Coforge interview question

The overal experience is good I had an interview as a Java backend developer Questions asked: 1. Implement Immutable class 2. Implement singleton class and what is the purpose 3. Write stream prints non repeated characters in a given string 4. Explain java main pillars 5. Why @functionalInterface annotation if both interface and functional interface having abstract methods 6. Difference between collections and streams 7. Intermediate and terminal operators 8. About interfaces