Java Interview Questions
| Q: | |
| A: | With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors. |
| Q: | |
| A: | The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help. |
| Q: | |
| A: | 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. |
| Q: | |
| A: | Map is Interface and Hashmap is class that implements that. |
| Q: | |
| A: | The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized. |
| Q: | |
| A: | Vector is synchronized whereas arraylist is not. |
| Q: | |
| A: | AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT. |
| Q: | |
| A: | 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. |
| Q: | |
| A: | Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator. |
| Q: | |
| 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. |
| Q: | |
| A: | 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. |
| Q: | |
| A: | 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). |
| Q: | ||
| A: | The program compiles properly but at runtime it will give "Main method not public." message. | |
| Q: |
What if the static modifier is removed from the signature of the main method? |
|
| A: | Program compiles. But at runtime throws an error "NoSuchMethodError". | |
| Q: |
What if I write static public void instead of public static void? |
|
| A: | Program compiles and runs properly. | |
| Q: |
What if I do not provide the String array as the argument to the method? |
|
| A: | Program compiles but throws a runtime error "NoSuchMethodError". | |
| Q: |
If I do not provide any arguments on the command line, then the String array of Main method will be empty or null? |
|
| A: | It is empty. But not null. | |
| Q: |
What environment variables do I need to set on my machine in order to be able to run Java programs? |
|
| A: | CLASSPATH and PATH are the two variables. | |
| Q: | ||
| A: | No the program fails to compile. The compiler says that the main method is already defined in the class. | |
| Q: | ||
| A: | No. It is by default loaded internally by the JVM. | |
| Q: | |
| A: | 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 possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· 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. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. |
| Q: | |
| A: | 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. |
| Q: | |
| A: | 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. |
| Q: |
Does Java provide any construct to find out the size of an object? |
|
| A: | No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java. | |
| Q: | ||
| A: | 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. | |
| Q: | ||
| A: | 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. | |
| Q: | ||
| A: | Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions. | |
| Q: | ||
| A: | Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time. | |
| Q: | ||
| A: | An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.). | |
| Q: | ||
| A: | Your class should extend class Exception, or some more specific type thereof. | |
| Q: | ||
| A: | An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates. | |
| Q: | ||
| A: | There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions. |
|
| Q: | ||
| A: | Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. | |
|
[Received from Venkateswara Manam]
|
||
| Q: | ||
| A: | With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors. |
|
|
[ Received from Venkateswara Manam]
|
||
| Q: | ||
| A: | It uses those low order bytes of the result that can fit into the size of the type allowed by the operation. | |
| Q: |
When a thread is created and started, what is its initial state? |
|
| A: | A thread is in the ready state after it has been created and started. | |
| Q: | ||
| A: | The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. | |
| Q: | ||
| A: | The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region. | |
| Q: | ||
| A: | This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. | |
| Q: | ||
| A: | Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. | |
| Q: | ||
| A: | At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable. An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members. It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial. |
|
| Q: | ||
| A: | While making a JDBC connection we go through the following steps : Step 1 : Register the database driver by using : Class.forName(/" driver classs for that specific database/" ); Step 2 : Now create a database connection using : Connection con = DriverManager.getConnection(url,username,password); Step 3: Now Create a query using : Statement stmt = Connection.Statement(/"select * from TABLE NAME/"); Step 4 : Exceute the query : stmt.exceuteUpdate(); |
|
| Q: | ||
| A: | 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. | |
|
[Received from P Rajesh]
|
||
| Q: | ||
| A: | All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface. |
|
|
[ Received from P Rajesh]
|
||
| Q: | ||
| A: | Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. |
|
| Q: | ||
| A: | Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in) | |
| Q: | ||
| A: | Only public and abstract modifiers are allowed for methods in interfaces. |
|
| Q: | ||
| A: | 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). |
|
| Q: | ||
| A: | 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. |
|
| Q: | ||
| A: | If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause. |
|
| Q: | ||
| A: | Yes, an empty .java file is a perfectly valid source file. | |
|
[Received from Sandesh Sadhale]
|
||
| Q: | ||
| A: | Yes, a .java file contain more than one java classes, provided at the most one of them is a public class. | |
| Q: | ||
| A: | 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. | |
| Q: | ||
| A: | No, main is not a keyword in Java. | |
| Q: | ||
| A: | No, next is not a keyword. | |
| Q: | ||
| A: | No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does. | |
| Q: | ||
| A: | No. To exit a program explicitly you use exit method in System object. | |
| Q: | ||
| A: | 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. |
|
| Q: | ||
| A: | 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.. | |
| Q: | ||
| A: | Yes, the main method can be declared final, in addition to being public static. | |
| Q: |
What will be the output of the following statement? |
|
| A: | It will print 13. | |
