Approximately 10 million developers across the globe are using Java right now for developing apps for 20 billion devices that support Java. Java is used extensively in desktop applications, mobile systems, and games. It is likewise used for creating apps for trending technologies such as Big Data to household gadgets like DTH boxes and mobiles. In case you would like to procure an IT sector job, it is essential for you to know Java properly.
Below, we have mentioned the top 20 core Java interview questions and answers, which will make you prepared for an interview in the best possible way. So, let us not waste time and look at these Java Interview questions for 5 years of experience in the following paragraphs.
1. What do you mean by Java programming language?
Being platform-independent, Java happens to be a high-level language. It is Sun Microsystems which is responsible for developing Java. This language is used for developing plenty of apps, games, and websites.
2. What are the features of Java?
Java programming language comes with the following features:
- Object-oriented: The object-oriented paradigm is followed by Java, using which weare able tomaintain the code as a combo of various kinds of objects that include behavior as well as data.
- Portable: The Java programming language can be executed on all the machines. Java program gets transformed to bytecode whichis capable of runningon each machine easily.
- Platform independent: Java happens to be a programming language that isplatform-independent. On the contrary, programming languages such as C as well as C++ require a platform for execution. Java is not dependent on the OS to be executed.
- Robust: Being a robust programming language, Java makes use of strong memory management. It becomes more powerful thanks to concepts such as Exception handling, Automatic garbage collection, and so forth.
- Secured: Explicit pointers are not used by Java which makes it quite secured. The concepts of Exception handling andByteCodeare also provided by Java, thus making it more fortified.
- Architecture Neutral: Java programming language does not depend on the architecture, and therefore, it happens to be architectural neutral. The data type size might vary in C as per the architecture, which is not present in Java.
3. What is meant by Java virtual machine?
One of the Java interview questions will be what exactly is meant by Java virtual machine. Java Virtual Machine is considered a virtual machine that allows the computer to operate the Java program. Acting similar to a runtime engine, it is responsible for calling the primary method existing in the Java code. Java virtual machine is the specification that one must implement in the computer system. JVM compiles the Java code to be a Bytecode that is close to the native code plus machine-independent.
4. How can you differentiate between JRE, JDK, and JVM?
As mentioned earlier, JVM stands for Java Virtual Machine. This abstract machine is accountable for providing the runtime environment where it is possible to execute Java bytecode. This specification specifies the functioning of JVM. Oracle plus other enterprises have provided its implementation, which is called JRE.
You will come across JVMs for lots of software and hardware platforms. As a result, JVM happens to be platform-independent. This runtime instance is created while running the Java class. You will come across 3 KVM notions, namely, specification, instance, and implementation.
JRE is the acronym for Java Runtime Environment, which is JVM’s implementation. The JRE is a collection of tools used for creating Java apps. It aids in providing the runtime environment. It exists physically and consists of a collection of libraries plus other files used by JVM at runtime.
JDK stands for Java Development Kit, which happens to be an application development environment used for creating Java applets and apps. Existing physically, it consists of JRE + development tools. Java Development Kit is an implementation of one of the following Java platforms:
- Enterprise Edition Java Platform
- Standard Edition Java Platform
- Micro Edition Java Platform
5. What do you mean by JIT compiler?
JIT (Just-In-Time) compiler aids in improving performance. By compiling bits of the bytecode having identical functionality, it helps to minimize the time required for compilation. By the term “compiler,” we refer to a translator from the JVM instruction set to the instruction set of a particular CPU.
6. How to differentiate between other platforms and the Java platform?
Our next advanced Java questions and answers for experienced Java developers will differentiate between other platforms and the Java platform.
While other platforms might be software-based or hardware-based platforms, Java happens to be a platform that is software-based. Java will be executed on top of hardware platforms, while other platforms can have hardware components only.
7. What are the benefits of Java packages?
Defining packages in Java comes with various advantages:
- Packages stay away from name clashes.
- It is also possible to have hidden classes which are utilized by the package and not visible outside.
- Easier access control is provided by the package.
- The related classes can be located easily.
8. How to differentiate a HashSet from aTreeSet?
Amongst the core Java interview questions and answers, the subsequent one will be differentiating between a HashSet and a TreeSet. Even though both of them make sure that there are no duplicates, you will come across certain attributes distinguishing the 2.
- Implementation: We make use of the hash table for storing elements in an unorganized manner when it comes to a HashSet. Nevertheless,TreeSetuses the red-black tree for storing elements in an orderly manner.
- Complexity/Performance: The time amortized complexity happens to beO(1) for a HashSet for appending, deleting, and retrieving elements. ForTreeSet, the time complexity to perform the identical operations is somewhat higher and is equivalent to O(log n). In general, HashSet’s performance is faster as compared to TreeSet.
- Methods: HashSet makes use of methods such asequals() andhashCode() for comparing between objects. On the other hand, TreeSet makes use of compare() and compareTo() methods for facilitating object comparisons.
- Objects type: It is feasible to store null and heterogeneous objects using HashSet. When it comes toTreeSet, the runtime exception takes place while inserting null objects or heterogeneous objects.
9. What is meant by constructor?
We can define the term “constructor” as a unique method employed for initializing an object’s state. It will be invoked once there is an instantiation of the class. Every time the new keyword creates the object, the class’s default constructor is called. It is essential for the constructor’s name to be identical to the class name. It is likewise imperative for the constructor not to have an explicit return type.
- What is implied by this keyword in Java?
This keyword happens to be a reference variable referring to the existing object. This keyword has many uses in Java. One can use it for referring to existing class properties like variables, instance methods, constructors, and so forth. It is also possible to pass it as an argument into the constructors or methods. It can likewise be returned as the existing class instance from the method.
- What is meant by composition?
This is definitely one of the most important Java interview questions. Holding a class’s reference within any other class is called composition. When any object contains another object, in case it is not possible for the contained object to exist without the container object’s existence, then it is known as composition. Put simply; it can be asserted that composition happens to be a case of aggregation representing a stronger connection between 2 objects. An example is a class consisting of students. It is not possible for a student to exist sans a class. A composition is present between students and class.
- How can you differentiate between composition and aggregation?
While a strong relationship is represented by composition, aggregation is the present relationship that is weak. For instance, although the bike comes with an indicator (aggregation), it has got an engine (composition).
- How to differentiate between super and this keyword?
The following are the primary differences between super and this keyword:
- While the super keyword is known topoint to the parent class contacts at all times, this keyword will be pointing to the current class context always.
- We mainly use the super keyword for making the base class variables initialized within the class constructor, which is derived, while this keyword is mainly utilized for differentiating between instance and local variables.
- It is imperative for the this and super to be the initial statement within the constructor, or else, an error will be thrown by the compiler.
- What do you mean by method overriding?
One of the common Java programming interview questions for developers happens to be what method overriding is. In case a particular implementation of any method is provided by a subclass which is offered already by the parent class, we call it Method Overriding. We use it for implementing the interface methods.
Method overriding rules:
- It is imperative for the method to have the identical name as the parent class.
- It is essential for the method to have the identical signature as the parent class.
- 2 classes ought to have an IS-A connection between them.
- Is it possible to override the static method?
It is not feasible to override the static method since they belong to the class and not the object.
- Why is it not possible to override the static method?
The reason for this is that the static method happens to be a part of the class. While the instance method is bound with the object and memory is received by static in the class area, the instance receives memory in a pile.
- Can a constructor be declared as final?
It is quite difficult to declare the constructor as final since it is not inherited. Constructors should not be declared as final since they are not ordinary methods. Nevertheless, an error will be thrown by the compiler in case you make an attempt to do so.
- Can the interface be declared as final?
No, an interface cannot be declared as final since some class has to implement it for providing its definition. Consequently, making an interface final is meaningless. Nevertheless, an error will be thrown by the compiler in case you make an attempt to do so.
- For what reasons are Java Strings immutable in nature?
This will be one of the most essential coding interview questions that you must be aware of. String objects, in basic Java, happen to be immutable in nature which implies that it is not possible to alter the state of the String object once it is created. A fresh string object is created by Java once you make an attempt to update that object’s value rather than updating the particular object’s values. Java String objects happen to be immutable since String objects are usually cached within the String pool. It is possible for the action from any client to affect the rest since String literals will be typically shared between a number of clients. It improves the security, synchronization, caching, as well as performance of the app.
- What are the various types of inheritance in Java?
4 inheritance types are supported by Java:
- Single inheritance: In this case, the properties of one class are inherited by the other, which implies that you will have only a single parent as well as a child class.
- Multilevel inheritance: Once we derive a class from another class that is likewise derived from one more class, this kind of inheritance is known as Multilevel inheritance.
- Hierarchical Inheritance: When more than one child class is present in a class, this type of inheritance is called hierarchical.
- Hybrid Inheritance: This happens to be a combo of several types of inheritance.
These Java interview questions mentioned above will be covering basic as well as advanced Java programming concepts intended for interviews and programming. Let us hope that this info will provide you with a proper insight into the core coding concepts of Java in detail. Apart from enriching your knowledge, these Java programming questions and answers will help you comprehend Java programming.