Suppose the Dog and Animal classes are imported and both have run and eat methods defined with no parameters. Given the following code:

11 Dog fido;
12 Animal simba = new Animal();
13 simba.run();
14 fido.eat();
15 fido.run();

Which line will cause an error?

Question

Suppose the Dog and Animal classes are imported and both have run and eat methods defined with no parameters.  Given the following code:

11 Dog fido;
12 Animal simba = new Animal();
13 simba.run();
14 fido.eat();
15 fido.run();

Which line will cause an error?

BlackTom AI · Accepted Answer

Begin by noting what the code is doing line by line and what the declarations imply.
Option analysis:
Option 1: 'none of these lines of code will cause an error' — This would be true only if every statement could execute without a problem. However, the line 11 declares a local variable fido of type Dog but does not initialize it. In many languages like Java, using an uninitialized local variable to call a method (as done in lines 14 and 15) is not allowed. So this blanket statement is unlikely to be correct given the subsequent uses of fido.
Option 2: 'I don’t know' — This is not a substantive choice about the code; it doesn’t address the actual behavior of the program and should not be considered a valid technical explanation.
Option 3: '14' — This is the line where fido.eat() is invoked on the uninitialized local variable fido. Since fido was declared but never assigned an object reference, calling a method on it constitutes a use of an uninitialized variable, which typically causes a compilation error. The lack of initialization is the root cause here.
Option 4: '15' — line 15 calls fido.run(), but since fido hasn't been initialized, this would also be problematic. However, some languages would error at line 14 first, so the actual first error encountered in a strictly single-pass compilation would be line 14. If the language allowed postponed checks, line 15 would also fail for the same reason as line 14.
Option 5: '13' — line 13 calls simba.run() on simba, which was created with new Animal(), so this is valid assuming run() exists and takes no parameters, which the problem states it does. This line should not cause an error.
Option 6: '12' — line 12 initializes simba with new Animal(), which is a proper object construction call. This line should not cause an error either.

Synthesis: The only problematic usage is the uninitialized local variable fido being used to call methods in lines 14 (and also 15). Since 14 is the first time fido is used, it is the line most likely to cause a compilation error in a language like Java where local variables must be initialized before use.

In summary, line 14 is the statement where the error occurs due to fido being declared but not initialized prior to its method calls. The other lines are consistent with the described setup: simba is created and used, and fido is declared (though not yet initialized) but not used until line 14.

CS_2114_Spring2025 01.00 CS 2114 Readiness questions (classic)

Suppose the Dog and Animal classes are imported and both have run and eat methods defined with no parameters. Given the following code: 11 Dog fido; 12 Animal simba = new Animal(); 13 simba.run(); 14 fido.eat(); 15 fido.run(); Which line will cause an error?

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