JAVA WRITTEN TEST QUESTIONS AND ANSWERS PDF

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Ans : An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object. What is the difference between the File and RandomAccessFile classes? Ans : The File class encapsulates the files and directories of the local file system.

The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file. What class allows you to read objects directly from a stream? Ans : The ObjectInputStream class supports the reading of objects from input streams. What value does read return when it has reached the end of a file? What value does readLine return when it has reached the end of a file?

Ans : The readLine method returns null when it has reached the end of a file. UTF-8 represents characters using 8, 16 and bit patterns. UTF uses bit and larger bit patterns. Which of the following are true? The InputStream and OutputStream classes are byte-oriented.

The Reader and Writer classes are character-oriented. The Reader and Writer classes are the preferred solution to serialized object output. Filters are supported on input, but not on output. Filters read from one stream and write to another.

A filter may alter data that is read from one stream and written to another. Any Unicode character is represented using bits. UTF-8 characters are represented using only 8-bits. UTF characters are represented using only bits.

The Serializable interface is used to identify objects that may be written to an output stream. The Externalizable interface is implemented by classes that control the way in which their objects are serialized.

The Serializable interface extends the Externalizable interface. The Externalizable interface extends the Serializable interface. Which of the following are true about the File class? Ans: In java, float takes 4 bytes in memory while Double takes 8 bytes in memory. Float is single precision floating point decimal number while Double is double precision decimal number. What is Final Keyword in Java?

Give an example. Ans: In java, a constant is declared using the keyword Final. Value can be assigned only once and after assignment, value of a constant can't be changed.

This method are faster than any other method,because they are resolved at complied time. When a class is declares as final,it cannot be subclassed.

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Example String,Integer and other wrapper classes. What is ternary operator? Ans: Ternary operator , also called conditional operator is used to decide which value to assign to a variable based on a Boolean value evaluation. It's denoted as? In the below example, if rank is 1, status is assigned a value of "Done" else "Pending". Ans: Using Math. What is default switch case? Give example. Ans: In a switch statement, default case is executed when no other switch condition matches. Default case is an optional case.

It can be declared only once all other switch cases have been coded. In the below example, when score is not 1 or 2, default case is used. What's the base class in Java from which all classes are derived? Ans: java.

Can main method in Java can return any data? Ans: In java, main method can't return any data and hence, it's always declared with a void return type. What are Java Packages? What's the significance of packages? Ans: In Java, package is a collection of classes and interfaces which are bundled together as they are related to each other.

Use of packages helps developers to modularize the code and group the code for proper re-use. Once code has been packaged in Packages, it can be imported in other classes and used.

Can we declare a class as Abstract without having any abstract method?

Ans: Yes we can create an abstract class by using abstract keyword before class name even if it doesn't have any abstract method. However, if a class has even one abstract method, it must be declared as abstract otherwise it will give an error. What's the difference between an Abstract Class and Interface in Java?

Ans: The primary difference between an abstract class and interface is that an interface can only possess declaration of public static methods with no concrete implementation while an abstract class can have members with any access specifiers public, private etc with or without concrete implementation. Another key difference in the use of abstract classes and interfaces is that a class which implements an interface must implement all the methods of the interface while a class which inherits from an abstract class doesn't require implementation of all the methods of its super class.

A class can implement multiple interfaces but it can extend only one abstract class. What are the performance implications of Interfaces over abstract classes?

Ans: Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces.

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Another key factor for developers to take into consideration is that any class can extend only one abstract class while a class can implement many interfaces. Use of interfaces also puts an extra burden on the developers as any time an interface is implemented in a class; developer is forced to implement each and every method of interface.

Does Importing a package imports its sub-packages as well in Java? Ans: In java, when a package is imported, its sub-packages aren't imported and developer needs to import them separately if required. For example, if a developer imports a package university. To load the classes from its sub-package say department , developer has to import it explicitly as follows: Import university.

Can we declare the main method of our class as private? Ans: In java, main method must be public static in order to run any application correctly.

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If main method is declared as private, developer won't get any compilation error however, it will not get executed and will give a runtime error. How can we pass argument to a function by reference instead of pass by value? Ans: In java, we can pass argument to a function only by value and not by reference. How an object is serialized in java? Ans: In java, to convert an object into byte stream by serialization, an interface with the name Serializable is implemented by the class.

All objects of a class implementing serializable interface get serialized and their state is saved in byte stream. When we should use serialization?

Ans: Serialization is used when data needs to be transmitted over the network. Using serialization, object's state is saved and converted into byte stream. The byte stream is transferred over the network and the object is re-created at destination. Ans: Try block needs to be followed by either Catch block or Finally block or both. Any exception thrown from try block needs to be either caught in the catch block or else any specific tasks to be performed before code abortion are put in the Finally block.

Is there any way to skip Finally block of exception even if some exception occurs in the exception block? Ans: If an exception is raised in Try block, control passes to catch block if it exists otherwise to finally block. Finally block is always executed when an exception occurs and the only way to avoid execution of any statements in Finally block is by aborting the code forcibly by writing following line of code at the end of try block: System.

When the constructor of a class is invoked? Ans: The constructor of a class is invoked every time an object is created with new keyword. For example, in the following class two objects are created using new keyword and hence, constructor is invoked two times.

Can a class have multiple constructors? Ans: Yes, a class can have multiple constructors with different parameters. Which constructor gets used for object creation depends on the arguments passed while creating the objects.

Can we override static methods of a class? Ans: We cannot override static methods. Static methods belong to a class and not to individual objects and are resolved at the time of compilation not at runtime.

Even if we try to override static method,we will not get an complitaion error,nor the impact of overriding when running the code. In the below example, what will be the output? Is String a data type in java?

Ans: String is not a primitive data type in java.

When a string is created in java, it's actually an object of Java. String class that gets created.

After creation of this string object, all built-in methods of String class can be used on the string object. In the below example, how many String Objects are created? String class are created. Why Strings in Java are called as Immutable? Ans: In java, string objects are called immutable as once value has been assigned to a string, it can't be changed and if changed, a new object is created.

In below example, reference str refers to a string object having value "Value one". What's the difference between an array and Vector? Ans: An array groups data of same primitive type and is static in nature while vectors are dynamic in nature and can hold data of different data types. What is multi-threading?

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Ans: Multi threading is a programming concept to run multiple tasks in a concurrent manner within a single program. Threads share same process stack and running in parallel. It helps in performance improvement of any program. Why Runnable Interface is used in Java? Ans: Runnable interface is used in java for implementing multi threaded applications. Runnable interface is implemented by a class to support multi threading.

What are the two ways of implementing multi-threading in Java? Ans: Multi threaded applications can be developed in Java by using any of the following two methodologies: 1.If we declare the constructor of a class as private, it will not be accessible by any other class and hence, no other class will be able to instantiate it and formation of its object will be limited to itself only.

There are no pointers in Java. What are Java Packages?

Ans: In java, when an object is not referenced any more, garbage collection takes place and the object is destroyed automatically. Which method is to get invoked will depend upon the parameters passed.

Which of these keywords is used by a class to use an interface defined previously? Which of these is an incorrect Statement? Ans: Interfaces are slower in performance as compared to abstract classes as extra indirections are required for interfaces.