JAVA INTERVIEW QUESTIONS

Question 1: What is the difference between an Interface and an Abstract class?
Answer:An abstract class can have instance methods that implement a default behavior.An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Question 2: What is the purpose of garbage collection in Java, and when is it used?
Answer:The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
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Question 3: Describe synchronization in respect to multithreading.
Answer:With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable
while another thread is in the process of using or updating same shared v ariable. This usually leads to
signif icant errors.

Question 4: Explain different way of using thread?
Answer:The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more adv antageous, 'cause when you are going for multiple inheritance..the only interface can
help.


Question 5: What are passby reference and passby value?
Answer:Pass By Reference means the passing the address itself rather than passing the v alue. Passby Value
means passing a copy of the value to be passed.

Question 6: What is HashMap and Map?
Answer:Map is Interface and Hashmap is class that implements that.

Question 7: Difference between HashMap and HashTable?
Answer:The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.(HashMap allows null values as key and v alue whereas Hashtable doesnt allow). HashMap does not
guarantee that the order of the map will remain constant over time. HashMap is non synchronized and
Hashtable is synchronized.



Question 8: Difference between Vector and ArrayList?
Answer:Vector is synchroniz ed whereas arraylist is not.

Question 9: Difference between Swing and AWT?
Answer:AWT are heavy-weight components. Swings are light-weight components. Hence swing work s faster than AWT.

Question 10: What is the difference between a constructor and a method?
Answer:A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

Question 11: What are Iterators?
Answer:Some of the collection classes prov ide traversal of their contents via a jav a.util.Iterator interface. This interface allows you to walk a collection of objects, operating on each object in turn. Remember when
using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained;generally it is not advisable to modify the collection itself while traversing an Iterator.

Question 12: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
Answer: public: Public class is visible in other packages, field is visible everywhere (class must be public too)
private: Priv ate variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected: Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided ev en to subclasses that reside in a different package f rom the class that owns the protec ted feature. default: What you get by default i.e. without any access modifier (ie, public priv ate or protected).It means that it is visible to all within a particular package.

Question 13: What is an abstract class?
Answer:Abstract class must be extended/subclassed (to be useful). It serv es as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prev ents it from being instantiated.

Question 14: What is static in java?
Answer:Static means one per class, not one for each object no matter how many instance of a class might exist.This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. Howev er, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

Question 15: What is final class?
Answer:A final class can't be extended. i.e. final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change v alue of a final variable (is a constant).

Question 16: What if the main method is declared as private?
Answer: The program compiles properly but at runtime it will give "Main method not public." message.

Question 17: What if the static modifier is removed from the signature of the main method?
Answer:Program compiles. But at runtime throws an error "NoSuchMethodError".

Question 18: What if I write static public void instead of public static void?
Answer:Program compiles and runs properly.

Question 19: What if I do not provide the String array as the argument to the method?
Answer:Program compiles but throws a runtime error "NoSuchMethodError".

Question 20: What is the first argument of the String array in main method?
Answer:The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

Question 21: If I do not provide any arguments on the command line, then the String array of Main method will be empty of null?
Answer:It is empty. But not null.

Question 22: How can one prove that the array is not null but empty?
Answer:Print args.length. It will print 0. That means it is empty. But if it would hav e been null then it would have thrown a NullPointerException on attempting to print args.length.

Question 23: What environment variables do I need to set on my machine in order to be able to run Java programs?
Answer:CLASSPATH and PATH are the two variables.

Question 24: Can an application have multiple classes having main method?
Answer:Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

Question 25: Can I have multiple main methods in the same class?
Answer:No the program fails to compile. The compiler says that the main method is already defined in the class.

Question 26: Do I need to import java.lang package any time? Why ?
Answer:No. It is by default loaded internally by the JVM.

Question 27: Can I import same package/class twice? Will the JVM load the package twice at runtime?
Answer:One can import the same package or same class multiple times. Neither compiler nor JVM complains about it. And the JVM will internally load the class only once no matter how many times you import the same class.

Question 28: What are Checked and UnChecked Exception?
Answer:A checked exception is some subclass of Exception (or Exception itself), excluding class Runtime Exception and its subclasses.Making an ex ception checked f orces client programmers to deal with the possibility that the exception will be thrown. eg, IOExc eption thrown by jav a.io.FileInputStream's read() method• Unchecked ex ceptions are RuntimeException and any of its subclasses. Class Error and its subclassesalso are unchecked. With an unchecked exception, howev er, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method• Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.