Thursday, September 8, 2011

Java : Basics Interview Questions

What is Java?
Java is a simple, secure, robust, complete object oriented and platform independent high level programming language and  It is also portable, high performance, multithreaded.

What is JVM?
JVM is a translator between the language and the underlying software and hardware.

What is the difference between an Interface and an Abstract class?

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.

A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers?
public : Public class is visible in other packages, field is visible everywhere (class must be public too)

private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.

protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.

default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

What is an abstract class?

A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.

A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

What is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.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 nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

What is final?
A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

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 if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error "NoSuchMethodError".

What if I write static public void instead of public static void?
Program compiles and runs properly.

What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".

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 I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
It is empty. But not null.

How can one prove that the array is not null but empty using one line of code?
Print args.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 args.length.

What environment variables do I need to set on my machine in order to be able to run Java programs?
CLASSPATH and PATH are the two variables.

Can an application have multiple classes having main method?
Yes it is possible. 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.

Can I have multiple main methods in the same class?
Yes, because main is also method so you can provide multiple main methods by overloading concept.
Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.

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 abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.

When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

What are different types of inner classes?
Nested top-level classes, Member classes, Local classes, Anonymous classes

Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class.

Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a

more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD 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,can not resolve symbol

symbol : class ABCD
location: package io

Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.

What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.

e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.

What is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.

Can a top level class be private or protected?
No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.

What type of parameter passing does Java support?
In Java the arguments are always passed by value .

Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.

Objects are passed by value or by reference?
Java only supports pass 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 .

Does Java provide any construct to find out the size of an object?
No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.

Give a simplest way to find out the time a method takes for execution without using any profiling tool?
Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.

What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

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.
Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement 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 body of a loop at least once.

What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

How are this() and super() used with constructors?
This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.

What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
Is Empty .java file a valid source file?
Yes, an empty .java file is a perfectly valid source file.

Can a .java file contain more than one java classes?
Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.

Is String a primitive data type in Java?
No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

What happens if you dont initialize an instance variable of any of the primitive types in Java?
Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.

What will be the initial value of an object reference which is defined as an instance variable?
The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

What are the different scopes for Java variables?

The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.

1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.

2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.

3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.
What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..
How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
Can a public class MyClass be defined in a source file named
No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.
Can main method be declared final?
Yes, the main method can be declared final, in addition to being public static.

What will be the output of the following statement?
System.out.println ("1" + 3);
It will print 13.

What will be the default values of all the elements of an array defined as an instance variable?
If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.

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