I have been asked about the list of Core Java Interview questions. So here I will list down some of the Core Java Interview Questions that I or my friends have faced in Interviews.

Beginners

1: Java is a platform-independent language. What does it mean? (beg)

2: How many methods are there in the Object class? (Beg)

There are a total of 9 methods

1. equals()
2. hashcode()
3. clone()
4. wait() (3 variant)
5. notify()
6. notifyAll()
7. toString()

3: Wait, notify and notify all these methods are related to Thread then why is it part of Object class?

These methods are related to Locks and LOCKS has nothing to do with Threads. Locks are associated with objects hence these methods are part of the Object class and not the Thread class.

4: Can we create a private class??

No Class can have only public or default access modifier.

5: What other modifiers are there which can be applied to class?

Answers: Non Access Modifiers that can be applied to Classes are

1. Final
2. Static
3. Abstract
4. Strictfp

8: What is the difference between == and equals method?

You better look here for detailed answer.

9: What is the difference between Comparable and Comparator interface?

10: What is the use of Marker Interfaces when there is no method in the Marker Interface? (Inter)

11: What is the difference between ArrayList and LinkedList in terms of performance? (Inter)

12: How(Steps) to Create Immutable Class? (Inter)

Some Rules need to be followed in order to create an Immutable class.
1- Make a constructor private
2- Provide a static method to get the instance to class.
3- Throw CloneNotSupported exception.
4- Make the class Final.

13: How to implement the Singleton pattern? (Inter)

Single instance per JVM 4 thing needs to follow in order to achieve it.
1- Make the constructor private.
2- Provide a Static method to get a singleton object
3- Make a private reference variable
4- Check for ref type in the static method if it is null then make a reference and assign it and return else just return.
5- make the static method synchronized. (when 2 threads access the same time they will see ref as null and end up creating two references so avoid that situation need to make the static method synchronized)
6- Provide the implementation for Clone() method and throw CloneNotSupportedException exception from a method (If singleton class extends some other class which is supporting the Clone method, in that case, we need to do this otherwise Clone method will create another instance).

Can we use an enum to create a singleton class? (Inter)

Class & Object

14: Why wait and Notify is in Object Class and not in Thread? (Inter)

In the Multi-Threaded model, Java uses locks to implement exclusive access to objects & the lock is related to an object and not Thread. These methods work on the locks of the object i.e. the reason why these methods are in Object class and not in Thread.

15: What is an inline class? (Inter)

16: What is Class

A class is a template for multiple objects with similar features(State & Behavior) and it is a blueprint for objects

17: What is Constructor

Special kind of method that determines how an object is initialized when created.

18: What is the difference between Constructor & Method?

Differences are

• The constructor needs to have the name of the Class. A method can have any name even the Class name.
• A constructor can not return any value like Method.
• A constructor will be invoked implicitly when an object of class is created. While the method needs to be invoked.

19: What is the use of Class.ForName and how it works. (Inter)

Every object has a corresponding class object. This class object instance is shared among all instances of that class type. Class.forName loads & Returns the Class object associated with the class/interface with the given string name. It will also load the class if it is not already loaded in JVM. From Java Docs Instances of the class, Class represents classes and interfaces in a running Java application. The class has no public constructor. Instead, Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader. It is mainly used in reflection to get the details of the class. Note: If Class has a Static block, anonymous block, and constructor then the only static block will get executed while executing this code.

20: How to increase JVM Heap size? (Inter)

in order to overcome the heap size problem -Xxm1024m tag can be used to increase the heap size.

21: Does the class ‘Class’ extends Object?

Java has a class named “Class” it also extends the Object class. It also implements the Serialization Interface. It represents classes and interfaces in a running Java application

Note: everything in Java except primitives extend the Object class.

22: What is compile-time loading and run-time loading? (??)

Inner Class

23: An inner class can not access the method variable in which it is defined but can access if it is final why? (Inter)

Inner classes are classes within Class. An inner class instance has a special relationship with the Outer class. This special relationship gives the inner class access to the member of the outer class as if they are part of the outer class.
An inner class instance has access to all members of the outer class(Public, Private & Protected)
For more Details Go to Core Java Inner Class Section.

24: Different type of Inner classes? (Inter)

Type of Inner Class

1- Static
2- Method Local
3- Anonymous
4- Other than above these normal inner class

Variable

25: Why the final instance variable doesn’t get the default value? what specialty occurred with the final variable??

Once the final variable is assigned the value can not be changed this is the reason why the Final variable doesn’t get a default value.

26: Use of intern() method in String object? (Inter)

Intern method checks if the given string object(using the equals method) is already present in the string pool or not. If yes method will return the String from the pool otherwise given String object is added to the pool and a reference to this String object is returned. In any case, this method can’t be used to check if a String object is already available in the String pool or not.

27: Java support pass-by value or pass-by reference?

Pass-by value.

Static & this

28: What is static in Java?

Static means class level. A static field would be class level and all instances of the class will have access to the same Single variable. Static variables and methods can be accessed without creating any reference.

29: Is there any way to make a static method from Superclass to Instance method in the subclass? (Inter)

No, We can only hide the static method from Superclass.

30: What are the different types of execute() methods available, which can be used with JDBC connection for executing proc/stmt? (Inter)

31: What are the pros and cons of using synchronization? (Inter)

Synchronization is used to restrict a particular section of code to be executed by a single thread one at a time only. But this feature also makes other Threads wait for one single thread to complete its processing which in turn delays the processing time.

32: Method name which can be used to call(Request) garbage collection? (Inter)

System.gc()
It Runs the garbage collector. It is equivalent to “Runtime.getRuntime().gc()“.

33: What is Final?

The Final is a modifier that can be applied on Class / Method / Variable.

1. A final class cannot be extended.
2. A final method cannot be overridden.
3. The final variable’s value can’t be changed once assigned.

34: Can we declare the Main method as private?

Yes, you can declare the main method as private.

35: What is the use of static keyword for the Main method and what will happen if we remove static keyword from signature?

The static method can be accessed without creating any object. In this way, the main method provides an entry point for Application. If we remove the static keywords it will not be access by JVM and the Application cannot be started.

37: Does importing of packages also import subpackage?

No.

38: What are the different steps in Serialization? (Inter)

39: In the process to serialize what happens to the object inside the class?

The object inside the object will be serialized or not si depend on if the given object is itself serialized or not.

40: What happens to the static field while serialization? (Inter)

41: What are wrapper classes and what is the use of it? (Inter)

In Java, everything except primitives is an object. These primitives don’t get the benefit of being an object. Wrapper classes are created to provide the same to primitives. The integer class is a wrapper for int primitive.

42: Execution of finally with respect to return and system exit? (Inter)

There will be only one case when finally block will not get executed and that would be System.exit()(If called from try block) other than this in every condition finally block will get executed.

43: What are Observer and Observable? (Inter)

44: What are the synchronized method and synchronized statement?

// This is synchronized metho
public synchronized void method(){
    System.out.println();
    }

public void method1(){
// This is synchronized block inside method
synchronized(this) {
    System.out.println("Hi");
    }
    System.out.println("Hello");
    }

45: Can an unreachable object become reachable again? (Yes via Final method) (Inter)

Yes, unreachable code can be accessed via the final block. Whenever garbage collection will occur the final block will be executed without fail so the object can be reachable in that final block.

46: What will be the default values of all the elements of an array defined as an instance variable? (Inter)

It will depend on the type of Array you are defining. If it is an Object type it will be null else it will contain the default value of that data type. e.g. for boolean, it will be false, for int, it is 0.

public void final methodName(){}