25 Java Interview Questions and Answers

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148. Question: How can i tell what state a thread is in ?
Answer : Prior to Java 5, isAlive() was commonly used to test a threads state. If isAlive() returned false the thread was either new or terminated but there was simply no way to differentiate between the two. Starting with the release of Tiger (Java 5) you can now get what state a thread is in by using the getState() method which returns an Enum of Thread.States. A thread can only be in one of the following states at a given point in time. NEW A Fresh thread that has not yet started to execute. RUNNABLE A thread that is executing in the Java virtual machine. BLOCKED A thread that is blocked waiting for a monitor lock. WAITING A thread that is wating to be notified by another thread. TIMED_WAITING A thread that is wating to be notified by another thread for a specific amount of time TERMINATED A thread whos run method has ended. The folowing code prints out all thread states. public class ThreadStates{ public static void main(String[] args){ Thread t = new Thread(); Thread.State e = t.getState(); Thread.State[] ts = e.values(); for(int i = 0; i < ts.length; i++){ System.out.println(ts[i]); } } }

149. Question: What methods java providing for Thread communications ?
Answer : Java provides three methods that threads can use to communicate with each other: wait, notify, and notifyAll. These methods are defined for all Objects (not just Threads). The idea is that a method called by a thread may need to wait for some condition to be satisfied by another thread; in that case, it can call the wait method, which causes its thread to wait until another thread calls notify or notifyAll.

150. Question: What is the difference between notify and notify All methods ?
Answer : A call to notify causes at most one thread waiting on the same object to be notified (i.e., the object that calls notify must be the same as the object that called wait). A call to notifyAll causes all threads waiting on the same object to be notified. If more than one thread is waiting on that object, there is no way to control which of them is notified by a call to notify (so it is often better to use notifyAll than notify).

151. Question: What is synchronized keyword? In what situations you will Use it?
Answer : Synchronization is the act of serializing access to critical sections of code. We will use this keyword when we expect multiple threads to access/modify the same data. To understand synchronization we need to look into thread execution manner. Threads may execute in a manner where their paths of execution are completely independent of each other. Neither thread depends upon the other for assistance. For example, one thread might execute a print job, while a second thread repaints a window. And then there are threads that require synchronization, the act of serializing access to critical sections of code, at various moments during their executions. For example, say that two threads need to send data packets over a single network connection. Each thread must be able to send its entire data packet before the other thread starts sending its data packet; otherwise, the data is scrambled. This scenario requires each thread to synchronize its access to the code that does the actual data-packet sending. If you feel a method is very critical for business that needs to be executed by only one thread at a time (to prevent data loss or corruption), then we need to use synchronized keyword. EXAMPLE Some real-world tasks are better modeled by a program that uses threads than by a normal, sequential program. For example, consider a bank whose accounts can be accessed and updated by any of a number of automatic teller machines (ATMs). Each ATM could be a separate thread, responding to deposit and withdrawal requests from different users simultaneously. Of course, it would be important to make sure that two users did not access the same account simultaneously. This is done in Java using synchronization, which can be applied to individual methods, or to sequences of statements. One or more methods of a class can be declared to be synchronized. When a thread calls an object's synchronized method, the whole object is locked. This means that if another thread tries to call any synchronized method of the same object, the call will block until the lock is released (which happens when the original call finishes). In general, if the value of a field of an object can be changed, then all methods that read or write that field should be synchronized to prevent two threads from trying to write the field at the same time, and to prevent one thread from reading the field while another thread is in the process of writing it. Here is an example of a BankAccount class that uses synchronized methods to ensure that deposits and withdrawals cannot be performed simultaneously, and to ensure that the account balance cannot be read while either a deposit or a withdrawal is in progress. (To keep the example simple, no check is done to ensure that a withdrawal does not lead to a negative balance.) public class BankAccount { private double balance; // constructor: set balance to given amount public BankAccount( double initialDeposit ) { balance = initialDeposit; } public synchronized double Balance( ) { return balance; } public synchronized void Deposit( double deposit ) { balance += deposit; } public synchronized void Withdraw( double withdrawal ) { balance -= withdrawal; } } Note: that the BankAccount's constructor is not declared to be synchronized. That is because it can only be executed when the object is being created, and no other method can be called until that creation is finished. There are cases where we need to synchronize a group of statements, we can do that using synchrozed statement. Java Code Example synchronized ( B ) { if ( D > B.Balance() ) { ReportInsuffucientFunds(); } else { B.Withdraw( D ); } }

152. Question: What is serialization ?
Answer : Serialization is the process of writing complete state of java object into output stream, that stream can be file or byte array or stream associated with TCP/IP socket

153. Question: What does the Serializable interface do ?
Answer : Serializable is a tagging interface; it prescribes no methods. It serves to assign the Serializable data type to the tagged class and to identify the class as one which the developer has designed for persistence. ObjectOutputStream serializes only those objects which implement this interface

154. Question: How do I serialize an object to a file ?
Answer : To serialize an object into a stream perform the following actions:
- Open one of the output streams, for exaample FileOutputStream - Chain it with the ObjectOutputStream - Call the method writeObject() providingg the instance of a Serializable object as an argument. - Close the streams
Java Code --------- try{ fOut= new FileOutputStream("c:\\emp.ser"); out = new ObjectOutputStream(fOut); out.writeObject(employee); //serializing System.out.println("An employee is serialized into c:\\emp.ser"); } catch(IOException e){ e.printStackTrace(); }

155. Question: How do I deserilaize an Object?
Answer : To deserialize an object, perform the following steps:
- Open an input stream - Chain it with the ObjectInputStream - Call the method readObject() and cast the returned object to the class that is being deserialized. - Close the streams Java Code try{ fIn= new FileInputStream("c:\\emp.ser"); 2. in = new ObjectInputStream(fIn); //de-serializing employee 3. Employee emp = (Employee) in.readObject(); 4. System.out.println("Deserialized " + emp.fName + " " + emp.lName + " from emp.ser "); } catch(IOException e){ e.printStackTrace(); }catch(ClassNotFoundException e){ e.printStackTrace(); }

156. Question: What is Externalizable Interface ?
Answer : Externalizable interface is a subclass of Serializable. Java provides Externalizable interface that gives you more control over what is being serialized and it can produce smaller object footprint. ( You can serialize whatever field values you want to serialize)
This interface defines 2 methods: readExternal() and writeExternal() and you have to implement these methods in the class that will be serialized. In these methods you'll have to write code that reads/writes only the values of the attributes you are interested in. Programs that perform serialization and deserialization have to write and read these attributes in the same sequence

157. Question: Explain garbage collection ?
Answer : Garbage collection is an important part of Java's security strategy. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects from the memory. The name "garbage collection" implies that objects that are no longer needed by the program are "garbage" and can be thrown away. A more accurate and up-to-date metaphor might be "memory recycling." When an object is no longer referenced by the program, the heap space it occupies must be recycled so that the space is available for subsequent new objects. The garbagecollector must somehow determine which objects are no longer referenced by the program and make available the heap space occupied by such unreferenced objects. In the process of freeing unreferenced objects, the garbage collector must run any finalizers of objects being freed

158. Question :How you can force the garbage collection ?
Answer : Garbage collection automatic process and can't be forced. We can call garbage collector in Java by calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected

159. Question :What are the field/method access levels (specifiers) and class access levels ?
Answer : Each field and method has an access level: private: accessible only in this class (package): accessible only in this package protected: accessible only in this package and in all subclasses of this class public: accessible everywhere this class is available Similarly, each class has one of two possible access levels: (package): class objects can only be declared and manipulated by code in this package public: class objects can be declared and manipulated by code in any package

160. Question: What are the static fields & static Methods ?
Answer : If a field or method defined as a static, there is only one copy for entire class, rather than one copy for each instance of class. static method cannot accecss non-static field or call non-static method
Example Java Code
static int counter = 0;
A public static field or method can be accessed from outside the class using either the usual notation:
Java-class-object.field-or-method-name
or using the class name instead of the name of the class object:
Java- class-name.field-or-method-name

161. Question: What are the Final fields & Final Methods ?
Answer : Fields and methods can also be declared final. A final method cannot be overridden in a subclass. A final field is like a constant: once it has been given a value, it cannot be assigned to again
Java Code
private static final int MAXATTEMPTS = 10;

162. Question: What are the uses of Serialization?
Answer : In some types of applications you have to write the code to serialize objects, but in many cases serialization is performed behind the scenes by various server-side containers. These are some of the typical uses of serialization: To persist data for future use. To send data to a remote computer using such client/server Java technologies as RMI or socket programming. To "flatten" an object into array of bytes in memory. To exchange data between applets and servlets. To store user session in Web applications . To activate/passivate enterprise java beans. To send objects between the servers in a cluster.

163. Question: what is a collection ?
Answer : Collection is a group of objects. java.util package provides important types of collections. There are two fundamental types of collections they are Collection and Map. Collection types hold a group of objects, Eg. Lists and Sets where as Map types hold group of objects as key, value pairs Eg. HashMap and Hashtable.

164. Question: For concatenation of strings, which method is good, StringBuffer or String ?
Answer : StringBuffer is faster than String for concatenation.

165. Question: What is Runnable interface ? Are there any other ways to make a java program as multithred java program?
Answer : There are two ways to create new kinds of threads: - Define a new class that extends the Thread class - Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor. - An advantage of the second approach is that the new class can be a subclass of any class, not just of the Thread class. Here is a very simple example just to illustrate how to use the second approach to creating threads: class myThread implements Runnable { public void run() { System.out.println("I'm running!"); } } public class tstRunnable { public static void main(String[] args) { myThread my1 = new myThread(); myThread my2 = new myThread(); new Thread(my1).start(); new Thread(my2).start(); }

166. Question: How can i tell what state a thread is in ?
Answer : Prior to Java 5, isAlive() was commonly used to test a threads state. If isAlive() returned false the thread was either new or terminated but there was simply no way to differentiate between the two. Starting with the release of Tiger (Java 5) you can now get what state a thread is in by using the getState() method which returns an Enum of Thread.States. A thread can only be in one of the following states at a given point in time. NEW A Fresh thread that has not yet started to execute. RUNNABLE A thread that is executing in the Java virtual machine. BLOCKED A thread that is blocked waiting for a monitor lock. WAITING A thread that is wating to be notified by another thread. TIMED_WAITING A thread that is wating to be notified by another thread for a specific amount of time TERMINATED A thread whos run method has ended. The folowing code prints out all thread states. public class ThreadStates{ public static void main(String[] args){ Thread t = new Thread(); Thread.State e = t.getState(); Thread.State[] ts = e.values(); for(int i = 0; i < ts.length; i++){ System.out.println(ts[i]); } } }

167. Question: What methods java providing for Thread communications ?
Answer : Java provides three methods that threads can use to communicate with each other: wait, notify, and notifyAll. These methods are defined for all Objects (not just Threads). The idea is that a method called by a thread may need to wait for some condition to be satisfied by another thread; in that case, it can call the wait method, which causes its thread to wait until another thread calls notify or notifyAll. Question: What is the difference between notify and notify All methods ? A call to notify causes at most one thread waiting on the same object to be notified (i.e., the object that calls notify must be the same as the object that called wait). A call to notifyAll causes all threads waiting on the same object to be notified. If more than one thread is waiting on that object, there is no way to control which of them is notified by a call to notify (so it is often better to use notifyAll than notify). Question: What is synchronized keyword? In what situations you will Use it? Synchronization is the act of serializing access to critical sections of code. We will use this keyword when we expect multiple threads to access/modify the same data. To understand synchronization we need to look into thread execution manner. Threads may execute in a manner where their paths of execution are completely independent of each other. Neither thread depends upon the other for assistance. For example, one thread might execute a print job, while a second thread repaints a window. And then there are threads that require synchronization, the act of serializing access to critical sections of code, at various moments during their executions. For example, say that two threads need to send data packets over a single network connection. Each thread must be able to send its entire data packet before the other thread starts sending its data packet; otherwise, the data is scrambled. This scenario requires each thread to synchronize its access to the code that does the actual data-packet sending. If you feel a method is very critical for business that needs to be executed by only one thread at a time (to prevent data loss or corruption), then we need to use synchronized keyword. EXAMPLE Some real-world tasks are better modeled by a program that uses threads than by a normal, sequential program. For example, consider a bank whose accounts can be accessed and updated by any of a number of automatic teller machines (ATMs). Each ATM could be a separate thread, responding todeposit and withdrawal requests from different users simultaneously. Of course, it would be important to make sure that two users did not access the same account simultaneously. This is done in Java using synchronization, which can be applied to individual methods, or to sequences of statements. One or more methods of a class can be declared to be synchronized. When a thread calls an object's synchronized method, the whole object is locked. This means that if another thread tries to call any synchronized method of the same object, the call will block until the lock is released (which happens when the original call finishes). In general, if the value of a field of an object can be changed, then all methods that read or write that field should be synchronized to prevent two threads from trying to write the field at the same time, and to prevent one thread from reading the field while another thread is in the process of writing it. Here is an example of a BankAccount class that uses synchronized methods to ensure that deposits and withdrawals cannot be performed simultaneously, and to ensure that the account balance cannot be read while either a deposit or a withdrawal is in progress. (To keep the example simple, no check is done to ensure that a withdrawal does not lead to a negative balance.) public class BankAccount { private double balance; // constructor: set balance to given amount public BankAccount( double initialDeposit ) { balance = initialDeposit; } public synchronized double Balance( ) { return balance; } public synchronized void Deposit( double deposit ) { balance += deposit; } public synchronized void Withdraw( double withdrawal ) { balance -= withdrawal; } } Note: that the BankAccount's constructor is not declared to be synchronized. That is because it can only be executed when the object is being created, and no other method can be called until that creation is finished. There are cases where we need to synchronize a group of statements, we can do that using synchrozed statement. Java Code Example synchronized ( B ) { if ( D > B.Balance() ) { ReportInsuffucientFunds(); } else { B.Withdraw( D ); } }

168. Question: What is serialization ?
Answer : Serialization is the process of writing complete state of java object into output stream, that stream can be file or byte array or stream associated with TCP/IP socket

169. Question: What does the Serializable interface do ?
Answer : Serializable is a tagging interface; it prescribes no methods. It serves to assign the Serializable data type to the tagged class and to identify the class as one which the developer has designed for persistence. ObjectOutputStream serializes only those objects which implement this interface

170. Question: How do I serialize an object to a file ?
Answer : To serialize an object into a stream perform the following actions:
- Open one of the output streams, for exxample FileOutputStream - Chain it with the ObjectOutputStream < - Call the method writeObject() providinng the instance of a Serializable object as an argument. - Close the streams Java Code --------- try{ fOut= new FileOutputStream("c:\\emp.ser"); out = new ObjectOutputStream(fOut); out.writeObject(employee); //serializing System.out.println("An employee is serialized into c:\\emp.ser"); } catch(IOException e){ e.printStackTrace(); }

171. Question: How do I deserilaize an Object?
Answer : To deserialize an object, perform the following steps: - Open an input stream - Chain it with the ObjectInputStream - Call the method readObject() and cast the returned object to the class that is being deserialized. - Close the streams Java Code try{ fIn= new FileInputStream("c:\\emp.ser"); in = new ObjectInputStream(fIn); //de-serializing employee Employee emp = (Employee) in.readObject(); System.out.println("Deserialized " + emp.fName + " " + emp.lName + " from emp.ser "); }catch(IOException e){ e.printStackTrace(); }catch(ClassNotFoundException e){ e.printStackTrace(); }<

172. Question: What is Externalizable Interface ?
Answer : Externalizable interface is a subclass of Serializable. Java provides Externalizable interface that gives you more control over what is being serialized and it can produce smaller object footprint. ( You can serialize whatever field values you want to serialize) This interface defines 2 methods: readExternal() and writeExternal() and you have to implement these methods in the class that will be serialized. In these methods you'll have to write code that reads/writes only the values of the attributes you are interested in. Programs that perform serialization and deserialization have to write and read these attributes in the same sequence.



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