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Headstrong Java Questions
Posted on :30-03-2016
Q1. What is a class?
A class is a blueprint, or prototype, that defines the variables and the methods common to all objects of a certain kind.
Q2. What is a object?
An object is a software bundle of variables and related methods.An instance of a class depicting the state and behavior at that particular time in real world.
Q3. What is a method?
Encapsulation of a functionality which can be called to perform specific tasks.
Q4. What is encapsulation? Explain with an example.
Encapsulation is the term given to the process of hiding the implementation details of the object. Once an object is encapsulated, its implementation details are not immediately accessible any more. Instead they are packaged and are only indirectly accessible via the interface of the object.
Q5. What is inheritance?
Explain with an example. Inheritance in object oriented programming means that a class of objects can inherit properties and methods from another class of objects.
Q6. What is polymorphism?
Explain with an example. In object-oriented programming, polymorphism refers to a programming languages ability to process objects differently depending on their data type or class. More specifically, it is the ability to redefine methods for derived classes. For example, given a base class shape, polymorphism enables the programmer to define different area methods for any number of derived classes, such as circles, rectangles and triangles. No matter what shape an object is, applying the area method to it will return the correct results. Polymorphism is considered to be a requirement of any true object-oriented programming language.
Q7. Is multiple inheritance allowed in Java? No, multiple inheritance is not allowed in Java.
Q8. What is interpreter and compiler?
Java interpreter converts the high level language code into a intermediate form in Java called as bytecode, and then executes it, where as a compiler converts the high level language code to machine language making it very hardware specific.
Q9. What is JVM?
The Java interpreter along with the run time environment required to run the Java application in called as Java virtual machine(JVM)
Q10. What are the different types of modifiers? There are access modifiers and there are other identifiers. Access modifiers are public, protected and private. Other are final and static.
Q11. What are the access modifiers in Java?
There are 3 access modifiers. Public, protected and private, and the default one if no identifier is specified is called friendly, but programmer cannot specify the friendly identifier explicitly.
Q12. What is a wrapper class?
They are classes that wrap a primitive data type so it can be used as a object.
Q13. What is a static variable and static method? Whats the difference between two?
The modifier static can be used with a variable and method. When declared as static variable, there is only one variable no matter how instances are created, this variable is initialized when the class is loaded. Static method do not need a class to be instantiated to be called, also a non static method cannot be called from static method.
Q14. What is garbage collection?
Garbage Collection is a thread that runs to reclaim the memory by destroying the objects that cannot be referenced anymore.
Q15. What is abstract class?
Abstract class is a class that needs to be extended and its methods implemented, a class has to be declared abstract if it has one or more abstract methods.
Q16. What is meant by final class, methods and variables?
This modifier can be applied to class method and variable. When declared as final class the class cannot be extended. When declared as final variable, its value cannot be changed if is primitive value, if it is a reference to the object it will always refer to the same object, internal attributes of the object can be changed.
Q17. What is interface?
Interface is a contact that can be implemented by a class, it has method that need implementation.
Q18. What is method overloading?
Overloading is declaring multiple method with the same name, but with different argument list.
Q19. What is method overriding?
Overriding has same method name, identical arguments used in subclass.
Q20. What is singleton class?
Singleton class means that any given time only one instance of the class is present, in one JVM.
Q21. What is the difference between an array and a vector?
Number of elements in an array are fixed at the construction time, whereas the number of elements in vector can grow dynamically.
Q22. What is a constructor?
In Java, the class designer can guarantee initialization of every object by providing a special method called a constructor. If a class has a constructor, Java automatically calls that constructor when an object is created, before users can even get their hands on it. So initialization is guaranteed.
Q23. What is casting?
Conversion of one type of data to another when appropriate. Casting makes explicitly converting of data.
Q24. What is the difference between final, finally and finalize?
The modifier final is used on class variable and methods to specify certain behaviour explained above. And finally is used as one of the loop in the try catch blocks, It is used to hold code that needs to be executed whether or not the exception occurs in the try catch block. Java provides a method called finalize( ) that can be defined in the class. When the garbage collector is ready to release the storage ed for your object, it will first call finalize( ), and only on the next garbage-collection pass will it reclaim the objects memory. So finalize( ), gives you the ability to perform some important cleanup at the time of garbage collection.
Q25. What is are packages?
A package is a collection of related classes and interfaces providing access protection and namespace management.
Q26. What is a super class and how can you call a super class?
Q27. When a class is extended that is derived from another class there is a relationship is created, the parent class is referred to as the super class by the derived class that is the child. The derived class can make a call to the super class using the keyword super. If used in the constructor of the derived class it has to be the first statement.
Q28. What is meant by a Thread?
Thread is defined as an instantiated parallel process of a given program.
Q29. What is multi-threading?
Multi-threading as the name suggest is the scenario where more than one threads are running.
Q30. What are two ways of creating a thread? Which is the best way and why?
Two ways of creating threads are, one can extend from the Java.lang.Thread and can implement the rum method or the run method of a different class can be called which implements the interface Runnable, and the then implement the run() method. The latter one is mostly used as first due to Java rule of only one class inheritance, with implementing the Runnable interface that problem is sorted out.
Q31. What is deadlock?
Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread. In Java, this resource is usually the object lock obtained by the synchronized keyword.
Q32. What are the three types of priority?
MAX_PRIORITY which is 10, MIN_PRIORITY which is 1, NORM_PRIORITY which is 5.
Q33. What is the use of synchronizations?
Every object has a lock, when a synchronized keyword is used on a piece of code the, lock must be obtained by the thread first to execute that code, other threads will not be allowed to execute that piece of code till this lock is released.
Q34. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. For example, a thread only executes a synchronized method after it has acquired the lock for the methods object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Q35. What are different ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, blocking on I/O, unsuccessfully attempting to acquire an objects lock, or invoking an objects wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
Q36. Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the classs Class object.
Q37. Whats new with the stop(), suspend() and resume() methods in new JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
Q38. What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.
Q39. What method is used to specify a containers layout?
The setLayout() method is used to specify a containers layout. For example, setLayout(new FlowLayout()); will be set the layout as FlowLayout.
Q40. Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
Q41. What state does a thread enter when it terminates its processing?
Q42. When a thread terminates its processing, it enters the dead state.
Q43. What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects. One example of class in Collections API is Vector and Set and List are examples of interfaces in Collections API.
Q44. What is the List interface?
The List interface provides support for ordered collections of objects. It may or may not allow duplicate elements but the elements must be ordered.
Q45. How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
Q46. What is the Vector class?
The Vector class provides the capability to implement a growable array of objects. The main visible advantage of this class is programmer neednt to worry about the number of elements in the Vector.