150 Top Advanced Java Job Interview Questions And Answers 2019

Advanced Java Interview Questions with Answers:-

1. Define what is a transient variable?
A transient variable is a variable that may not be serialized.

2. Which containers use a border Layout as their default layout?
The Window, Frame and Dialog classes use a border layout as their default layout.

3. Why do threads block on I/O?
Threads block on I/O (that enters the waiting state) so that other threads may execute while the I/O Operation is performed.

4. How are the Observer and Observable used?
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.

5. Is null a keyword?
The null is not a keyword.

6. Define what is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.

7. Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.

8. Define what state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.

9. Define what is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.

10. 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.

11. Define what is the List interface?
The List interface provides support for ordered collections of objects.

12. 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.

13. Define what is the Vector class?
The Vector class provides the capability to implement a growable array of objects

14. Define 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.

15. Define what is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.

16. Define 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.

17. Which method of the Component class is used to set the position and size of a component?
setBounds() method is used to set the position and size of a component.

18. Define what is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

19. Which javutil classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.

20. Is the size of a keyword?
The size of the operator is not a keyword.

21. Define what is wrapped, classes?
Wrapper classes are classes that allow primitive types to be accessed as objects.

22. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

23. Define 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).

24. Define what is the immediate superclass of the Applet class?
Panel.

25. Define what is final?
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 the value of a final variable (is a constant).

26. Define what if the main method is declared as private?
The program compiles properly but at runtime, it will give “Main method not public.” message.

27. Define what if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error “NoSuchMethodError”.

28. Define what if I write static public void instead of a public static void?
The program compiles and runs properly.

29. Define what if I do not provide the String array as the argument to the method?
The program compiles but throws a runtime error “NoSuchMethodError”.

30. Define what is the first argument of the String array in the main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

31. If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
It is empty. But not null.

32. How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

33. Define what environment variables do I need to set on my machine in order to be able to run Java programs?
CLASSPATH and PATH are the two variables.

34. Can an application have multiple classes having the main method?
Yes, it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is no conflict amongst the multiple classes having the main method.

35. Can I have multiple main methods in the same class?
No, the program fails to compile. The compiler says that the main method is already defined in the class.

36. Do I need to import java?lang package any time? Why?
No. It is by default loaded internally by the JVM.

37. Can I import the same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

38. Define what are Checked and UnChecked Exception?
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 has 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.

39. Define what is Overriding?
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 a superclass. Methods may be overridden to be more public, not more private.

40. Define what are different types of inner classes?
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. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables. There can also be inner interfaces. All of these are of the nested top-level variety. Member classes – Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes – Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface. Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes – Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

41. Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?

  • Yes, the imports are checked for the semantic validity at compile time. The code containing the above line of import will not compile.
  • It will throw an error saying, cannot resolve symbol: class ABCD location: package to import java.io.ABCD;

42. Does importing a package imports the subpackages as well? e.g. Does import com.MyTest.* also import com.MyTest.UnitTests.*?
No, you will have to import the sub packages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of its sub packages.

43. Define what is the difference between declaring a variable and defining a variable?
In a declaration, we just mention the type of the variable and its name. We do not initialize it. But defining means declaration + initialization.

e.g String s; is just a declaration while String s = new String (“abcd”), Or String s = “abcd”; are both definitions.

44. Define what is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.

45. Can a top level class be private or protected? A: No. A top-level class.s cannot be private or protected. It can have either “public” or no modifier. If it
does not have a modifier it is supposed to have a default access. If a top-level class is declared as private the compiler will complain that the “modifier private is not allowed here”. This means that a top-level class cannot be private. Same is the case with protected.

46. Define what type of parameter passing does Java support?
In Java, the arguments are always passed by value.

47. Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.

48. Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.

49. Define what is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

50. How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a file output stream. This will save the object to a file.

51. Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

52. How can I customize the serialization process? i.e. how can one have control over the serialization process?

  • Yes, it is possible to have control over the serialization process. The class should implement the Externalizable interface.
  • This interface contains two methods namely readExternal and writeExternal.
  • You should implement these methods and write the logic for customizing the serialization process.

53. Define what is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized. Moreover, if the state of an object is to be saved, objects need to be serialized.

54. Define what is Externalizable interface?

  1. Externalizable is an interface which contains two methods readExternal and writeExternal.
  2. These methods give you control over the serialization mechanism.
  3. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

55. When you serialize an object, Define what happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized along with the original object.

56. Define what one should take care of while serializing the object?

One should make sure that all the included objects are also serializable. If any of the objects are not serializable then it throws a NotSerializableException.

57. Define what happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesn’t necessarily read and write to the stream. These are 1. Serialization ignores static fields because they are not part of ay particular state. 2. Base class fields are only handled if the base class itself is serializable. 3. Transient fields.

58. How does an exception permeate through the code?
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. The 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.

59. Define what are the different ways to handle exceptions?
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 handle those exceptions.

60. Define what is the basic difference between the 2 approaches to exception handling.
1> try-catch block and 2> specifying the candidate exceptions in the throws clause?

61. When should you use which approach?
In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in the best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with its own exceptions, then do not use this approach. In this case, use the second approach. In the second approach, we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

62. Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a final block. And Define whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

63. If I write return at the end of the try block, will the finally block still execute?
Yes even if you write return as the last statement in the try block and no exception occurs, the final block will execute. The final block will execute and then the control return.

64. If I write System. exit (0); at the end of the try block, will the finally block still execute?
No, in this case, the final block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

65. How are the Observer and Observable used?
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.

66. Define what is synchronization and why is it important?
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.

67. 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.

68. Define what are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

69. How are Java source code files named?
If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.

70. Define 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.

71. Can a Byte object be cast to a double value?
No. An object cannot be cast to a primitive value.

72. Define 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 classes outer class. A static inner class does not have any object instances.

73. Define what is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not constants.

74. 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.

75. Define what is an objects lock and which objects 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 classes lock is acquired on the class’s Class object.

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76. Define what is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.

77. How are the elements of a BorderLayout organized?
The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.

78. Define 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.

79. Explain the different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, Because when you are going for multiple inheritances. The only interface can help.

80. Difference between Swing and AWT?
AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT.

81. Difference between HashMap and HashTable?
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 doesn’t allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

82. Define what is HashMap and Map?
A map is Interface and Hashmap is a class that implements that.

83. Define what is final?
A final class cant is extended ie., final class may not be subclassed. A final method cant is overridden when its class is inherited. You cant change value of a final variable (is a constant).

84. Define what if the main method is declared as private?
The program compiles properly but at runtime, it will give “Main method not public.” message.

85. Define what if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error “NoSuchMethodError”.

86. Define what if I write static public void instead of a public static void?
Program compiles and runs properly.

87. Define what if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error “NoSuchMethodError”.

88. Can I have multiple main methods in the same class?
No, the program fails to compile. The compiler says that the main method is already defined in the class.

89. Do I need to import javlang package any time? Why?
No. It is by default loaded internally by the JVM.

90. Can I import the same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

91. Define what is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.

92. When can an object reference be cast to an interface reference?
An object reference is cast to an interface reference when the object implements the referenced interface.

93. Define what is the difference between a Window and a Frame?
The Frame class extends Window to define the main application window that can have a menu bar.

94. Which class is extended by all other classes?
The Object class is extended by all other classes.

95. Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected.

96. Is the ternary operator written x: y? z or x? y: z?
It is written x? y: z.

97. Define what is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.

98. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.

99. Define what happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object’s lock, it enters the waiting state until the lock becomes available.

100. Define what is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

101. Define what classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

102. 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.

103. Define what is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.

104. Define what is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used to associate keys with values.

105. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.

106. For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.

107. Define what is the purpose of the System class?
The purpose of the System class is to provide access to system resources.

108. Which TextComponent method is used to set a TextComponent to the read-only state?
setEditable().

109. How are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.

110. Is &&= a valid Java operator?
No. It is not a valid java operator.

111. Name the eight primitive Java types.
The eight primitive types are a byte, char, short, int, long, float, double, and boolean.

112. Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object’s design.

113. Define what is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

114. Is “ABC” a primitive value?
The String literal “ABC” is not a primitive value. It is a String object.

115. Define what is the relationship between an event-listener interface and an event-adapter class?
An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.

116. Define 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.

117. Define what modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.

118. Is a class a subclass of itself?
A class is a subclass of itself.

119. Define what is the highest-level event class of the event-delegation model?
The java util. An EventObject class is a highest-level class in the event-delegation class hierarchy.

120. Define what event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.

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