26. What are the types of ResultSet?
| Answer: |
The type of a ResultSet object determines the level of its functionality in two areas: the ways in which the cursor can be manipulated, and how concurrent changes made to the underlying data source are reflected by the ResultSet object. The sensitivity of a ResultSet object is determined by one of three different ResultSet types: TYPE_FORWARD_ONLY: The result set cannot be scrolled; its cursor moves forward only, from before the first row to after the last row. The rows contained in the result set depend on how the underlying database generates the results. That is, it contains the rows that satisfy the query at either the time the query is executed or as the rows are retrieved. TYPE_SCROLL_INSENSITIVE: The result can be scrolled; its cursor can move both forward and backward relative to the current position, and it can move to an absolute position. The result set is insensitive to changes made to the underlying data source while it is open. It contains the rows that satisfy the query at either the time the query is executed or as the rows are retrieved. TYPE_SCROLL_SENSITIVE: The result can be scrolled; its cursor can move both forward and backward relative to the current position, and it can move to an absolute position. The result set reflects changes made to the underlying data source while the result set remains open. |
27. What is difference between wait and sleep methods in java?
| Answer: |
sleep(): It is a static method on Thread class. It makes the current thread into the "Not Runnable" state for specified amount of time. During this time, the thread keeps the lock (monitors) it has acquired. wait(): It is a method on Object class. It makes the current thread into the "Not Runnable" state. Wait is called on a object, not a thread. Before calling wait() method, the object should be synchronized, means the object should be inside synchronized block. The call to wait() releases the acquired lock. |
28. What is servlet context? | ||
|
29. What happens if one of the members in a class does not implement Serializable interface?
| Answer: |
When you try to serialize an object which implements Serializable interface, incase if the object includes a reference of an non serializable object then NotSerializableException will be thrown. |
30. What is race condition?
| Answer: |
A race condition is a situation in which two or more threads or processes are reading or writing some shared data, and the final result depends on the timing of how the threads are scheduled. Race conditions can lead to unpredictable results and subtle program bugs. A thread can prevent this from happening by locking an object. When an object is locked by one thread and another thread tries to call a synchronized method on the same object, the second thread will block until the object is unlocked. |
31. How to get current time in milli seconds?
| Answer: |
System.currentTimeMillis() returns the current time in milliseconds. It is a static method, returns long type. |
32. How can you convert Map to List?
| Answer: |
We know that Map contains key-value pairs, whereas a list contains only objects. Since Entry class contains both key-value pair, Entry class will helps us to convert from Map (HashMap) to List (ArrayList). By using Map.entrySet() you will get Set object, which intern you can use it to convert to list object. |
| Code: |
public static void main(String a[]){
Map<String, String> wordMap = new HashMap<String, String>();
Set<Entry<String, String>> set = wordMap.entrySet();
List<Entry<String, String>> list = new ArrayList<Entry<String, String>>(set);
}
|
33. What is strictfp keyword?
| Answer: |
By using strictfp keyword, we can ensure that floating point operations take place precisely. |
34. What is System.out in Java?
| Answer: |
Here out is an instance of PrintStream. It is a static member variable in System class. This is called standard output stream, connected to console. |
35. What is difference between ServletOuptputStream and PrintWriter?
| Answer: |
ServletOutputStream: ServletResponse.getOutputStream() returns a ServletOutputStream suitable for writing binary data in the response. The servlet container does not encode the binary data, it sends the raw data as it is. PrintWriter: ServletResponse.getWriter() returns PrintWriter object which sends character text to the client. The PrintWriter uses the character encoding returned by getCharacterEncoding(). If the response's character encoding has not been specified then it does default character encoding. |
36. What is java static import?
| Answer: |
By using static imports, we can import the static members from a class
rather than the classes from a given package. For example, Thread class has
static sleep method, below example gives an idea:
import static java.lang.Thread;
public class MyStaticImportTest {
public static void main(String[] a) {
try{
sleep(100);
|
37.When to use String and StringBuffer?
| Answer: |
We know that String is immutable object. We can not change the value of a String object once it is initiated. If we try to change the value of the existing String object then it creates new object rather than changing the value of the existing object. So incase, we are going to do more modificatios on String, then use StringBuffer. StringBuffer updates the existing objects value, rather creating new object. |
38. What is difference between StringBuffer and StringBuilder?
| Answer: |
The only difference between StringBuffer and StringBuilder is StringBuffer is thread-safe, that is StringBuffer is synchronized. |
39. What is wrapper class in java?
| Answer: |
Everything in java is an object, except primitives. Primitives are int, short, long, boolean, etc. Since they are not objects, they cannot return as objects, and collection of objects. To support this, java provides wrapper classes to move primitives to objects. Some of the wrapper classes are Integer, Long, Boolean, etc. |
40. Is Iterator a Class?
| Answer: |
Iterator is an interface. It is not a class. It is used to iterate through each and every element in a list. Iterator is implemented Iterator design pattern. |
41. What is java classpath?
| Answer: |
The classpath is an environment variable. It is used to let the compiler know where the class files are available for import. |
42. Can a class in java be private?
| Answer: |
We can not declare top level class as private. Java allows only public and default modifier for top level classes in java. Inner classes can be private. |
43. What is the initial state of a thread when it is started?
| Answer: |
When the thread is createdn and started, initially it will be in the ready state. |
44. What is the super class for Exception and Error?
| Answer: |
The super class or base class for Exception and Error is Throwable. |
45. What is Class.forName()?
| Answer: |
Class.forName() loads the class into the ClassLoader. |
46. Can interface be final?
| Answer: |
No. We can not instantiate interfaces, so in order to make interfaces useful we must create subclasses. The final keyword makes a class unable to be extended. |
47. What is the difference between exception and error?
| Answer: |
An error is an irrecoverable condition occurring at runtime like out of memory error. These kind of jvm errors cannot be handled at runtime. Exceptions are because of condition failures, which can be handled easily at runtime. |
48. What is default value of a local variables?
| Answer: |
The local variables are not initialized to any default values. We should not use local variables with out initialization. Even the java compiler throws error. |
49. What is local class in java?
| Answer: |
In java, local classes can be defined in a block as in a method body or local block. |
50. Can we initialise uninitialized final variable?
| Answer: |
Yes. We can initialise blank final variable in constructor, only in construtor. The condition here is the final variable should be non-static. |
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