Java Interview Questions and Answers

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Java Interview Questions and Answers

In this article, I am going to discuss the most frequently asked Top 100 Java Interview Questions and Answers. At the end of this article, you will able to answer most of the Java Interview Questions.

What is the difference between an Interface and an Abstract class?

An Abstract class declares to have at least one instance method that is declared abstract which will be implemented by the subclasses. An abstract class can have instance methods that implement a default behavior. An interface can only declare constants and instance methods, but cannot implement a default behavior.

What is the purpose of garbage collection in java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Describe synchronization with respect to multithreading.

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating the same shared variable. This usually leads to significant errors.

Explain the different ways of using threads?

The thread could be implemented by using a runnable interface or by inheriting from the Thread class. The former is more advantageous, ‘cause when you are going for multiple inheritances, the only interface can help.

What is pass by reference and pass by value?

Pass By Reference means passing the address itself rather than passing the value. Pass by values means passing a copy of the value to be passed.

What are HashMap and Map?

The map is Interface and HashMap is a class that implements that.

Difference between HashMap and HashTable?

This is one of the frequently asked Java Interview Questions and Answers. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is non synchronized and Hashtable is synchronized.

Difference between Vector and Arraylist?

Vector is synchronized whereas ArrayList is not.

Difference between Swing and AWT?

AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT.

What is the difference between a constructor and a method?

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

What are Iterators?

This is one of the frequently asked Java Interview Questions and Answers. Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This Interface allows you to walk a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally, it is not advisable to modify the collection itself while traversing an Iterator.

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

Public: Public class is visible in other packages, the field is visible everywhere (class must be public too)

Private: Private variables or methods may be used only by an instance of the same class that declares the variables or method, A private feature may only be accessed by the class that owns the feature.

Protected: Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.

Default: What you get by default i.e., without any access modifier (i.e., public, private, or protected). It means that it is visible to all within a particular package.

What is an abstract class?

This is one of the frequently asked Java Interview Questions and Answers. The abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (i.e., you may not call its constructor), an abstract class may contain static data. Any class with an abstract method is automatically abstract even if it has no abstract methods. This prevents it from being instantiated.

What is static in java?

This is one of the frequently asked Java Interview Questions and Answers. Static means one per class, not one for each object no matter how many instances of a class might exist. This means that you can use them without creating an instance of a class. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can’t override a static method with a non-static method. In other words, you can’t change a static method into an instance method in a subclass.

What is the final?

This is one of the frequently asked Java Interview Questions and Answers. A final class can’t be extended i.e. the final class may not be subclassed. A final method can’t be overridden when its class is inherited. You can’t change the value of a final variable (is a constant).

What if the main method is declared as private?

The program compiles properly but at runtime, it will give a “Main method not public.” message.

What if the static modifier is removed from the signature of the main method?

Program compiles. But at runtime throws an error “NoSuchMethodError”.

What if I write static public void instead of the public static void?

The program compiles and runs properly.

What if I do not provide the String array as the argument to the method?

The program compiles but throws a runtime error “NoSuchMethodError”.

What is the first argument of the String array in the main method?

The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

If I do not provide any arguments on the command line, then the String array of the Main method will be empty or null?

It is empty. But not null.

How can one prove that the array is not null but empty?

Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

What environment variables do I need to set on my machine in order to be able to run Java programs?

CLASSPATH and PATH are the two variables.

Can an application have multiple classes having the main method?

Yes, it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is no conflict amongst the multiple classes having the main method.

Can I have multiple main methods in the same class?

No, the program fails to compile. The compiler says that the main method is already defined in the class.

Do I need to import java.lang package anytime? Why?

No. It is by default loaded internally by JVM.

Can I import the same package/class twice? Will the JVM load the package twice at runtime?

One can import the same package or same class multiple times. Neither compiler nor JVM complains it. And the JVM will internally load the class only once no matter how many times you import the same class.

What is Checked and Unchecked Exception?

This is one of the frequently asked Java Interview Questions and Answers. A checked exception is some subclass of Exception (or Exception itself), excluding class runtime exception and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException has thrown by java.io.fileInputStream’s read() method. Unchecked exceptions are Runtime Exception and any of its subclasses. Class Error and its subclasses also are unchecked exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String’s charAT() method. Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

What is Overriding?

When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.

When the method is invoked for an object of the class, it is the new definition of the method that is called and not the method definition from the superclass. Methods may be overridden to be more public, not more private.

What are the different types of inner classes?

This is one of the frequently asked Java Interview Questions and Answers. Nested top-level classes, Member classes, Local classes, Anonymous classes

Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. Eg, outer.inner. Top-level inner classes implicitly have access only to static variables. There can also be inner interfaces. All of these are of the nested top-level variety.

Member classes- Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.

Local classes- Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface. Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes- Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Are the imports checked for validity at compile time? E.g. will the code containing an import such as java.lang.ABCD compile?

Yes, the imports are checked for semantic validity at compile time. The code containing the above line of import will not compile. It will throw an error saying, cannot resolve symbol: class ABCD location: package io import java.io.ABCD;

Does importing a package imports the subpackages as well? E.g. does import com.MyTest.*?

No, you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of its sub-packages.

What is the difference between declaring a variable and defining a variable?

In the declaration, we just mention the type of the variable and its name. we do not initialize it. But defining means declaration + initialization. eg. String s; is just a declaration while String s = new String (“abcd”); or String s = “abcd”; are both definitions.

What is the default value of an object reference declared as an instance variable?

Null unless we define it explicitly.

Can a top-level be private or protected?

This is one of the frequently asked Java Interview Questions and Answers. No, A top-level class cannot be private or protected. It can have either a “public” or no modifier. If it does not have a modifier it is supposed to have a default access. If a top-level class is declared as private the compiler will complain that the “modifier private is not allowed here”. This means that a top-level class can not be private. The same is the case with protected.

What type of parameter passing does Java support?

In Java, the arguments are always passed by value.

Primitive data types are passed by reference or pass by value?

Primitive data types are passed values.

Objects are passed by value or by reference?

Java only supports pass by value, with objects, the object reference itself is passed by value and so both the original reference and parameter copy, refer to the same object.

What is serialization?

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

How do I serialize an object to a file?

The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

Which methods of Serializable interface should I implement?

The serializable interface is an empty interface, it does not contain any methods. So, we do not implement any methods.

How can I customize the serialization process? i.e. how can one have control over the serialization process?

Yes, it is possible to have control over the serialization process. The class should implement the Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

What is the common usage of serialization?

Whenever an object is to be sent over the network, objects need to be serialized. Moreover, if the state of an object is to be saved, objects need to be serialized.

What is an Externalizable interface?

Externalizable is an interface that contains two methods readExternal and writeExternal. These methods give you control over the serialization mechanism. Thus, if your class implements this interface, you can customize the serialization process by implementing these methods.

What happens to the object references included in the object?

This is one of the frequently asked Java Interview Questions and Answers. The serialization mechanism generates an object graph for serialization. Thus, it determines whether the included object references are serializable or not. This is a recursive process. Thus, when an object is serialized, all the included objects are also serialized along with the original object.

What one should take care of while serializing the object?

One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

What happens to the static fields of a class during serialization? Are these fields serialized as a part of each serialized object?

yes, the static fields do get serialized. If the static field is an object then it must have implemented the Serializable interface. The static fields are serialized as a part of every object. But the commonness of the static fields across all the instances is maintained even after serialization.

Does Java provide any construct to find out the size of an object?

No. There is no size of operator in Java. So there is no way to determine the size of an object directly in java.

Does importing a package imports the subpackages as well? e.g. Does import com.MyTest.* also import com.Mytest.UnitTest.*?

Read the system time just before the method is invoked and immediately after the method returns. Take the time difference, which will give you the time taken by a method for execution.

To put it in code…

Long start = System.currentTimeMillis ();

method ();

long end = system.currentTimeMillis ();

System.out.printin (“Time taken for execution is” + (end – start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method that is big enough, in the sense the one which is doing a considerable amount of processing.

What are wrapper classes?

Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, double, etc.

Why do we need wrapper classes?

This is one of the frequently asked Java Interview Questions and Answers. It is sometimes easier to deal with primitives as objects. Moreover, most of the collection classes store objects and not primitive data types. And also, the wrapper classes provide many utility methods also. Because of these reasons we need wrapper classes. And since we create instances of these classes, we can store them in any of the collection classes and pass them around as a collection. Also, we can pass them around as method parameters where a method expects an object.

What are checked exceptions?

The checked exception are those which the Java compiler forces you to catch. e.g. IOException has checked Exceptions.

What are runtime exceptions?

Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

What is the difference between an error and an exception?

This is one of the frequently asked Java Interview Questions and Answers. An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you cannot repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or NullPointerException will take place if you try using a null reference. In most cases, it is possible to recover from an exception (probably by giving the user feedback for entering proper values, etc.).

How to create custom exceptions?

Your class should extend class Exception or some more specific type thereof.

If I want an object of my class to be thrown as an exception object, what should I do?

The class should extend from the Exception class. Or you can extend your class from some more precise exception type also.

If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

One cannot do anything in this scenario. Because Java does not allow multiple inheritances and does not provide any exception interface as well.

What happens to an unhandled exception?

One cannot do anything in this scenario. Because Java does not allow multiple inheritances and does not provide any exception interface as well.

How does an exception permeate through the code?

This is one of the frequently asked Java Interview Questions and Answers. An unhandled exception moves up the method stack in search of matching when an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. The same procedure is repeated if the caller method is included in a try-catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

What are the different ways to handle exceptions?

There are two ways to handle exceptions:

1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. And
2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.

What is the basic difference between the 2 approaches to exception handling…1> try-catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach?

In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in the best position to decide what should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with its own exceptions, then do not use this approach. In this case, use the second approach. In the second approach, we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

Is it necessary that each try block must be followed by a catch block?

It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

If I write return at the end of the try block, will the finally block still execute?

Yes, even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

If I write System.exit (0); at the end of the try block, will the finally block still execute?

No in this case the finally block will not execute because when you say Sytem.exit (0); the control immediately goes out of the program, and thus finally never executes.

How are the Observer and Observable used?

Objects that subclass the Observable class maintain a list of observers. When an Observable object updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

What is synchronization and why is it important?

This is one of the frequently asked Java Interview Questions and Answers. With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object’s value. This often leads to significant errors.

How does Java handle integer overflows and underflows?

It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Does garbage collection guarantee that a program will not run out of memory?

Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

What is the difference between preemptive scheduling and time slicing?

This is one of the frequently asked Java Interview Questions and Answers. Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

When a thread is created and started, what is its initial state?

A thread is in the ready state after it has been created and started.

What is the purpose of finalization?

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

What is the Locale class?

The Locale class is used to tailor program output to the conventions of a particular geographic, political. Or cultural region.

What is the difference between a while statement and a do statement?

A while statement checks at the beginning of a loop to see whether the next iteration of a loop should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

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

A static variable is associated with the class as a whole rather than with specific instances of the class. Non-static variables take on unique values with each object instance.

How is this() and super() used with constructors?

this() is used to invoke a constructor of the same class. Super() is used to invoke a superclass constructor.

What are synchronized methods and synchronized statements?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method’s object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

What is Multithreading in Java?

This is one of the frequently asked Java Interview Questions and Answers. It is a process of creating multiple threads in the Java Stack Area (JSA) for executing multiple tasks concurrently to finish their execution in a short time by using the processor’s ideal time effectively.

Multithreading means executing multiple threads at the same time simultaneously where each thread is called a separated task of the program that executes separately.

Java is a multi-threaded programming language which means we can develop a multi-threaded program using Java. A multi-threaded program contains two or more parts that can run concurrently and each part can handle a different task at the same time making optimal use of the available resources especially when your computer has multiple CPUs. Hence, it is also known as Concurrency in Java. 

Advantages of Java Multithreading:

1. The users are not blocked because threads are independent, and we can perform multiple operations at times
2. As such the threads are independent, the other threads won’t get affected if one thread meets an exception.

Why we need multithreading in Java?

We need multithreading mainly for two reasons.

1. To complete the execution of independent multiple tasks in a short time
2. To effectively utilize the CPU ideal time.

What is the Main Thread?

For every Java program, there will be a default thread created by JVM which is nothing but Main Thread. The entry point for Main Thread is the main() method.

What is JDBC?

JDBC is a java based data access technology from Sun Microsystems. JDBC is an open specification that contains rules and guidelines that have to be followed by the vendor for developing JDBC drivers.

How JDBC Works?

This is one of the frequently asked Java Interview Questions and Answers. Developed as an alternative to the C-based ODBC (Open Database Connectivity) API, JDBC offers a programming-level interface that handles the mechanics of Java applications communicating with a database or RDBMS.

The JDBC interface consists of two layers:

1. The JDBC API supports communication between the Java application and the JDBC manager.
2. The JDBC driver supports communication between the JDBC manager and the database driver.

JDBC is the common API that your application code interacts with. Beneath that is the JDBC-compliant driver for the database you are using.

What is Garbage Collection in Java?

This is one of the frequently asked Java Interview Questions and Answers. Garbage collection is the process of looking at heap memory, identifying which objects are in use and which are not, and deleting the unused objects. An in-use object, or a referenced object, means that some part of your program still maintains a pointer to that object. An unused object, or unreferenced object, is no longer referenced by any part of your program. So the memory used by an unreferenced object can be reclaimed. In reality, Garbage Collection tracks each and every object available in the JVM heap space and removes unused ones.

In a programming language like C, allocating and deallocating memory is a manual process. In Java, the process of deallocating memory is handled automatically by the garbage collector.

How does Garbage Collection work in Java?

This is one of the frequently asked Java Interview Questions and Answers. Java garbage collection is an automatic process. The programmer does not need to explicitly mark objects to be deleted. However, we can request the JVM for garbage collection of an object but ultimately it depends on the JVM to call garbage collector.

Let’s start with the heap, which is the area of memory used for dynamic allocation. In most configurations, the operating system allocates the heap in advance to be managed by the JVM while the program is running. This has a couple of important ramifications:

Object creation is faster because global synchronization with the operating system is not needed for every single object. An allocation simply claims some portion of a memory array and moves the offset pointer forward. The next allocation starts at this offset and claims the next portion of the array.

When an object is no longer used, the garbage collector reclaims the underlying memory and reuses it for future object allocation. This means there is no explicit deletion and no memory is given back to the operating system.

In simple words, GC works in two simple steps known as Mark and Sweep:

1. Mark – it is where the garbage collector identifies which pieces of memory are in use and which are not.
2. Sweep – this step removes objects identified during the “mark” phase.

Can the Garbage Collection be forced explicitly?

No, the Garbage Collection can not be forced explicitly. We may request JVM for garbage collection by calling System.gc() method. But This does not guarantee that JVM will perform the garbage collection.

Advantages of Java Garbage Collection

• It is done automatically by JVM.
• It makes java memory-efficient because the garbage collector removes the unreferenced objects from heap memory.
• The programmer doesn’t need to worry about manual memory allocation/deallocation handling because unused memory space is automatically handled by GC.

Disadvantages of Garbage Collection in Java

• Programmers have no control over the scheduling of CPU time dedicated to freeing objects that are no longer needed
• There is no guarantee that any one of the GC methods will definitely run Garbage Collector.
• Automatized memory management will not be as efficient as the proper manual memory allocation/deallocation.

Here, in this article, I try to explain Java Interview Questions and Answers. I hope you enjoy this Java Interview Questions and Answers article.