Important Core Java interview questions

1. What is the Collections API? - The Collections API is a set of classes and interfaces that support operations on collections of objects
2. What is the List interface? - The List interface provides support for ordered collections of objects.
3. What is the Vector class? - The Vector class provides the capability to implement a growable array of objects
4. What is an Iterator interface? - The Iterator interface is used to step through the elements of a Collection
5. Which java.util classes and interfaces support event handling? - The EventObject class and the EventListener interface support event processing
6. What is the GregorianCalendar class? - The GregorianCalendar provides support for traditional Western calendars
7. What is the Locale class? - The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region
8. What is the SimpleTimeZone class? - The SimpleTimeZone class provides support for a Gregorian calendar
9. What is the Map interface? - The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values
10. What is the highest-level event class of the event-delegation model? - The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy
11. What is the Collection interface? - The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates
12. What is the Set interface? - The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements
13. What is the purpose of the enableEvents() method? - The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.
14. What is the ResourceBundle class? - The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.
15. What is the difference between yielding and sleeping? - When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.
16. When a thread blocks on I/O, what state does it enter? - A thread enters the waiting state when it blocks on I/O.
17. When a thread is created and started, what is its initial state? - A thread is in the ready state after it has been created and started.
18. What invokes a thread’s run() method? - After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.
19. What method is invoked to cause an object to begin executing as a separate thread? - The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.
20. What is the purpose of the wait(), notify(), and notifyAll() methods? - The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object’s wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object’s notify() or notifyAll() methods.
21. What are the high-level thread states? - The high-level thread states are ready, running, waiting, and dead
22. What happens when a thread cannot acquire a lock on an object? - If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.
23. How does multithreading take place on a computer with a single CPU? - The operating system’s task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
24. What happens when you invoke a thread’s interrupt method while it is sleeping or waiting? - When a task’s interrupt() method is executed, the task enters the ready state. The next time the task enters the running state, an InterruptedException is thrown.
25. What state is a thread in when it is executing? - An executing thread is in the running state
26. What are three ways in which a thread can enter the waiting state? - A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
27. What method must be implemented by all threads? - All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
28. What are the two basic ways in which classes that can be run as threads may be defined? - A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.
29. How can you store international / Unicode characters into a cookie? - One way is, before storing the cookie URLEncode it. URLEnocder.encoder(str); And use URLDecoder.decode(str) when you get the stored cookie.