The most common question asks in java interview

Here are a most important Java interview questions

1. What is the difference between an abstract class and an interface in Java?

An abstract class is a class that can have both abstract (without a body) and non-abstract (with a body) methods, whereas an interface can only have abstract methods. A class can extend only one abstract class, but it can implement multiple interfaces. An abstract class can have instance variables and a constructor, while an interface cannot have any of those.

Here are some key differences between an abstract class and an interface in Java:

• An abstract class can have instance variables, a constructor, and non-abstract methods, while an interface cannot.
• A class can extend only one abstract class, but it can implement multiple interfaces.
• An abstract class can have any visibility (public, private, etc.), while an interface is always public.
• An abstract class can have any number of abstract and non-abstract methods, while an interface can have only abstract methods (and also constants, since Java 8).
• An abstract class can have inner classes (including non-static and static ones), while an interface cannot have inner classes.

2.What is the difference between a constructor and a method in Java?

A constructor is a special type of method in Java that is used to initialize an object. It has the same name as the class and is invoked when an object of the class is created. A constructor does not have a return type, not even void.

On the other hand, a method is a block of code that performs a specific task and may or may not return a value. A method has a return type or void if it doesn’t return a value.

Here are some key differences between a constructor and a method in Java:

• A constructor has the same name as the class and is used to create an object of the class, while a method has a different name and is used to perform a specific task.
• A constructor does not have a return type, not even void, while a method has a return type or void if it doesn’t return a value.
• A constructor is invoked automatically when an object of the class is created, while a method is called explicitly by the code.
• A constructor is used to initialize the state of an object, while a method is used to perform a specific task.

3.Can a class be both abstract and final in Java?

A final class is a class that cannot be subclassed, which means that it cannot be extended by any other class. On the other hand, an abstract class is a class that cannot be instantiated and is meant to be extended (subclassed) by other classes.

If you try to declare a class as both abstract and final, the compiler will throw an error because the two are mutually exclusive. An abstract class is meant to be extended, while a final class is not meant to be extended.

For example:

However, it is possible to have an abstract method in a final class. In this case, the final class cannot be subclassed, but the abstract method must be implemented in a subclass if the subclass is not also declared as final.

4.What is the difference between a static method and a non-static method in Java?

In Java, a static method is a method that belongs to a class rather than an instance of the class. It can be invoked without creating an instance of the class and can only access static variables of the class.

On the other hand, a non-static method (also called an instance method) is a method that belongs to an instance of a class. It requires an instance of the class to be created before it can be invoked and can access both static and non-static variables of the class.

Here are some key differences between static and non-static methods in Java:

• A static method belongs to a class and can be invoked without creating an instance of the class, while a non-static method belongs to an instance of a class and requires an instance to be created before it can be invoked.
• A static method can only access static variables of the class, while a non-static method can access both static and non-static variables.
• A static method is declared with the static keyword, while a non-static method is not declared with the static keyword.

Here’s an example of a static method:

And here’s an example of a non-static method:

What is the difference between a while loop and a do-while loop in Java?

In Java, a while loop is a control flow statement that allows you to execute a block of code repeatedly as long as a boolean condition is true. The loop checks the condition before the block of code is executed.

On the other hand, a do-while loop is also a control flow statement that allows you to execute a block of code repeatedly. However, the loop checks the condition after the block of code is executed. This means that the block of code will be executed at least once, even if the condition is false.

Here are some key differences between a while loop and a do-while loop in Java:

• A while loop checks the condition before the block of code is executed, while a do-while loop checks the condition after the block of code is executed.
• A while loop will not execute the block of code at all if the condition is initially false, while a do-while loop will execute the block of code at least once.

Here’s an example of a while ‘loop’

And here’s an example of a do-while loop:

5.What is the difference between a for loop and a while loop in Java?

In Java, a for loop is a control flow statement that allows you to execute a block of code repeatedly for a given number of times. It consists of three parts: an initialization statement, a boolean condition, and an increment/decrement statement.

On the other hand, a while loop is a control flow statement that allows you to execute a block of code repeatedly as long as a boolean condition is true. It consists of only one part: a boolean condition.

Here are some key differences between a for loop and a while loop in Java:

• A for loop has three parts: an initialization statement, a boolean condition, and an increment/decrement statement, while a while loop has only one part: a boolean condition.
• A for loop is used when the number of iterations is known beforehand, while a while loop is used when the number of iterations is not known beforehand or when the loop needs to run an indefinite number of times.

Here’s an example of a for loop:

And here’s an example of a while loop:

6.What is the difference between a public class and a default class in Java?

In Java, a class can have one of four access levels: public, private, protected, or default (no access modifier).

A public class is a class that can be accessed from anywhere, regardless of the package it is in. It is declared with the public keyword.

On the other hand, a default class (also called a package-private class) is a class that can only be accessed from within the package it is in. It does not have an access modifier.

Here are some key differences between a public class and a default class in Java:

• A public class can be accessed from anywhere, while a default class can only be accessed from within the package it is in.
• A public class is declared with the public keyword, while a default class does not have an access modifier.

Here’s an example of a public class:

And here’s an example of a default class:

7.What is the difference between a try-catch block and a try-with-resources block in Java?

In Java, a try-catch block is a control flow statement that allows you to handle exceptions that may be thrown in a block of code. It consists of a try block, which contains the code that may throw an exception, and one or more catch blocks, which handle the exception if it is thrown.

On the other hand, a try-with-resources block is similar to a try-catch block, but it also includes a mechanism for automati

cally closing any resources that are opened within the try block. The resources are declared in a try statement and are automatically closed when the block completes, whether it completes normally or abruptly due to an exception.

Here are some key differences between a try-catch block and a try-with-resources block in Java:

• A try-catch block handles exceptions that may be thrown in a block of code, while a try-with-resources block also includes a mechanism for automatically closing resources that are opened within the try block.
• A try-with-resources block can only be used with resources that implement the AutoCloseable interface, while a try-catch block can be used with any code that may throw an exception.

Here’s an example of a try-catch block:

And here’s an example of a try-with-resources block:

8.What is the difference between a Java array and an ArrayList in Java?

In Java, an array is a fixed-size data structure that can store a homogeneous (elements of the same type) collection of elements. An array is created with a specified size and cannot change its size once created.

On the other hand, an ArrayList is a resizable-array implementation of the List interface. It is an object that can store a collection of elements and can dynamically increase or decrease its size as elements are added or removed.

Here are some key differences between an array and an ArrayList in Java:

• An array has a fixed size and cannot change its size once created, while an ArrayList has a dynamic size and can change its size as elements are added or removed.
• An array can store only homogeneous elements, while an ArrayList can store both homogeneous and heterogeneous elements (elements of different types).
• An array has faster access times compared to an ArrayList, but it is more difficult to use and requires manual resizing. An ArrayList is easier to use but has slower access times compared to an array.

Here’s an example of creating and initializing an array:

And here’s an example of creating and initializing an ArrayList:

9.What is the difference between a HashMap and a Hashtable in Java?

In Java, a HashMap is a class that implements the Map interface and stores key-value pairs in a hash table. It is an unordered collection and allows null values and null keys.

On the other hand, a Hashtable is a legacy class that was introduced in Java 1.0 and implements the Map interface. It is similar to a HashMap, but it is synchronized (thread-safe) and does not allow null values or null keys.

Here are some key differences between a HashMap and a Hashtable in Java:

• A HashMap is unsynchronized (not thread-safe) and allows null values and null keys, while a Hashtable is synchronized (thread-safe) and does not allow null values or null keys.
• A HashMap is faster and more efficient than a Hashtable, but it is not thread-safe. A Hashtable is slower and less efficient than a HashMap, but it is thread-safe.
• A HashMap is the recommended implementation of the Map interface, unless thread-safety is required.

Here’s an example of creating and initializing a HashMap:

And here’s an example of creating and initializing a Hashtable:

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