntimeException .

So the real question COMES HERE:

This code fails (no idea why, presumably infinite loop):

public static void moveRobot(RobotConnectionManager robotConnectionManager, int toX, int toY) {
    boolean success = false;
    for (int i = 0; !success && (i < 3); ++i) {
        try (RobotConnection connection = robotConnectionManager.getConnection()) {
            connection.moveRobotTo(toX, toY);
            success = true;
        }
    }
    if (!success) {
        throw new RobotConnectionException("3 attempts failed");
    }
}   

And this one was accepted as working (I cannot see the real difference =( )

public static void moveRobot(RobotConnectionManager robotConnectionManager, int toX, int toY) {
    boolean success = false;
    for (int i = 0; !success && (i < 3); ++i) {
        try (RobotConnection connection = robotConnectionManager.getConnection()) {
            connection.moveRobotTo(toX, toY);
            success = true;
        } catch (RobotConnectionException e) {}
    }
    if (!success) {
        throw new RobotConnectionException("3 attempts failed");
    }
}

Answer 1

In your first method, you don't catch RobotConnectionException . Consequently, it can fail at most once, rather than the required exactly 3 times .

Answer 2

The difference is that in the first case, you say "open and clean up the connection, but I don't know how to deal with exceptions: let them propagate up the call chain to something which can handle it":

try (RobotConnection connection = robotConnectionManager.getConnection()) {
  // ...
}

whereas in the second case, you say "open and clean up the connection, but if an exception occurs, I will deal with it myself; the action I will take is to do nothing":

try (RobotConnection connection = robotConnectionManager.getConnection()) {
  // ...
} catch (RobotConnectionException e) {}

Answer 3

If a RobotConnectionException is thrown in the first code snippet, then it might come from inside the try statement. Since you do not catch it, you will not necessary make three attempts.

More information on java exceptions: http://docs.oracle.com/javase/tutorial/essential/exceptions/