Final answer:
The mystery method in Java may return a substring of str1 that is equal to str2 if str2 is found in str1. It cannot return a string without any common characters with str2, and it might return str1 if str2 is empty or the lengths match. The code snippet has an error with an empty pair of parentheses.
Step-by-step explanation:
The student is asking about the behavior of the mystery method in Java, which takes two strings as arguments. There's a mistake in the provided code, as it includes an empty pair of parentheses where an integer value should be, making the code incomplete and thus it will not compile. However, assuming the method is meant to return a substring of str1 starting at the index where str2 is found, here are some possibilities:
- I. It may return a string that is equal to str2 if str2 is found in str1 and the substring length equals the length of str2.
- II. It cannot return a string that has no characters in common with str2, because the method starts the substring at the index where str2 is found in str1.
- III. It may return a string that is equal to str1 if str2 is an empty string, or if the substring length is equal to the remaining length of str1 after the index.
To clarify, in case str2 is not found within str1, the indexOf method will return -1, leading to an exception or incorrect behaviour if this is not properly handled in code.