Java interview questions - level advanced

This is the third in a series of articles on Java interview questions. These questions require subtle, but important, details of certain parts of Java to be correctly answered. These questions are less obvious than the intermediate questions.

1. Why do we favour the use of StringBuilder over String concatenation?
   String concatenation used to be expensive because it creates more String objects. StringBuilder does not create as many objects and is unsynchronized. In later versions of Java, the compiler optimizes concatenation operator to use StringBuilder automatically on compilation, so it doesn’t matter if you use the “+” operator or StringBuilder.
2. Why is it important to implement hashCode() if we override equals()?
   It is important to override hashCode because the java collections framework relies on hashcodes. Ultimately if two objects are equal they must return the same hashCode, however it is not required that two objects that are unequal produce distinct results, but collections might be more efficient if they do.
3. What is the contract for equals() method?
   It is reflexive: for any non-null reference value x, x.equals(x) should return true. It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified. For any non-null reference value x, x.equals(null) should return false.
4. If I have two object of the same type. If I pass the first object to the second object, can the second object directly access the first objects private member fields?
   Yes you can – you do not need to use getter methods or any other “fancy” solution. Private is class private, not object private.
5. Outline how you would use a Future interface in Java?
   A future is used to get the result of an synchronous result from a ExecutorService. The ExecutorService will gather the return values of call() and call back the main application via the future interface
6. What is strictfp? Where would you use it/side effects?
   strictfp makes the JVM put restrictions on the floating point datatypes to ensure they are portable. Some processors are able to make more accurate floating point calculations and the JVM spec allows these to be taken advantage of off. If you use it your calculations will be less accurate, but will be equally wrong on all platforms.
7. How many times does finalise get called?
   Finalise gets called exactly once. If I manage to re-reference the object in my finalise method, making it no long eligible for garbage collection - the next time it comes around to garbage collecting the object finalise will not be called.
8. Explain the substring memory leak problem in Java 1.6:
   http://www.dzone.com/links/r/the_introduction_of_memory_leak_in_java.html
9. Whats the difference between Comparable and Comparator?
   Comparable has one method and makes a class comparable to another. The comparator takes two objects of the same type and applies logic to compare one to another.