Core Java Interview Questions and Answers Part 5


Q: What is a static block?

A: It is used to initialize the static data member, It is executed before the main method at the time of class loading.

Q: Define composition?

A: Holding the reference of the other class within some other class is known as composition.

Q: What is function overloading?

A: If a class has multiple functions by the same name but different parameters, it is known as Method Overloading.

Q: What is function overriding?

A: If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding.

Q: Difference between Overloading and Overriding

A: Method overloading increases the readability of the program. Method overriding provides the specific implementation of the method that is already provided by its superclass parameter must be different in case of overloading, a parameter must be same in case of overriding.

Q: What is a final class?

A: Final classes are created so the methods implemented by that class cannot be overridden. It can’t be inherited.

Q: What is NullPointerException?

A: A NullPointerException is thrown when calling the instance method of a null object, accessing or modifying the field of a null object etc.

Q: What are the ways in which a thread can enter the waiting state?

A: A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Q: How does multithreading take place on a computer with a single CPU?

A: The operating system’s task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

Q: What invokes a thread’s run() method?

A: After a thread is started, via its start() method of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.

Q: Does it matter in what order catch statements for FileNotFoundException and IOException are written?

A: Yes, it does. The FileNoFoundException is inherited from the IOException. Exception’s subclasses have to be caught first.

Q: What is the difference between yielding and sleeping

A: When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

Q: Why Vector class is used?

A: The Vector class provides the capability to implement a growable array of objects. Vector proves to be very useful if you don’t know the size of the array in advance, or you just need one that can change sizes over the lifetime of a program.

Q: How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?

A: Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18-bit patterns. UTF-16 uses 16-bit and larger bit patterns.

Q: What are the Wrapper classes?

A: These are classes that allow primitive types to be accessed as objects. Example: Integer, Character, Double, Boolean etc.

Q: What is the difference between a Window and a Frame?

A: The Frame class extends Window to define a main application window that can have a menu bar.

Q: Which package has lightweight components?

A: javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame, and JWindow are lightweight components.

Q: What is the difference between the paint() and repaint() methods?

A: The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

Q: What is the purpose of File class?

A: It is used to create objects that provide access to the files and directories of a local file system.

Q: What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?

A: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

Q: Which class should you use to obtain design information about an object?

A: The Class class is used to obtain information about an object’s design and java.lang.Class class instance represent classes, interfaces in a running Java application.

Q: What is the difference between static and non-static variables?

A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Q: What are Serialization and Deserialization?

A: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

Q: What are the use cases?

A: It is part of the analysis of a program and describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance.

Q: Explain the use of subclass in a Java program?

A: Subclass inherits all the public and protected methods and the implementation. It also inherits all the default modifier methods and their implementation.

Q: How to add menu shortcut to the menu item?

A: If there is a button instance called b1, you may add menu shortcut by calling b1.setMnemonic(‘F’), so the user may be able to use Alt+F to click the button.


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