Thread interview questions: 

1. Why wait, notify and notifyAll is defined in Object Class and not on Thread class in Java
2. How to implement Thread in Java
3. What do you know about Concurrent interface in java.util package
4. Checked and unchecked exceptions
5. Difference between wait, sleep, yield in Java
6. Why wait and notify requires to be called from synchronized blocks
7. What is race condition
8. Can you write Produce Consument problem
9. Can you write Blocking Queue example
10. What is deadlock and how to avoid it
11. Difference between Thread and Runnable in Java
12. volatile keyword in Java
13. What is double check locking pattern
14. The Executor framework example

Collection interview question:

1. How TreeMap works in Java

       a) How to use Comparator and Comparable in Java

2. How HashMap works in Java

       a) How to override hashCode method

       b) How to override equals method 
             - Best practices to avoid NPE

3. How HashSet works in Java
4. Difference between HashMap and HashTable
5. Difference between List and Set in Java
6. What is Iterator and ListIterator
7. What is the difference between fail-fast and fail-safe Iterator
8. Different between synchronized collection and Concurrent collection
9. Difference between Iterator and Enumeration
10. How do you sort objects on collection
11. Difference between ArrayList and Vector (How ArrayList works)
12. Difference between HashSet and HashMap

Some interesting interview questions

1. Here I found many interesting interview questions about Spring.
2. Tricky question: "Why multiple inheritance is not supported in Java"
3. How HashMap works in Java
4. Why String is immutable or final in Java
5. What is String Pool in Java
6. Why character array is better than String for storing password in Java
7. How substring method works in Java
8. How Thread.join() method works in Java
9. What is deadlock and how to avoid it in Java
10. How volatile in Java works
11. What is Race Condition in Java
12. Understanding a Java thread dump
13. Difference between start and run method in Thread
14. Design pattern interview questions
15. Some information about Java Heap Space
16. Short brief about WeakHashmap
17. How Garbage Collection works in Java
18. Difference between start and run method in Thread