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Java interview questions

  • How to retrieve a key and Value from a map ? view answer
  • Why can an interface supertype call methods that belong to Object class? view answer
  • Q. What is difference between shallow copy and deep copy ? view answer
  • Q.Why to override equals() and hashCode()? and How i can implement both equals() and hashCode() for Set ? view answer
  • Q.How to sort list of objects ( User Defined) using comparator? in Descending Order. view answer
  • Q. How to sort list of objects ( User Defined) using comparator? view answer
  • What is the difference between int and Interger? view answer
  • Singleton Double-checked locking in Java? view answer
  • What are the parameters to follow Creating and Destroying Objects in Java? view answer
  • Q.What are the different scopes for Java variables? view answer
  • Q.What method must be implemented by all threads? view answer
  • Q.Can an unreachable object become reachable again? view answer
  • Q.What are the steps in the JDBC connection? view answer
  • Q.How are this() and super() used with constructors? view answer
  • Q.What is the difference between static and non-static variables? view answer
  • Q.What is the purpose of finalization? view answer
  • Q.Does garbage collection guarantee that a program will not run out of memory? view answer
  • Q.What is synchronization and why is it important? view answer
  • Q.What is daemon thread and which method is used to create the daemon thread? view answer
  • Q.What are synchronized methods and synchronized statements? view answer
  • Q.If I write System.exit (0); at the end of the try block, will the finally block still execute? view answer
  • Q.If I write return at the end of the try block, will the finally block still execute? view answer
  • Q.Is it necessary that each try block must be followed by a catch block? view answer
  • Q.What is the basic difference between the 2 approaches to exception handling. 1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach? view answer
  • Q.What are the different ways to handle exceptions? view answer
  • Q.What is the difference between error and an exception? view answer
  • Q: What is wrapper class? Explain with example? view answer
  • Q.What is serialization? Explain with example? view answer
  • Q.What one should take care of while serializing the object? view answer
  • Q. When you serialize an object, what happens to the object references included in the object? view answer
  • Q.What happens to the static fields of a class during serialization? view answer
  • Q. Objects are passed by value or by reference? view answer
  • Q. Primitive data types are passed by reference or pass by value? view answer
  • Q. What type of parameter passing does Java support? view answer
  • Q.Can a top level class be private or protected? view answer
  • Q.What is the default value of an object reference declared as an instance variable? view answer
  • Q.What is the difference between declaring a variable and defining a variable? view answer
  • Q. What are different types of inner classes? view answer
  • Q. What is Overriding? view answer
  • Q.What are Checked and UnChecked Exception? view answer
  • Q. What is final? view answer
  • Q.What is static in java? view answer
  • What is an abstract class? view answer
  • Q. What are the modifiers in Java ? view answer
  • Q.Difference between HashMap and HashTable? view answer
  • Q.Difference between Vector and ArrayList? view answer
  • Q.Difference between ArrayList and LinkedList? view answer
  • Q.What are pass by reference and passby value in Java? view answer
  • Q. When we go for Abstract and Interface in Java? view answer
  • Q.What is the difference between interface and abstract class? view answer
  • Q.What are the Garbage collection algorithms in Java? view answer
  • Q.What is garbage collection and the purpose of garbage collection in Java? view answer
  • Q. What is the difference between an Interface and an Abstract class? view answer

!!! Java interview questions !!!

What are the parameters to follow Creating and Destroying Objects in Java?



Item 1: Consider providing static factory methods instead of
constructors
public static Boolean valueOf(boolean b) {
return (b ? Boolean.TRUE : Boolean.FALSE);
}
advantage of static factory methods is that, unlike constructors, they are not
required to create a new object each time they're invoked.
This allows immutable classes
(Item 13) to use preconstructed instances or to cache instances as they're constructed and to
dispense these instances repeatedly so as to avoid creating unnecessary duplicate objects.
The Boolean.valueOf(boolean) method illustrates this technique: It never creates an object.
This technique can greatly improve performance if equivalent objects are requested
frequently, especially if these objects are expensive to create.
it allows an immutable class to ensure that no two equal instances exist:
a.equals(b) if and only if a==b. If a class makes this guarantee, then its clients can use
the == operator instead of the equals(Object) method, which may result in a substantial
performance improvement
implements this
optimization, and the String.intern method implements it in a limited form.

advantage of static factory methods is that, unlike constructors, they can return
an object of any subtype of their return type. This gives you great flexibility in choosing
the class of the returned object.
One application of this flexibility is that an API can return objects without making their
classes public. Hiding implementation classes in this fashion can lead to a very compact API.

Item 2: Enforce the singleton property with a private constructor

Item 4: Avoid creating duplicate objects
It is often appropriate to reuse a single object instead of creating a new functionally equivalent
object each time it is needed. Reuse can be both faster and more stylish. An object can always
be reused if it is immutable
As an extreme example of what not to do, consider this statement:
String s = new String("silly"); // DON'T DO THIS!
The statement creates a new String instance each time it is executed, and none of those
object creations is necessary. The argument to the String constructor ("silly") is itself a
String instance, functionally identical to all of the objects created by the constructor. If this
usage occurs in a loop or in a frequently invoked method, millions of String instances can be
created needlessly.
The improved version is simply the following:
String s = "No longer silly";
Item 5: Eliminate obsolete object references
So where is the memory leak? If a stack grows and then shrinks, the objects that were popped
off the stack will not be garbage collected, even if the program using the stack has no more
references to them. This is because the stack maintains obsolete references to these objects.
An obsolete reference is simply a reference that will never be dereferenced again. In this case,
any references outside of the ?active portion? of the element array are obsolete. The active
portion consists of the elements whose index is less than size.

public Object pop() {
if (size == 0)
throw new EmptyStackException();
return elements[--size];
}
The fix for this sort of problem is simple: Merely null out references once they become
obsolete. In the case of our Stack class, the reference to an item becomes obsolete as soon as
it's popped off the stack. The corrected version of the pop method looks like this:
public Object pop() {
if (size==0)
throw new EmptyStackException();
Object result = elements[--size];
elements[size] = null; // Eliminate obsolete reference
return result;
}
Item 6: Avoid finalizers
There is no guarantee that finalizers will be executed promptly. It can take
arbitrarily long between the time that an object becomes unreachable and the time that its
finalizer is executed. This means that nothing time-critical should ever be done by a
finalizer. For example, it is a grave error to depend on a finalizer to close open files because
open file descriptors are a limited resource. If many files are left open because the JVM is
tardy in executing finalizers, a program may fail because it can no longer open files.





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