Core Java - Interview Questions and Answers
1. What is the most important feature of Java?
Java is a platform independent language.
2. What do you mean by platform independence?
Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).
3. What is a JVM?
JVM is Java Virtual Machine which is a run time environment for the compiled java class files.
4. Are JVM's platform independent?
JVM's are not platform independent. JVM's are platform specific run time implementation provided by the vendor.
5. What is the difference between a JDK and a JVM?
JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.
6. What is a pointer and does Java support pointers?
Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn't support the usage of pointers.
7. What is the base class of all classes?
java.lang.Object
8. Does Java support multiple inheritance?
Java doesn't support multiple inheritance.
9. Is Java a pure object oriented language?
Java uses primitive data types and hence is not a pure object oriented language.
10. Are arrays primitive data types?
In Java, Arrays are objects.
11. What is difference between Path and Classpath?
Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.
12. What are local variables?
Local varaiables are those which are declared within a block of code like methods. Local variables should be initialised before accessing them.
13. What are instance variables?
Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.
14. How to define a constant variable in Java?
The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.
static final int PI = 2.14; is an example for constant.
static final int PI = 2.14; is an example for constant.
15. Should a main() method be compulsorily declared in all java classes?
No not required. main() method should be defined only if the source class is a java application.
16. What is the return type of the main() method?
Main() method doesn't return anything hence declared void.
17. Why is the main() method declared static?
main() method is called by the JVM even before the instantiation of the class hence it is declared as static.
18. What is the arguement of main() method?
main() method accepts an array of String object as arguement.
19. Can a main() method be overloaded?
Yes. You can have any number of main() methods with different method signature and implementation in the class.
20. Can a main() method be declared final?
Yes. Any inheriting class will not be able to have it's own default main() method.
21. Does the order of public and static declaration matter in main() method?
No. It doesn't matter but void should always come before main().
22. Can a source file contain more than one class declaration?
Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.
23. What is a package?
Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.
24. Which package is imported by default?
java.lang package is imported by default even without a package declaration.
25. Can a class declared as private be accessed outside it's package?
Not possible.
26. Can a class be declared as protected?
A class can't be declared as protected. only methods can be declared as protected.
27. What is the access scope of a protected method?
A protected method can be accessed by the classes within the same package or by the subclasses of the class in any package.
28. What is the purpose of declaring a variable as final?
A final variable's value can't be changed. final variables should be initialized before using them.
29. What is the impact of declaring a method as final?
A method declared as final can't be overridden. A sub-class can't have the same method signature with a different implementation.
30. I don't want my class to be inherited by any other class. What should i do?
You should declared your class as final. But you can't define your class as final, if it is an abstract class. A class declared as final can't be extended by any other class.
31. Can you give few examples of final classes defined in Java API?
java.lang.String, java.lang.Math are final classes.
32. How is final different from finally and finalize()?
final is a modifier which can be applied to a class or a method or a variable. final class can't be inherited,final method can't be overridden and final variable can't be changed.
finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.
finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.
33. Can a class be declared as static?
We can not declare top level class as static, but only inner class can be declared static.
public class Test
{
static class InnerClass
{
public static void InnerMethod()
{ System.out.println("Static Inner Class!"); }
}
public static void main(String args[])
{
Test.InnerClass.InnerMethod();
}
}
//output: Static Inner Class!
34. When will you define a method as static?
When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.
35. What are the restriction imposed on a static method or a static block of code?
A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.
36. I want to print "Hello" even before main() is executed. How will you acheive that?
Print the statement inside a static block of code. Static blocks get executed when the class gets loaded into the memory and even before the creation of an object. Hence it will be executed before the main() method. And it will be executed only once.
37. What is the importance of static variable?
static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.
38. Can we declare a static variable inside a method?
Static varaibles are class level variables and they can't be declared inside a method. If declared, the class will not compile.
39. What is an Abstract Class and what is it's purpose?
A Class which doesn't provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.
40. Can a abstract class be declared final?
Not possible. An abstract class without being inherited is of no use and hence will result in compile time error.
41. What is use of a abstract variable?
Variables can't be declared as abstract. only classes and methods can be declared as abstract.
42. Can you create an object of an abstract class?
Not possible. Abstract classes can't be instantiated.
43. Can a abstract class be defined without any abstract methods?
Yes it's possible. This is basically to avoid instance creation of the class.
44. Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?
No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.
45. Can a method inside a Interface be declared as final?
No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.
46. Can an Interface implement another Interface?
Intefaces doesn't provide implementation hence a interface cannot implement another interface.
47. Can an Interface extend another Interface?
Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.
48. Can a Class extend more than one Class?
Not possible. A Class can extend only one class but can implement any number of Interfaces.
49. Why is an Interface be able to extend more than one Interface but a Class can't extend more than one Class?
Basically Java doesn't allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn't have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.
50. Can an Interface be final?
Not possible. Doing so so will result in compilation error.
51. Can a class be defined inside an Interface?
Yes it's possible.
52. Can an Interface be defined inside a class?
Yes it's possible.
53. What is a Marker Interface?
An Interface which doesn't have any declaration inside but still enforces a mechanism.
54. Which object oriented Concept is achieved by using overloading and overriding?
Polymorphism.
55. Why does Java not support operator overloading?
Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.
56. Can we define private and protected modifiers for variables in interfaces?
No.
57. What is Externalizable?
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)
58. What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
59. What is a local, member and a class variable?
Variables declared within a method are "local" variables.
Variables declared within the class i.e not within any methods are "member" variables (global variables).
Variables declared within the class i.e not within any methods and are defined as "static" are class variables.
60. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
61. What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.
62. Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
63. What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.
64. What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
65. What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.
66. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.
67. Which class is extended by all other classes?
The Object class is extended by all other classes.
68. Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
69. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
70. What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
71. What is the return type of a program's main() method?
void.
72. If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.
73. What do you understand by private, protected and public?
These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.
74. What is Downcasting ?
Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy
75. What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
76. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits.
UTF-8 represents characters using 8, 16, and 18 bit patterns.
UTF-16 uses 16-bit and larger bit patterns.
77. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and comments).
78. What is a native method?
A native method is a method that is implemented in a language other than Java.
79. What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.
80. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
81. What is the range of the char type?
The range of the char type is 0 to 216 - 1 (i.e. 0 to 65535.)
82. What is the range of the short type?
The range of the short type is -(215) to 215 - 1. (i.e. -32,768 to 32,767)
83. Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain.
84. 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 likeSystem.out.println() work. out is a static field in the java.lang.System class.
85. Is null a keyword?
The null value is not a keyword.
86. Which characters may be used as the second character of an identifier, but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
87. Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
88. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.
89. If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.
90. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.
91. Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
92. What restrictions are placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
93. What is the difference between a while statement and a do while statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do whilestatement will always execute the body of a loop at least once.
94. What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
95. When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.
96. If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
97. What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.
98. Are true and false keywords?
The values true and false are not keywords.
99. What happens when you add a double value to a String?
The result is a String object.
100. What is the diffrence between inner class and nested class?
When a class is defined within a scope od another class, then it becomes inner class. If the access modifier of the inner class is static, then it becomes nested class.
101. Can an abstract class be final?
An abstract class may not be declared as final.
102. What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.
103. What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.
104. To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
105. What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.
106. What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.
107. What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
108. What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
109. What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.
110. What are the practical benefits, if any, of importing a specific class rather than an entire package (e.g. import java.net.* versus import java.net.Socket)?
It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use "Timer", I get an error while compiling (the class name is ambiguous between both packages). Let's say what you really wanted was thejavax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMapinstead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and otherjavax.swing classes without using fully qualified class names in.
111. Can a method be overloaded based on different return type but same argument type ?
No, because the methods can be called without using their return type in which case there is ambiquity for the compiler.
112. What happens to a static variable that is defined within a method of a class ?
Can't do it. You'll get a compilation error.
113. How many static initializers can you have ?
As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.
114. What is the difference between method overriding and overloading?
Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments
115. What is constructor chaining and how is it achieved in Java ?
A child object constructor always first needs to construct its parent (which in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement.
116. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
117. Which Java operator is right associative?
The = operator is right associative.
118. Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
119. What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies (switch, for, do, or while). Acontinue statement is used to end the current loop iteration and return control to the loop statement.
120. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following: for(;;);
121. To what value is a variable of the String type automatically initialized?
The default value of an String type is null.
122. What is the difference between a field variable and a local variable?
A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.
123. 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.
124. What does it mean that a class or member is final?
A final class cannot be inherited. A final method cannot be overridden in a subclass. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared.
125. What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or it also should be declared abstract.
126. What is a transient variable?
Transient variable is a variable that may not be serialized.
127. 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.
128. What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
129. Is sizeof a keyword?
The sizeof operator is not a keyword.
Java interview questions and answers
1. What is garbage collection? What is the process that is responsible for doing that in java? - Reclaiming the unused memory by the invalid objects. Garbage collector is responsible for this process
2. What kind of thread is the Garbage collector thread? - It is a daemon thread.
3. What is a daemon thread? - These are the threads which can run without user intervention. The JVM can exit when there are daemon thread by killing them abruptly.
4. How will you invoke any external process in Java? - Runtime.getRuntime().exec(….)
5. What is the finalize method do? - Before the invalid objects get garbage collected, the JVM give the user a chance to clean up some resources before it got garbage collected.
6. What is mutable object and immutable object? - If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float, …)
7. What is the basic difference between string and stringbuffer object? - String is an immutable object. StringBuffer is a mutable object.
8. What is the purpose of Void class? - The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the primitive Java type void.
9. What is reflection? - Reflection allows programmatic access to information about the fields, methods and constructors of loaded classes, and the use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions.
10. What is the base class for Error and Exception? - Throwable
11. What is the byte range? -128 to 127
12. What is the implementation of destroy method in java.. is it native or java code? - This method is not implemented.
13. What is a package? - To group set of classes into a single unit is known as packaging. Packages provides wide namespace ability.
14. What are the approaches that you will follow for making a program very efficient? - By avoiding too much of static methods avoiding the excessive and unnecessary use of synchronized methods Selection of related classes based on the application (meaning synchronized classes for multiuser and non-synchronized classes for single user) Usage of appropriate design patterns Using cache methodologies for remote invocations Avoiding creation of variables within a loop and lot more.
15. What is a DatabaseMetaData? - Comprehensive information about the database as a whole.
16. What is Locale? - A Locale object represents a specific geographical, political, or cultural region
17. How will you load a specific locale? - Using ResourceBundle.getBundle(…);
18. What is JIT and its use? - Really, just a very fast compiler… In this incarnation, pretty much a one-pass compiler — no offline computations. So you can’t look at the whole method, rank the expressions according to which ones are re-used the most, and then generate code. In theory terms, it’s an on-line problem.
19. Is JVM a compiler or an interpreter? - Interpreter
20. When you think about optimization, what is the best way to findout the time/memory consuming process? - Using profiler
21. What is the purpose of assert keyword used in JDK1.4.x? - In order to validate certain expressions. It effectively replaces the if block and automatically throws the AssertionError on failure. This keyword should be used for the critical arguments. Meaning, without that the method does nothing.
22. How will you get the platform dependent values like line separator, path separator, etc., ? - Using Sytem.getProperty(…) (line.separator, path.separator, …)
23. What is skeleton and stub? what is the purpose of those? - Stub is a client side representation of the server, which takes care of communicating with the remote server. Skeleton is the server side representation. But that is no more in use… it is deprecated long before in JDK.
24. What is the final keyword denotes? - final keyword denotes that it is the final implementation for that method or variable or class. You can’t override that method/variable/class any more.
25. What is the significance of ListIterator? - You can iterate back and forth.
26. What is the major difference between LinkedList and ArrayList? - LinkedList are meant for sequential accessing. ArrayList are meant for random accessing.
27. What is nested class? - If all the methods of a inner class is static then it is a nested class.
28. What is inner class? - If the methods of the inner class can only be accessed via the instance of the inner class, then it is called inner class.
29. What is composition? - Holding the reference of the other class within some other class is known as composition.
30. What is aggregation? - It is a special type of composition. If you expose all the methods of a composite class and route the method call to the composite method through its reference, then it is called aggregation.
31. What are the methods in Object? - clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString
32. Can you instantiate the Math class? - You can’t instantiate the math class. All the methods in this class are static. And the constructor is not public.
33. What is singleton? - It is one of the design pattern. This falls in the creational pattern of the design pattern. There will be only one instance for that entire JVM. You can achieve this by having the private constructor in the class. For eg., public class Singleton { private static final Singleton s = new Singleton(); private Singleton() { } public static Singleton getInstance() { return s; } // all non static methods … }
34. What is DriverManager? - The basic service to manage set of JDBC drivers.
35. What is Class.forName() does and how it is useful? - It loads the class into the ClassLoader. It returns the Class. Using that you can get the instance ( “class-instance”.newInstance() ).
36. Inq adds a question: Expain the reason for each keyword of
36.1.public static void main(String args[])
Basic Java interview questions
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
18. What is the use of preparedstatement? - Preparedstatements are precompiled statements. It is mainly used to speed up the process of inserting/updating/deleting especially when there is a bulk processing.
19. What is callable statement? Tell me the way to get the callable statement? - Callablestatements are used to invoke the stored procedures. You can obtain the callablestatement from Connection using the following methods prepareCall(String sql) prepareCall(String sql, int resultSetType, int resultSetConcurrency)
20. In a statement, I am executing a batch. What is the result of the execution? - It returns the int array. The array contains the affected row count in the corresponding index of the SQL.
21. Can a abstract method have the static qualifier? - No
22. What are the different types of qualifier and what is the default qualifier? - public, protected, private, package (default)
23. What is the super class of Hashtable? - Dictionary
24. What is a lightweight component? - Lightweight components are the one which doesn’t go with the native call to obtain the graphical units. They share their parent component graphical units to render them. Example, Swing components
25. What is a heavyweight component? - For every paint call, there will be a native call to get the graphical units. Example, AWT.
26. What is an applet? - Applet is a program which can get downloaded into a client environment and start executing there.
27. What do you mean by a Classloader? - Classloader is the one which loads the classes into the JVM.
28. What are the implicit packages that need not get imported into a class file? - java.lang
29. What is the difference between lightweight and heavyweight component? - Lightweight components reuses its parents graphical units. Heavyweight components goes with the native graphical unit for every component. Lightweight components are faster than the heavyweight components.
30. What are the ways in which you can instantiate a thread? - Using Thread class By implementing the Runnable interface and giving that handle to the Thread class.
31. What are the states of a thread? - 1. New 2. Runnable 3. Not Runnable 4. Dead
32. What is a socket? - A socket is an endpoint for communication between two machines.
33. How will you establish the connection between the servlet and an applet? - Using the URL, I will create the connection URL. Then by openConnection method of the URL, I will establish the connection, through which I can be able to exchange data.
34. What are the threads will start, when you start the java program? - Finalizer, Main, Reference Handler, Signal Dispatcher
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