Java Interview Questions : Series 5

 

78. What is the purpose of package in

Java?

A package is used to encapsulate a group of classes, interfaces and

sub-packages. Often, it is a hierarchical structure of storing

information. It is easier to organize the related classes and subpackages

in this manner.

A Package also provides access protection for classes and

interfaces. A package also helps in removing naming collision.

79. What is java.lang package?

In Java, java.lang package contains the classes that are fundamental

to the design of Java programming language. The most important

class in this package is Object class.

It also contains wrapper classes like- Integer, Boolean, Character

etc. It provides Math class for mathematical operations.

80. Which is the most important class

in Java?

It is an open-ended question with many answers. In my view, Object

class is the most important class of Java programming language. It

is the root of all the classes in Java. It provides some very

important and fundamental methods.

81. Is it mandatory to import java.lang

package every time?

No. By default, JVM loads it internally.

82. Can you import same package or

class twice in your class?

If we import same package multiple times in a class, compiler

includes it only once. So neither JVM nor Compiler gives any

error/warning on including a package multiple times.

If you have two classes with same name, then you may get name

collision on importing the class erroneously.

JVM internally loads the class only one time.

83. What is a static import in Java?

Static import is similar to normal import declaration. Normal

import allows us to import classes from packages without using

package qualifier. Static import allows us to import static members

from a class without using class qualifier.

84. What is the difference between

import static com.test.Fooclass and

import com.test.Fooclass?

First import is a static import and the second import is normal

import of a class. First import allows us to import static members of

class.

85. What is Locale in Java?

A Locale object represents a specific geographical, political, or

cultural region. It is used to locale-sensitive operations in Java.

It helps is following the local conventions of a country, native or

region. These conventions can be for formatting the dates, money,

numbers etc.

86. How will you use a specific Locale

in Java?

To use a specific Locale, we need to load that Locale. We can use

ResourceBundle.getBundle("Locale.UK") method to load a Locale.

87. What is the serialization?

Serialization is a process converting an object into a byte array.

This byte array represents the class, version and internal state of the

object. JVM can use this byte array to transmit/read the object over

a network.

88. What is the purpose of

serialization?

Some of the uses of serialization are:

1. Communication: It is used for transmitting an object over

network between two machines.

2. Persistence: We can store the object’s state in a database

and retrieve it from database later on.

3. Caching: Serialization can be used for caching to improve

performance. We may need 10 minutes to build an object,

but it may take just 10 seconds to de-serialize the object.

4. Cross JVM Synchronization: It can be used in same way

across multiple JVM that follow different architecture.

89. What is Deserialization?

Deserialization is the process of reconstructing the object from the

serialized state. It is the reverse process of serialization.

90. What is Serialization and

Deserialization conceptually?

Serialization is to convert Object data into a stream of bytes

Deserialization is to convert a stream of bytes back into a copy of

the original object.

91. Why do we mark a data member

transient?

Member variables of an object are marked transient to indicate that

they should not be serialized.

During serialization process the transient variables are not

considered part of the persistent state of an object.

92. Is it allowed to mark a method as

transient?

No, Java does not allow marking a method as transient. The

transient keyword is valid only for member variables.

93. How does marking a field as

transient makes it possible to serialize

an object?

Let say we have a class ABC that implements Serializable

interface, but it contains a member variable object of class XYZ

that does not implement Serializable interface. Due to this it is not

possible to Serialize the class ABC.

To solve this issue, we can mark the member variable XYZ as

Transient in class ABC. This will allow us to serialize the class

ABC.

94. What is Externalizable interface

in Java?

Externalizable interface extends Serializable interface in Java. It is

used for giving the Class control over saving and restoring the

contents of its instances.

A class implements methods writeExternal() and readExternal() to

store and restore the object.

95. What is the difference between

Serializable and Externalizable

interface?

Serializable is a marker interface but Externalizable is not a marker

interface.

When we implement Serializable interface, the class is serialized

automatically by default. We can override writeObject() and

readObject()methods to control more complex object Serialization

process.

In case of Externalizable, we use readExternal() and

writeExternal() methods to give control to class for class's

serialization process.

Serializable interface is based on recursive algorithm.

Serializable gives you two options. One option is to provide custom

way of serialization, the other default way. In Externalizable, you

have to always implement readExternal() and writeExternal()

methods.

A public no-arg constructor is needed while using Externalizable

interface.

In Serialization, we need to define serialVersionUID. If it is not

explicitly defined it will be generated automatically based on all the

fields, methods of the class.

96. What is Reflection in Java?

Reflection is Java language's ability to inspect and dynamically call

classes, methods, attributes etc. at Runtime. It helps in examining or

modifying the Runtime behavior of a class at Runtime.

97. What are the uses of Reflection in

Java?

Reflection is often used in Testing, Debugging and in Integrated

Development Environment (IDE).

Reflection allows you to write programs that do not have to "know"

everything at compile time. It makes programs more dynamic, since

they can be tied together at runtime.

Many modern frameworks like Spring etc. use Reflection. Some

modern languages like Python etc. also use Reflection.

JAVA API for XML Parsing (JAXP) also uses Reflection.

98. How can we access private

method of a class from outside the

class?

We can use Reflection to access private method of a class from

outside the class. IN Java, we use getDeclaredMethod() to get

instance of a private method. Then we mark this method accessible

and finally invoke it.

In following sample code, we are accessing private method

message() of class Foo by Reflection.

FileName: Foo.java

public class Foo {

private void message(){System.out.println("hello java"); }

}

FileName: FooMethodCall.java

import java.lang.reflect.Method;

public class FooMethodCall{

public static void main(String[] args)throws Exception{

Class c = Class.forName("Foo");

Object o= c.newInstance();

Method m =c.getDeclaredMethod("message", null);

m.setAccessible(true);

m.invoke(o, null);

}

}

99. How can we create an Object

dynamically at Runtime in Java?

We can use Reflection to create an Object dynamically at Runtime

in Java. We can use Class.newInstance() or

Constructor.newInstance() methods for creating such Objects.