Core Java Interview Questions and Answers Part 4
Q: What is an Interface?
A: An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
Q: Give some features of Interface?
A: It includes:
- An interface cannot be instantiated.
- An interface does not contain any constructors.
- All of the methods in an interface are abstract.
Q: Define Packages in Java?
A: A Package can be defined as a grouping of related types(classes, interfaces, enumerations, and annotations ) providing access protection and namespace management.
Q: Why Packages are used?
A: Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations, and annotations, etc., easier.
Q: What do you mean by Multi-threaded program?
A: A multi-threaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.
Q: What are the two ways in which Thread can be created?
A: Thread can be created by implementing the Runnable interface, extending the Thread class.
Q: What is an applet?
A: An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
Q: An applet extend which class?
A: An applet extends java.applet.Applet class.
Q: Explain garbage collection in Java?
A: It uses garbage collection to free the memory. By cleaning those objects that are no longer referenced by any of the program.
Q: Define immutable object?
A: An immutable object can’t be changed once it is created.
Q: Explain the usage of this() with constructors?
A: It is used with variables or methods and used to call a constructor of the same class.
Q: Explain Set Interface?
A: It is a collection of an element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.
Q: Explain TreeSet?
A: It is a Set implemented when we want elements in a sorted order.
Q: What is the Comparable Interface?
A: It is used to sort collections and arrays of objects using the Collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.
Q: Difference between throw and throws?
A: It includes:
- The throw is used to trigger an exception where as throws is used in the declaration of exception.
- Without throws, a Checked exception cannot be handled whereas a checked exception can be propagated with throws.
Q: Explain the following line used under Java Program:
public static void main (String args[ ])
A: The following shows the explanation individually:
- public: it is the access specifier.
- static: it allows main() to be called without instantiating a particular instance of a class.
- void: it affirms the compiler that no value is returned by main().
- main(): this method is called at the beginning of a Java program.
- String args: args parameter is an instance array of class String
Q: Define JRE i.e. Java Runtime Environment?
A: Java Runtime Environment is an implementation of the Java Virtual Machine which executes Java programs. It provides the minimum requirements for executing a Java application.
Q: What is JAR file?
A: JAR files is Java Archive fles and it aggregates many files into one. It holds Java classes in a library. JAR files are built on ZIP file format and have .jar file extension.
Q: What is a WAR file?
A: This is Web Archive File and used to store XML, java classes, and JavaServer Pages. which is used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, XML files, static Web pages etc.
Q: Define JIT compiler?
A: It improves the runtime performance of computer programs based on bytecode.
Q: What is the difference between object-oriented programming language and object-based programming language?
Q: What is the purpose of default constructor?
A: The Java compiler creates a default constructor only if there is no constructor in the class.
Q: Can a constructor be made final?
A: No, this is not possible.
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