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Core Java Interview Questions and Answers

Ques 36. What's the difference between == and equals method?

Ans. equals checks for the content of the string objects while == checks for the fact that the two String objects point to same memory location ie they are same references.

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Ques 37. What are abstract classes, abstract methods?
Ans. Simply speaking a class or a method qualified with "abstract" keyword is an abstract class or abstract method. You create an abstract class when you want to manipulate a set of classes through a common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism. If you have an abstract class, objects of that class almost always have no meaning. That is, abstract class is meant to express only the interface and sometimes some default method implementations, and not a particular implementation, so creating an abstract class object makes no sense and are not allowed ( compile will give you an error message if you try to create one). An abstract method is an incomplete method. It has only a declaration and no method body. Here is the syntax for an abstract method declaration: abstract void f(); If a class contains one or more abstract methods, the class must be qualified an abstract. (Otherwise, the compiler gives you an error message.). It's possible to create a class as abstract without including any abstract methods. This is useful when you've got a class in which it doesn't make sense to have any abstract methods, and yet you want to prevent any instances of that class. Abstract classes and methods are created because they make the abstractness of a class explicit, and tell both the user and the compiler how it was intended to be used.
For example:
abstract class Instrument { 
	int i; // storage allocated for each 
	public abstract void play(); 
	public String what() { 
		return "Instrument";
	public abstract void adjust(); 

class Wind extends Instrument { 
	public void play() { 
	public String what() { 
		return "Wind"; 
	public void adjust() {}
Abstract classes are classes for which there can be no instances at run time. i.e. the implementation of the abstract classes are not complete. Abstract methods are methods which have no defintion. i.e. abstract methods have to be implemented in one of the sub classes or else that class will also become Abstract.
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Ques 38. Java says "write once, run anywhere". What are some ways this isn't quite true?
Ans. As long as all implementaions of java are certified by sun as 100% pure java this promise of "Write once, Run everywhere" will hold true. But as soon as various java core implemenations start digressing from each other, this won't be true anymore. A recent example of a questionable business tactic is the surreptitious behavior and interface modification of some of Java's core classes in their own implementation of Java. Programmers who do not recognize these undocumented changes can build their applications expecting them to run anywhere that Java can be found, only to discover that their code works only on Microsoft's own Virtual Machine, which is only available on Microsoft's own operating systems.
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Ques 39. What is the difference between a Vector and an Array. Discuss the advantages and disadvantages of both?
Ans. Vector can contain objects of different types whereas array can contain objects only of a single type.
- Vector can expand at run-time, while array length is fixed.
- Vector methods are synchronized while Array methods are not
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Ques 40. What are java beans?
Ans. JavaBeans is a portable, platform-independent component model written in the Java programming language, developed in collaboration with industry leaders. It enables developers to write reusable components once and run them anywhere ' benefiting from the platform-independent power of Java technology. JavaBeans acts as a Bridge between proprietary component models and provides a seamless and powerful means for developers to build components that run in ActiveX container applications. JavaBeans are usual Java classes which adhere to certain coding conventions:
1. Implements interface
2. Provides no argument constructor
3. Provides getter and setter methods for accessing it'??s properties
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