Java Introduction

Question – How many types of memory areas allocated by JVM?

JVM Javaintervewquestion.in

1.Class Loader – is a subsystem of JVM that is used to load class files into Memory areas

2.Class/Method Area – stores per class structures such as the field and method data, runtime constant pool and the code for methods

3.Heap – Runtime data area in which all objects are allocated.

4.Stack – Stores frames, Holds local variables and variables results, also pays a part in method invocation and return.New frame is created each time a method is invoked, frame is destroyed when its method invocation completes. Each thread has a private JVM stack

5.PC/Program Counter Register – Holds the address of the JVM instruction currently being executed.

6.Native Method Stack – Having all the native methods used in the application. (Native methods : is a method that is implemented in a language other than Java.)

Question – What if the main method is declared as private?

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

Question – What is meant by pass by reference and pass by value in Java?

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

Question – If you’re overriding the method equals() of an object, which other method you might also consider?

hashCode().

Question – What is Byte Code? Or What gives java it’s “write once and run anywhere” nature?

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.

Java Introduction

Question – Explain the reason for each keyword of public static void main(String args[])?

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.

String args[] : 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.

Question – What is string constant pool ?

There are two ways to create String Object
a. String Literal
b. Using new keyword

Literal is created by double quote.

E.g

    String jiq = "JavaInterviewQuestion";

String literal value jump into string constant pool. Each time you create a string literal, JVM checks the string pool first. If the string value already exist into the pool, a reference to the pooled instance returns. If string value does not exist into the pool, a new String Object instantiates, then is placed in the pool.

E.g

	String jiq1 = "JavaInterviewQuestion";
	String jiq2 = "JavaInterviewQuestion"; // No new object will be created

StringGIF

Importent – String Objects are stored in a special memory area known as string constant pool inside the Heap memory

Question – What are the differences between == and .equals() ?

Comparing Object references with the == Operators [e.g a == b]

Compares references, not values. The use of == with object references is generally limited to the following:
1. Comparing to see if a reference is null.
2. You want to know if two references are to the same object

Comparing Object values with the equals() Method [e.g a.equals(b) ]

Compares values for equality. Because this method is defined in the Object class, from which all other classes are derived, it’s automatically defined for every class.
However, it doesn’t perform an intelligent comparison for most classes unless the class overrides it.
It has been defined in a meaningful way for most Java core classes. If it’s not defined for a (user) class, it behaves the same as ==.

Question – What if the static modifier is removed from the signature of the main method?
What if I do not provide the String array as the argument to the method?

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

Question – What is the difference between final, finally and finalize?

– declare constant.
finally – handles exception.
finalize – helps in garbage collection.
Variables defined in an interface are implicitly final.
A final class can’t be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.

Question – What is the Java API?

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

Question – What is the GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars.

Question – What is the ResourceBundle class?

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.

Question – Why there are no global variables in Java?

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

Question – How to convert String to Number in java program?

The valueOf() function of Integer class is is used to convert string to Number.
Here is the code example:
String numString = “1000”;
int id=Integer.valueOf(numString).intValue();

Question – What is the SimpleTimeZone class?

The SimpleTimeZone class provides support for a Gregorian calendar.

Question – What is the difference between a while statement and a do statement?

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.

Question – What is the Locale class?

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

Question – What is implicit casting?

Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not work for all scenarios.
Example
:int i = 1000;
long j = i; //Implicit casting

Question – What is explicit casting?

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

Question – Is sizeof a keyword in java?

The sizeof operator is not a keyword.

Question – What is a native method?

A native method is a method that is implemented in a language other than Java.In System.out.println(),

Question – what is System, out and println?

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

Question – What are Java Access Specifiers?
What is the difference between public, private, protected and default Access Specifiers?
What are different types of access modifiers?

Access specifiers are keywords that determine the type of access to the member of a class. These keywords are for allowing<br /> privileges to parts of a program such as functions and variables.
These are:
Public : accessible to all classes
Protected : accessible to the classes within the same package and any subclasses.
Private : accessible only to the class to which they belong
Default : accessible to the class to which they belong and to subclasses within the same package

Question – Which class is the superclass of every class?

Object.

Question – Name primitive Java types.

The 8 primitive types are byte, char, short, int, long, float, double, and boolean.

Question – What is the difference between static and non-static variables?
What are class variables?
What is static in java?
What is a static 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.

Question – What is the difference between the boolean & operator and the && operator?

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 &amp;&amp; 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.

Question – How does Java handle integer overflows and underflows?

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

Question – What is the difference between declaring a variable and defining a variable?

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

Question – What do you understand by a variable?

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.

Question – Can an application have multiple classes having main method?

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.

Question – When is static variable loaded?
Is it at compile time or runtime?
When exactly a static block is loaded in Java?

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.

Question – Can I have multiple main methods in the same class?

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) {}

Question – What is phantom memory?

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

Question – Can a method be static and synchronized?

A static method can be synchronized. If you do so, the JVM will obtain a lock on the java.lang. <br /> Class instance associated with the object. It is similar to saying:<br />
synchronized(XYZ.class) {<br />
}

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