Top 50 Jvm Interview Questions You Must Prepare 19.May.2024

Reflection is the process of introspecting the features and state of a class at runtime and dynamically manipulate at run time. This is supported using Reflection API with built-in classes like Class, Method, Fields, Constructors etc. Example: Using Java Reflection API we can get the class name, by using the getName method.

The entry point method main is used to the provide a standard convention for starting Java programs. The choice of the method name is somewhat arbitrary, but is partly designed to avoid clashes with the Thread start() andRunnable run() methods, for example.

Yes, the JVM maintains a cache by itself. It creates the Objects on the HEAP, but references to those objects are on the STACK.

Inheritance is the process by which one object acquires the properties of another object. Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places

The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.

In Java the arguments (primitives and objects) are always passed by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.

The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.

A native method is a method that is implemented in a language other than Java.

For very simple programs it is possible to write a main method that only uses static variables and methods. For more complex systems, the main method is used to create an instance of itself, or another primary class, as the basis of the application. The primary application object reference uses instance methods to create and interact with other objects, do the work and return when the application terminates.
public class SimpleClass {
public void doSomething() {
// Instance method statements
}
public static main(final String[] args) {
SimpleClass instance = new SimpleClass();
instance.doSomething();
}
}

A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement (post test) 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 loop body at least once.

The GregorianCalendar provides support for traditional Western calendars.

The Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integral and floating-point operations may take place. In the numerical promotion process the byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

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

If an expression involving the boolean & operator is evaluated, both operands are evaluated, whereas the && operator is a short cut operator. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. If the first operand evaluates to false, the evaluation of the second operand is skipped.

We can have multiple overloaded main methods but there can be only one main method with the following signature :
public static void main(String[] args) {}
No the program fails to compile. The compiler says that the main method is already defined in the class.

Yes, the main method can be called from a separate class. First you must prepare the string array of arguments to pass to the method, then call the method through a static reference to the host class, MaxFactors in the example below.
String[] arguments = new String[] {"123"};
MaxFactors.main(arguments);

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

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

The Java Virtual Machine is software that can be ported onto various hardware-based platforms

Yes. 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 not conflict amongst the multiple classes having main method.

A static variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static variables i.e. there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class. These are declared outside a class and stored in static memory. Class variables are mostly used for constants. Static variables are always called by the class name. This variable is created when the program starts and gets destroyed when the programs stops. The scope of the class variable is same an instance variable. Its initial value is same as instance variable and gets a default value when its not initialized corresponding to the data type. Similarly, a static method is a method that belongs to the class rather than any object of the class and doesn’t apply to an object or even require that any objects of the class have been instantiated.

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.

Non-static variables take on unique values with each object instance.

Command line arguments are passed to the application's main method by the Java runtime system before theapplication class or any supporting objects are instantiated. It would be much more complex to define and construct arbitrary object types to pass to the main method and primitive values alone are not versatile enough to provide the range of input data that strings can. String arguments can be parsed for primitive values and can also be used for arbitrary text input, file and URL references.

Pass by reference me, passing the address itself rather than passing the value. Pass by value me passing a copy of the value.

The SimpleTimeZone class provides support for a Gregorian calendar.

JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor.

Phantom memory is false memory. Memory that does not exist in reality.

Yes, any Java method can be overloaded, provided there is no final method with the same signature already. The Java interpreter will only invoke the standard entry point signature for the main method, with a string array argument, but your application can call its own main method as required.

The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy

Global variables are globally accessible. Java does not support globally accessible variables due to following reasons:
• The global variables breaks the referential trparency.
• Global variables creates collisions in namespace.

Explicit casting in the process in which the complier are specifically informed to about trforming the object.
Example
long i = 700.20;
int j = (int) i; //Explicit casting.

Static variable are loaded when classloader brings the class to the JVM. It is not necessary that an object has to be created. Static variables will be allocated memory space when they have been loaded. The code in a static block is loaded/executed only once i.e. when the class is first initialized. A class can have any number of static blocks. Static block is not member of a class, they do not have a return statement and they cannot be called directly. Cannot contain this or super. They are primarily used to initialize static fields.

The static void main(String[]) method is a basic convention of the Java programming language that provides an entry point into the runtime system. The main method must be declared static because no objects exist when you first invoke the Java Virtual Machine (JVM), so there are no references to instance methods. The JVM creates the initial runtime environment in which this static method can be called, if you remove the static modifier, it will throw aNoSuchMethodException.

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

System is a predefined final class, out is a PrintStream object and println is a built-in overloaded method in the out object.

Yes, the static void main(String[]) method can be declared final.

Java programs that take input from the command line declare a special static method called main, which takes aString array as an argument and returns void. The example program below loops through any arguments passed to the program on the command line and lists their values.

JVM is Java Virtual Machine which is a run time environment for the compiled java class files.

There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.

When you invoke the Java Virtual Machine on a class without any arguments, the class' main method receives aString array of zero length. Thus, the method signature is fulfilled. Provided the main method does not make any reference to elements in the array, or checks the array length before doing so, no exception will occur.

This is a special static method signature that is used to run Java programs from a command line interface (CLI). There is nothing special about the method itself, it is a standard Java method, but the Java interpreter is designed to call this method when a class reference is given on the command line, as below.

The process of converting one data type to another is called Casting. There are two types of casting in Java; these are implicit casting and explicit casting.

All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.

In declaration we only mention the type of the variable and its name without initializing it. Defining me declaration + initialization. E.g. String s; is just a declaration while String s = new String (”bob”); Or String s = “bob”; are both definitions.

Print array.length. It will print @That me it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print array.length.

public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public.

static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static.

void: main does not return anything so the return type must be void

The argument String indicates the argument type which is given at the command line and arg is an array for string given during command line.

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 we do not provide any arguments on the command line, then the String array of main method will be empty but not null.

Variable is a named memory location that can be easily referred in the program. The variable is used to hold the data and it can be changed during the course of the execution of the program.