getMessage() on Exception object does not provides null

Question

With a code like this

public static void main(String[] args) {
    Exception one = new Exception("my cause");
    System.out.println("A) " + one.getMessage());

    System.out.println();

    Exception two = new Exception(one);
    System.out.println("B) " + two.getMessage());
    System.out.println("C) " + two.getCause().getMessage());

    System.out.println();

    Exception three = new Exception("my message", one);
    System.out.println("D) " + three.getMessage());
    System.out.println("E) " + three.getCause().getMessage());

    System.out.println();

    Exception fourth = new Exception(null, one);
    System.out.println("F) " + fourth.getMessage());
    System.out.println("G) " + fourth.getCause().getMessage());
}

The output is this one

A) my cause

B) java.lang.Exception: my cause
C) my cause

D) my message
E) my cause

F) null
G) my cause

See the difference between B and F

In both cases I did NOT provided a message, but the difference is that in the B case the null value is not forced.

It seems that for the B case, when a message is not specified, the getMessage() method provides the format className: cause.getMessage() But I would except to have a null value (as is for the F case).

Is there a way to get null value (like F) if I call the getMessage on an Exception that has been created providing only the cause and not the message?

Answer 1

Take a look at Exception's JavaDoc. For the constructor that takes only a Throwable:

Constructs a new exception with the specified cause and a detail message of (cause==null ? null : cause.toString()) (which typically contains the class and detail message of cause). This constructor is useful for exceptions that are little more than wrappers for other throwables (for example, PrivilegedActionException).

So, in your B case, since the cause is not null, you get the value of cause.toString() as the container exception's message.

If that constructor was used to create the exception, then by the time you catch the exception, it's too late - it already has a detail message as specified above. You can't get the "null" as the detail message is not null. You can compare it to the cause's toString() and deduce that it should have been null, but that's a kludge and theoretically, the cause's message could change over time and be different at the time of the catch.

Answer 2

Basing on @RealSkeptic reply I created a method like this

public static String getMessageOrNull(Throwable t) {
    String message = t.getMessage();

    if (t.getCause() != null && message.equals(t.getCause().toString())) {
        message = null;
    }

    return message;
}

It may not be the best approach but for my case works just fine.

Answer 3

You could simply build it the same way, hiding it in a static method :

public static Exception getException(Throwable cause){
    return new Exception(null, cause);
}

Or you define your own class that will use the Exception(String, Throwable) constructor like

public MyExceptoin extends Exception{

    public MyException(Throwable cause){
        super(null, cause);
    }
}

This would be simpler to use later.