Q1. Describe, in general, how java's garbage collector works ?
Ans. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by
objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java's dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected.
Q2. Does garbage collection guarantee that a program will not run out of memory?
Ans. Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection
Q3. What are the methods of Object Class ?
Ans. clone() - Creates and returns a copy of this object.
equals() - Indicates whether some other object is "equal to" this one.
finalize() - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object
getClass() - Returns the runtime class of an object.
hashCode() - Returns a hash code value for the object.
toString() - Returns a string representation of the object.
notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program.
Q4. Explain java.lang.OutOfMemoryError ?
Ans. This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.
Q5. Should we override finalize method ?
Ans. Finalize is used by Java for Garbage collection. It should not be done as we should leave the Garbage Collection to Java itself.
Q6. Do we have garbage collector in C++ ?
Ans. No.
Q7. What is the difference between final, finally and finalize() ?
Ans. final - constant variable, restricting method overloading, restricting class subclassing.
finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release
other system resources like database connections, statements etc.
finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.
Q8. What are strong, soft, weak and phantom references in Java ?
Ans. Garbage Collector won’t remove a strong reference.
A soft reference will only get removed if memory is low.
A weak reference will get removed on the next garbage collection cycle.
A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to be notified that an object is about to be collected.
Q9. What is a daemon thread? Give an Example ?
Ans. These are threads that normally run at a low priority and provide a basic service to a program or programs when activity on a machine is reduced. garbage collector thread is daemon thread.
Q10. Can we call the garbage collector explicitly ?
Ans. Yes, We can call garbage collector of JVM to delete any unused variables and unreferenced objects from memory using gc( ) method. This gc( ) method appears in both Runtime and System classes of java.lang package.