Wednesday, February 2, 2011

Some FAQ about Java

What if the main method is declared as private?
The program compiles properly but at runtime it will give “Main method not public.” message.

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.

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

hashCode()

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.

Expain 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

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.

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

Or

what is difference between == and equals

Or

Difference between == and equals method

Or

What would you use to compare two String variables - the operator == or the method equals()?

Or

How is it possible for two String objects with identical values not to be equal under the == operator?

The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory.

== compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal.

public class EqualsTest {

public static void main(String[] args) {

String s1 = “abc”;
String s2 = s1;
String s5 = “abc”;
String s3 = new String(”abc”);
String s4 = new String(”abc”);
System.out.println(”== comparison : ” + (s1 == s5));
System.out.println(”== comparison : ” + (s1 == s2));
System.out.println(”Using equals method : ” + s1.equals(s2));
System.out.println(”== comparison : ” + s3 == s4);
System.out.println(”Using equals method : ” + s3.equals(s4));
}
}

Output
== comparison : true
== comparison : true
Using equals method : true
false
Using equals method : true

What if the static modifier is removed from the signature of the main method?

Or

What if I do not provide the String array as the argument to the method?

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

Why oracle Type 4 driver is named as oracle thin driver?

Oracle provides a Type 4 JDBC driver, referred to as the Oracle “thin” driver. This driver includes its own implementation of a TCP/IP version of Oracle’s Net8 written entirely in Java, so it is platform independent, can be downloaded to a browser at runtime, and does not require any Oracle software on the client side. This driver requires a TCP/IP listener on the server side, and the client connection string uses the TCP/IP port address, not the TNSNAMES entry for the database name.

What is the difference between final, finally and finalize? What do you understand by the java final keyword?

Or

What is final, finalize() and finally?

Or

What is finalize() method?

Or

What is the difference between final, finally and finalize?

Or

What does it mean that a class or member is final?

o final - declare constant
o finally - handles exception
o 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.

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.

What is the GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars.

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.

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 transparency
* Global variables creates collisions in namespace.

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();

What is the SimpleTimeZone class?

The SimpleTimeZone class provides support for a Gregorian calendar.

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.

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.

Describe the principles of OOPS.

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

Explain the Inheritance principle.

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

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

Is sizeof a keyword in java?

The sizeof operator is not a keyword.

What is a native method?

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

In System.out.println(), 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.

What are Encapsulation, Inheritance and Polymorphism

Or

Explain the Polymorphism principle. Explain the different forms of Polymorphism.

Polymorphism in simple terms means one name many forms. Polymorphism enables one entity to be used as a general category for different types of actions. The specific action is determined by the exact nature of the situation.

Polymorphism exists in three distinct forms in Java:
• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface

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

What is the Java Virtual Machine (JVM)?

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

What do you understand by downcasting?

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

What are Java Access Specifiers?

Or

What is the difference between public, private, protected and default Access Specifiers?

Or

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
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

Which class is the superclass of every class?

Object.

Name primitive Java types.

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

What is the difference between static and non-static variables?

Or

What are class variables?

Or

What is static in java?

Or

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.

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 && 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.

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.

What if I write static public void instead of public static void?

Program compiles and runs properly.

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.

What type of parameter passing does Java support?

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.

Explain the Encapsulation principle.

Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. Objects allow procedures to be encapsulated with their data to reduce potential interference. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.

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.

What do you understand by numeric promotion?

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.

What do you understand by casting in java language? What are the types of casting?

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.

What is the first argument of the String array in main method?

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.

How can one prove that the array is not null but empty?

Print array.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print array.length.

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.

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.

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) {}

No the program fails to compile. The compiler says that the main method is already defined in the class.

Explain working of Java Virtual Machine (JVM)?

JVM is an abstract computing machine like any other real computing machine which first converts .java file into .class file by using Compiler (.class is nothing but byte code file.) and Interpreter reads byte codes.

How can I swap two variables without using a third variable?

Add two variables and assign the value into First variable. Subtract the Second value with the result Value. and assign to Second variable. Subtract the Result of First Variable With Result of Second Variable and Assign to First Variable. Example:

int a=5,b=10;a=a+b; b=a-b; a=a-b;

An other approach to the same question

You use an XOR swap.

for example:

int a = 5; int b = 10;
a = a ^ b;
b = a ^ b;
a = a ^ b;

What is data encapsulation?

Encapsulation may be used by creating ‘get’ and ’set’ methods in a class (JAVABEAN) which are used to access the fields of the object. Typically the fields are made private while the get and set methods are public. Encapsulation can be used to validate the data that is to be stored, to do calculations on data that is stored in a field or fields, or for use in introspection (often the case when using javabeans in Struts, for instance). Wrapping of data and function into a single unit is called as data encapsulation. Encapsulation is nothing but wrapping up the data and associated methods into a single unit in such a way that data can be accessed with the help of associated methods. Encapsulation provides data security. It is nothing but data hiding.

What is reflection API? How are they implemented?

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.

Does JVM maintain a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap maintained by the JVM? Why

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

What is phantom memory?

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

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.
Class instance associated with the object. It is similar to saying:

synchronized(XYZ.class) {

}

What is difference between String and StringTokenizer?

A StringTokenizer is utility class used to break up string.

Example:

StringTokenizer st = new StringTokenizer(”Hello World”);

while (st.hasMoreTokens()) {

System.out.println(st.nextToken());

}

Output:

Hello

World

Do I need to import java.lang package any time? Why?

Or

Which package is always imported by default?

No. It is by default loaded internally by the JVM. The java.lang package is always imported by default.

Can I import same package/class twice? Will the JVM load the package twice at runtime?

One can import the same package or same class multiple times. Neither compiler nor JVM complains anything about it. And the JVM will internally load the class only once no matter how many times you import the same class.

Does importing a package imports the sub packages as well? E.g. Does importing com.bob.* also import com.bob.code.*?

No you will have to import the sub packages explicitly. Importing com.bob.* will import classes in the package bob only. It will not import any class in any of its sub package’s.

What is a Java package and how is it used?

Or

Explain the usage of Java packages.

A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
For example: The Java API is grouped into libraries of related classes and interfaces; these libraries are known as package.

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.BOB compile?

Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying, cannot resolve symbol.

Explain the user defined Exceptions?

User defined Exceptions are custom Exception classes defined by the user for specific purpose. A user defined exception can be created by simply sub-classing an Exception class or a subclass of an Exception class. This allows custom exceptions to be generated (using throw clause) and caught in the same way as normal exceptions.
Example:

class CustomException extends Exception {

}

What classes of exceptions may be caught by a catch clause?

A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types. Errors are generally irrecoverable conditions

What is the difference between exception and error?

Error’s are irrecoverable exceptions. Usually a program terminates when an error is encountered.

What is the difference between throw and throws keywords?

The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as an argument. The exception will be caught by an enclosing try-catch block or propagated further up the calling hierarchy. The throws keyword is a modifier of a method that denotes that an exception may be thrown by the method. An exception can be rethrown.

What class of exceptions are generated by the Java run-time system?

The Java runtime system generates Runtime Exceptions and Errors.

What is the base class for Error and Exception?

Throwable

What are Checked and Unchecked Exceptions?

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the exception may be thrown. Checked exceptions must be caught at compile time. Example: IOException.
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn’t force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. Example: ArrayIndexOutOfBoundsException. Errors are often irrecoverable conditions.

Does the code in finally block get executed if there is an exception and a return statement in a catch block?

Or

What is the purpose of the finally clause of a try-catch-finally statement?

The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught. If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(0) statement is executed earlier or on system shut down earlier or the memory is used up earlier before the thread goes to finally block.

try{
//some statements
}
catch{
//statements when exception is caught
}
finally{
//statements executed whether exception occurs or not
}

Does the order of placing catch statements matter in the catch block?

Yes, it does. The FileNoFoundException is inherited from the IOException. So FileNoFoundException is caught before IOException. Exception’s subclasses have to be caught first before the General Exception

What are Java Applets used for?

Java Applets are usually used to add small, interactive components or enhancements to a webpage. These may consist of buttons, scrolling text, or stock tickers, but they can also be used to display larger programs like word processors or games. Java's incredible portability (the ability to run on many different operating systems and browsers) is what makes it ideal for use on the World Wide Web.

What are all the listeners in java and explain ?

An event listener is an object used to handling a particular kind of event. Event Handling Interfaces * ActionListener * AdjustmentListener * ComponentListener * ContainerListener * FocusListener * ItemListener * KeyListener * MouseListener * MouseMotionListener * WindowListener

Ways of creating a Thread

There are two ways of creating a thread.

1. Extending the Thread class

Ex: public class ThreadDemo extends Thread {
.....
}

2. Implementing the Runnable Interface

Ex: public class ThreadDemo implements Runnable {
...........
}

What is the difference between Abstract class and Interface
Or
When should you use an abstract class, when an interface, when both?
Or
What is similarities/difference between an Abstract class and Interface?
Or
What is the difference between interface and an abstract class?

1. Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. An abstract class can contain no abstract methods also i.e. abstract class may contain concrete methods. A Java Interface can contain only method declarations and public static final constants and doesn't contain their implementation. The classes which implement the Interface must provide the method definition for all the methods present.

2. Abstract class definition begins with the keyword "abstract" keyword followed by Class definition. An Interface definition begins with the keyword "interface".

3. Abstract classes are useful in a situation when some general methods should be implemented and specialization behavior should be implemented by subclasses. Interfaces are useful in a situation when all its properties need to be implemented by subclasses

4. All variables in an Interface are by default - public static final while an abstract class can have instance variables.

5. An interface is also used in situations when a class needs to extend an other class apart from the abstract class. In such situations its not possible to have multiple inheritance of classes. An interface on the other hand can be used when it is required to implement one or more interfaces. Abstract class does not support Multiple Inheritance whereas an Interface supports multiple Inheritance.

6. An Interface can only have public members whereas an abstract class can contain private as well as protected members.

7. A class implementing an interface must implement all of the methods defined in the interface, while a class extending an abstract class need not implement any of the methods defined in the abstract class.

8. The problem with an interface is, if you want to add a new feature (method) in its contract, then you MUST implement those method in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass

9. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast

10.Interfaces are often used to describe the peripheral abilities of a class, and not its central identity, E.g. an Automobile class might
implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.

Note: There is no difference between a fully abstract class (all methods declared as abstract and all fields are public static final) and an interface.

Note: If the various objects are all of-a-kind, and share a common state and behavior, then tend towards a common base class. If all they
share is a set of method signatures, then tend towards an interface.

Similarities:
Neither Abstract classes nor Interface can be instantiated.

What does it mean that a method or class is abstract?

An abstract class cannot be instantiated. Only its subclasses can be instantiated. A class that has one or more abstract methods must be declared abstract. A subclass that does not provide an implementation for its inherited abstract methods must also be declared abstract. You indicate that a class is abstract with the abstract keyword like this:

public abstract class AbstractClass

Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the class. It exists only to be overridden in subclasses. Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses
or itself be declared abstract. Only the method’s prototype is provided in the class definition. Also, a final method can not be abstract and vice versa. Methods specified in an interface are implicitly abstract.
. It has no body. For example,

public abstract float getInfo()

What must a class do to implement an interface?

The class must provide all of the methods in the interface and identify the interface in its implements clause.

What is an abstract method?

An abstract method is a method whose implementation is deferred to a subclass.

What is interface? How to support multiple inhertance in Java?

Or

What is a cloneable interface and how many methods does it contain?

An Interface are implicitly abstract and public. Interfaces with empty bodies are called marker interfaces having certain property or behavior. Examples:java.lang.Cloneable,java.io.Serializable,java.util.EventListener. An interface body can contain constant declarations, method prototype declarations, nested class declarations, and nested interface declarations.

Interfaces provide support for multiple inheritance in Java. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Example of Interface:
public interface sampleInterface {
public void functionOne();

public long CONSTANT_ONE = 1000;
}

What is an abstract class?
Or
Can you make an instance of an abstract class?

Abstract classes can contain abstract and concrete methods. Abstract classes cannot be instantiated directly i.e. we cannot call the constructor of an abstract class directly nor we can create an instance of an abstract class by using “Class.forName().newInstance()” (Here we get java.lang.InstantiationException). However, if we create an instance of a class that extends an Abstract class, compiler will initialize both the classes. Here compiler will implicitly call the constructor of the Abstract class. Any class that contain an abstract method must be declared “abstract” and abstract methods can have definitions only in child classes. By overriding and customizing the abstract methods in more than one subclass makes “Polymorphism” and through Inheritance we define body to the abstract methods. Basically an abstract class serves as a template. Abstract class must be extended/subclassed for it to be implemented. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated. Abstract class is a class that provides some general functionality but leaves specific implementation to its inheriting classes.

Example of Abstract class:

abstract class AbstractClassExample{

protected String name;
public String getname() {
return name;
}
public abstract void function();
}

Example: Vehicle is an abstract class and Bus Truck, car etc are specific implementations

No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed.
If you have an abstract class and you want to use a method which has been implemented, you may
need to subclass that abstract class, instantiate your subclass and then call that method.

What is meant by "Abstract Interface"?

Firstly, an interface is abstract. That means you cannot have any implementation in an interface.
All the methods declared in an interface are abstract methods or signatures of the methods.

How to define an Interface?

In Java Interface defines the methods but does not implement them. Interface can include constants.
A class that implements the interfaces is bound to implement all the methods defined in Interface.
Example of Interface:

public interface SampleInterface {
public void functionOne();

public long CONSTANT_ONE = 1000;
}

Can Abstract Class have constructors? Can interfaces have constructors?

Abstract class's can have a constructor, but you cannot access it through the object, since you cannot instantiate abstract class. To access the constructor create a sub class and extend the abstract class which is having the constructor.

Example
public abstract class AbstractExample {
public AbstractExample(){
System.out.println("In AbstractExample()");
}
}

public class Test extends AbstractExample{
public static void main(String args[]){
Test obj=new Test();
}
}

If interface & abstract class have same methods and those methods contain no implementation, which one would you prefer?

Obviously one should ideally go for an interface, as we can only extend one class. Implementing an interface for a class is very much effective rather than extending an abstract class because we can extend some other useful class for this subclass.

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