Java interview questions & answers -Set 2

• What is meant by Inheritance and what are its advantages?- Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
• What is the difference between this() and super()?- this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
• What is the difference between superclass and subclass?- A super class is a class that is inherited whereas sub class is a class that does the inheriting.
• What modifiers may be used with top-level class?- public, abstract and final can be used for top-level class.
• What are inner class and anonymous class?- Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private. Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
• What is a package?- A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
• What is a reflection package?- java. lang. reflect package has the ability to analyze itself in runtime.
• What is interface and its use?- Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object’s programming interface without revealing the actual body of the class.
• What is an abstract class?- An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
• What is the difference between Integer and int?- a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
• What is a cloneable interface and how many methods does it contain?- It is not having any method because it is a TAGGED or MARKER interface.
• What is the difference between abstract class and interface?- a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can’t have subclasses.
• Can you have an inner class inside a method and what variables can you access?- Yes, we can have an inner class inside a method and final variables can be accessed.
• What is the difference between String and String Buffer?- a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
• What is the difference between Array and vector?- Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
• What is the difference between exception and error?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing. The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.
• What is the difference between process and thread?- Process is a program in execution whereas thread is a separate path of execution in a program.
• What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?- Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state. notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.
• What is the class and interface in java to create thread and which is the most advantageous method?- Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
• What are the states associated in the thread?- Thread contains ready, running, waiting and dead states.