1. What is a Marker Interface?
An interface with no methods. Example: Serializable, Remote, Cloneable
2. What interface do you implement to do the sorting?
Comparable
3. What is the eligibility for a object to get cloned?
It must implement the Cloneable interface
4. What is the purpose of abstract class?
It is not an instantiable class. It provides the concrete implementation for some/all the methods. So that they can reuse the concrete functionality by inheriting the abstract class.
5. What is the difference between interface and abstract class?
Abstract class defined with methods. Interface will declare only the methods. Abstract classes are very much useful when there is a some functionality across various classes. Interfaces are well suited for the classes which varies in functionality but with the same method signatures.
6. What do you mean by RMI and how it is useful?
RMI is a remote method invocation. Using RMI, you can work with remote object. The function calls are as though you are invoking a local variable. So it gives you a impression that you are working really with a object that resides within your own JVM though it is somewhere.
7. What is the protocol used by RMI?
RMI-IIOP
8. What is a hashCode?
hash code value for this object which is unique for every object.
9. What is a thread?
Thread is a block of code which can execute concurrently with other threads in the JVM.
10. What is the algorithm used in Thread scheduling?
Fixed priority scheduling.
11. What is hash-collision in Hashtable and how it is handled in Java?
Two different keys with the same hash value. Two different entries will be kept in a single hash bucket to avoid the collision.
12. What are the different driver types available in JDBC?
1. A JDBC-ODBC bridge 2. A native-API partly Java technology-enabled driver 3. A net-protocol fully Java technology-enabled driver 4. A native-protocol fully Java technology-enabled driver For more information: Driver Description
13. Is JDBC-ODBC bridge multi-threaded?
No
14. Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection?
No
15. What is the use of serializable?
To persist the state of an object into any perminant storage device.
16. What is the use of transient?
It is an indicator to the JVM that those variables should not be persisted. It is the users responsibility to initialize the value when read back from the storage.
17. What are the different level lockings using the synchronization keyword?
Class level lock Object level lock Method level lock Block level lock
Wednesday, May 06, 2009
Thursday, April 30, 2009
Java Fundamental-I
1. How could Java classes direct program messages to the system console, but error messages, say to a file?
The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:
Stream st =
new Stream (new
FileOutputStream ("techinterviews_com.txt"));
System.setErr(st);
System.setOut(st);
2. What’s the difference between an interface and an abstract class?
An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.
3. Why would you use a synchronized block vs. synchronized method?
Synchronized blocks place locks for shorter periods than synchronized methods.
4. Explain the usage of the keyword transient?
This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).
5. How can you force garbage collection?
You can’t force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.
6. How do you know if an explicit object casting is needed?
If you assign a superclass object to a variable of a subclass’s data type, you need to do explicit casting. For example:
Object a;Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.
7. What’s the difference between the methods sleep() and wait()
The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
8. Can you write a Java class that could be used both as an applet as well as an application?
Yes. Add a main() method to the applet.
9. What’s the difference between constructors and other methods?
Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.
10. Can you call one constructor from another if a class has multiple constructors
Yes. Use this() syntax.
11. Explain the usage of Java packages.
This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.
12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?
You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let’s say a class Employee belongs to a package com.xyz.hr; and is located in the file c:/dev/com.xyz.hr.Employee.java. In this case, you’d need to add c:/dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee
13. What’s the difference between J2SDK 1.5 and J2SDK 5.0?
There’s no difference, Sun Microsystems just re-branded this version.
14. What would you use to compare two String variables - the operator == or the method equals()?
I’d use the method equals() to compare the values of the Strings and the = = to check if two variables point at the same instance of a String object.
15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Yes, it does. The FileNoFoundException is inherited from the IOException. Exception’s subclasses have to be caught first.
16. Can an inner class declared inside of a method access local variables of this method?
It’s possible if these variables are final.
17. What can go wrong if you replace && with & in the following code:
String a=null;
if (a!=null && a.length()>10)
{...}
A single ampersand here would lead to a NullPointerException.
18. What’s the main difference between a Vector and an ArrayList
Java Vector class is internally synchronized and ArrayList is not.
The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:
Stream st =
new Stream (new
FileOutputStream ("techinterviews_com.txt"));
System.setErr(st);
System.setOut(st);
2. What’s the difference between an interface and an abstract class?
An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.
3. Why would you use a synchronized block vs. synchronized method?
Synchronized blocks place locks for shorter periods than synchronized methods.
4. Explain the usage of the keyword transient?
This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).
5. How can you force garbage collection?
You can’t force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.
6. How do you know if an explicit object casting is needed?
If you assign a superclass object to a variable of a subclass’s data type, you need to do explicit casting. For example:
Object a;Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.
7. What’s the difference between the methods sleep() and wait()
The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
8. Can you write a Java class that could be used both as an applet as well as an application?
Yes. Add a main() method to the applet.
9. What’s the difference between constructors and other methods?
Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.
10. Can you call one constructor from another if a class has multiple constructors
Yes. Use this() syntax.
11. Explain the usage of Java packages.
This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.
12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?
You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let’s say a class Employee belongs to a package com.xyz.hr; and is located in the file c:/dev/com.xyz.hr.Employee.java. In this case, you’d need to add c:/dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee
13. What’s the difference between J2SDK 1.5 and J2SDK 5.0?
There’s no difference, Sun Microsystems just re-branded this version.
14. What would you use to compare two String variables - the operator == or the method equals()?
I’d use the method equals() to compare the values of the Strings and the = = to check if two variables point at the same instance of a String object.
15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Yes, it does. The FileNoFoundException is inherited from the IOException. Exception’s subclasses have to be caught first.
16. Can an inner class declared inside of a method access local variables of this method?
It’s possible if these variables are final.
17. What can go wrong if you replace && with & in the following code:
String a=null;
if (a!=null && a.length()>10)
{...}
A single ampersand here would lead to a NullPointerException.
18. What’s the main difference between a Vector and an ArrayList
Java Vector class is internally synchronized and ArrayList is not.
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