Java core

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

What is function overriding?

Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.

What is protected access modifier?

It is used to initialize the static data member, It is excuted before main method at the time of classloading.

What is static block?

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

What is Serialization and deserialization?

These are variables declared with in a class, outside of any method with the static word.

What is a class variable?

variable defined inside methods, constructors or blocks are called local variables. That variable will be declared and initialized within the method and it will be destroyed when the method has completed.

What is a local variable?

Singleton class control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.

What is a singleton class?

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.

What are use cases?

It is a high level programming language oriented objects style. Java runs on a different platforms as windows, Mac OS, unix

What do you know about Java?

Java provides access modifiers to set access levels for classes, variables, methods and constructors. A member has package or default accessibility when no accessibility modifier is specified.

What do you mean by Access Modifier?

It is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.

What do you mean by Checked Exceptions?

A multithreaded 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.

What do you mean by Multithreaded program?

Constructor gets invoked when a new object is created. Every class has a constructor. If we don't explicitly write a constructor for a class the Java compiler builds a default constructor for that class.

What do you mean by constructor?

Object is a runtime entity and it's state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanics for object to object communication.

What do you mean by object?

Java provides these modifiers for providing functionalities other than Access Modifiers, synchronized used to indicate that a method can be accessed by only one thread at a time.

What do you mean by synchronized Non Access Modifier?

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

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

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.

What is the difference between yielding and sleeping?

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this Encapsulation gives maintainability, flexibility and extensibility to our code.

What is the primary benefit of Encapsulation?

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

What is the purpose of File class?

The java compiler creates a default constructor only if there is no constructor in the class.

What is the purpose of default constructor?

local variable and instance variable, and class variable.

What kind of variables a class can consist of?

While creating your own exception − All exceptions must be a child of Throwable. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class. You want to write a runtime exception, you need to extend the RuntimeException class.

What things should be kept in mind while creating your own exceptions in Java?

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.

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

No, this is not possible.

Can a constructor be made final?

Default value of byte datatype is 0.

What is the default value of byte datatype in Java?

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.

Define Packages in Java?

A class is a blue print from which individual objects are created. A class can contains fields and methods to describe the behavior of an object.

Define class?

Postfix operators i.e () [] . is at the highest precedence.

According to Java Operator precedence, which operator is considered to be with highest precedence?

Yes, use this() syntax.

Can you call one constructor from another if a class has multiple constructors?

It is the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.

Define Inheritance?

It improves the runtime performance of computer programs based on bytecode.

Define JIT compiler?

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;

Define JRE i.e. Java Runtime Environment?

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

Define composition?

An immutable object can't be changed once it is created.

Define immutable object?

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

Difference between Overloading and Overriding?

It includes: Throw is used to trigger an exception where as throws is used in declaration of exception. Without throws, Checked exception cannot be handled where as checked exception can be propagated with throws.

Difference between throw and throws?

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

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

It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.

Explain Runtime Exceptions?

It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

Explain Set Interface?

It is a Set implemented when we want elements in a sorted order.

Explain TreeSet?

It uses garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program.

Explain garbage collection in Java?

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 affirns 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

Explain the following line used under Java Program − public static void main (String args[ ])

It is used with variables or methods and used to call constructer of same class.

Explain the usage of this() with constructors?

Sub class inherits all the public and protected methods and the implementation. It also inherits all the default modifier methods and their implementation.

Explain the use of sublass in a Java program?

It includes − Interface cannot be instantiated An interface does not contain any constructors. All of the methods in an interface are abstract.

Give some features of Interface?

Java uses just in time compiler to enable high performance. Just in time compiler it is a program that turns Java bytecode, which is a program that contains instructions that must be interpreted into instructions that can be sent directly to the processor.

How Java enabled high performance?

The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.

How finally used under Exception Handling?

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.

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

Yes, since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation. Sí, dado que la herencia hereda todo de la superclase y la interfaz, puede hacer que la subclase sea demasiado agrupada y, a veces, propensa a errores cuando se sobreescribe dinámicamente o se sobrecarga dinámicamente en alguna situación.

Is there any limitation of using Inheritance?

Object oriented Platform independent Robust Interpreted Multi-threaded

List any 5 features of Java

import, super, finally, etc

List some Java keywords(Unlike C, C++) keywords.

An object is first declared, then instantiated and then is initialized.

List the three steps for creating an object for a class

Netbeans, Eclipse

List to Java IDE's

Variables used in a switch statement can only be a string, enum, byte, short, int, or char.

Variables used in a switch statement can be used with which datatypes?

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

What are Wrapper classes?

windows, Unix -linux, Mac OS

What are the supported platforms by Java programming language?

Thread can be created by: implementing Runnable interface, extending the Thread class.

What are the two ways in which Thread can be created?

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.

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

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.

What invokes a thread's run() method?

These classes cannot be instantiated and are either partially implemented or not at all implemented. This class contains one or more abstract methods which are simply method declarations without a body.

What is Abstract class?

It refers to the ability to make a class abstract in OOP. It helps to reduce the complexity and also improves the maintainability of the system.

What is Abstraction?

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.

What is Comparable Interface?

It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.

What is Encapsulation?

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.

What is JAR file?

Its compiler generates an architecture-neutral object file format, which made the compiled code to be executable on many processors with the presence of Java run time system

What is Java architecture neutral?

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

What is NullPointerException?

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

What is Polymorphism?

When Java is compiled into platform specific machine, rather into platform independent bytecode. This byte code is distributed over the web and interpreted by virtual machine (JVM) on whatever platform it is being run.

What is a Java virtual machine and how is considered in context of Java's platform independent feature?

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.

What is a WAR file?

Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.

What is a static variable?

A transient variable is a variable that may not be serialized during Serialization and which is initialized by its default value during de-serialization,

What is a transient variable?

An exception is a problem that arises during the execution of a program. Exceptions are caught by handlers positioned along the thread's method invocation stack.

What is an Exception?

An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

What is an Interface?

Instances variables are variables within a class but outside any method. These variables are instantiated when the class is loaded.

What is an instance variable?

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

What is final class?

It is possible to define a method that will be called just before an object's final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.

What is finalize() method?

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

What is function overloading?

Synchronization is the capability to control the access of multiple threads to shared resources. synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race. Synchronization control the access of multiple threads to shared resources. Sychronized key workd that handled that.

What is synchronization?

For float its 0.0f and for double it's 0.0d

What is the default value of float and double datatype in Java?

Use StringBuilder whenever possible because it is faster than StringBuffer. But, if thread safety is necessary then use StringBuffer objects.

What is the difference between StringBuffer and StringBuilder class?

Object based programming languages follow all the features of OOPs except Inheritance. JavaScript is an example of object based programming languages.

What is the difference between object oriented programming language and object based programming language?

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. Una variable estática se asocia con la clase como un todo en lugar de con instancias específicas de una clase. Las variables no estáticas adquieren valores únicos con cada instancia de objeto.

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

Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

What's the difference between constructors and other methods?

The code sleep(2000); puts thread aside for exactly two seconds. The code wait(2000), causes a wait of up to two second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

What's the difference between the methods sleep() and wait()?

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.

When Abstract methods are used?

The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.

When ArithmeticException is thrown?

This data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.

When a byte datatype is used?

When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.

When is the ArrayStoreException thrown?

This method is used to get the primitive data type of a certain String.

When parseInt() method can be used?

If the method overrides one of its superclass's methods, overridden method can be invoked through the use of the keyword super. It can be also used to refer to a hidden field.

When super keyword is used?

An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.

When throw keyword is used?

If a method does not handle a checked exception, the method must declare it using the throwskeyword. The throws keyword appears at the end of a method's signature.

When throws keyword is used?

The Exception class has two main subclasses : IOException class and RuntimeException Class.

Which are the two subclasses under Exception class?

java.util.regex package is used for this purpose.

Which package is used for pattern matching with regular expressions?

It is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve access to objects on run-time. Java can carry a lot of run time information can be used to verify and resolve access to objects in run time.

Why Java is considered dynamic?

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.

Why Packages are used?

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.

Why Vector class is used?

The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.

Why is String class considered immutable?

The String class is considered as immutable, so that once it is created a String object cannot be changed. If there is a necessity to make a lot of modifications to Strings of characters then StringBuffer should be used.

Why is StringBuffer called mutable?

java.util.regex consists of three classes − Pattern class, Matcher class and PatternSyntaxException class.

java.util.regex consists of which classes?