Core Java Interview Questions
1. What is Java?
Java is the high-level, object-oriented, robust, secure programming language, platform-independent, high performance, Multithreaded, and portable programming language. It was developed by James Gosling in June 1991. It can also be known as the platform as it provides its own JRE and API.
2. What do you mean by Object?
An object consists of methods and class which represent it’s formal and perform operations. A java program covers a lot of objects initiating each other their jobs. This concept is a like part of core java. To get in-Depth knowledge on Java you can enroll for live Java Online Training
3. What do you understand by Java virtual machine?
Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.
4. Why Java is platform independent?
Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system.
5. What are wrapper classes in Java?
Wrapper classes convert the Java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class. Refer to the below image which displays different primitive type, wrapper class and constructor argument.
6. What is JIT compiler?
Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU. To get in-Depth knowledge on JIT compiler you can enroll for live Java Online Training
7. What if I write static public void instead of public static void?
The program compiles and runs correctly because the order of specifiers doesn’t matter in Java.
8. What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references.
9. What is the purpose of static methods and variables?
The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.
For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.
10. What is the constructor?
The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.
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