Java Interview Questions - 20

Java Interview Questions - 20

1.How many methods in the Serializable interface?

There is no method in the Serializable interface. The Serializable interface
acts as a marker, telling the object serialization tools that your class is serializable.

2.What comes to mind when someone mentions a shallow copy in Java?
Object cloning.
If you're overriding the method equals() of an object, which other method

you might also consider?

3.You are planning to do an indexed search in a list of objects. Which of

the two Java collections should you use: ArrayList or LinkedList?

4.What is Collection API ?
The Collection API is a set of classes and interfaces that support operation

on collections of objects. These classes and interfaces are more flexible,

more powerful, and more regular than the vectors, arrays, and hashtables

if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and

Example of interfaces: Collection, Set, List and Map.

5.Is Iterator a Class or Interface? What is its use?
Answer: Iterator is an interface which is used to step through the elements

of a Collection.

6.What is similarities/difference between an Abstract class and Interface?
Differences are as follows:
Interfaces provide a form of multiple inheritance. A class can extend only

one other class. Interfaces are limited to public methods and constants

with no implementation. Abstract classes can have a partial

implementation, protected parts, static methods, etc.
A Class may implement several interfaces. But in case of abstract class, a

class may extend only one abstract class. Interfaces are slow as it requires

extra indirection to to find corresponding method in in the actual class.

Abstract classes are fast.
Neither Abstract classes or Interface can be instantiated.

7.How many methods in the Externalizable interface?
There are two methods in the Externalizable interface. You have to

implement these two methods in order to make your class externalizable.

These two methods are readExternal() and writeExternal().

8.What is the difference between Serializalble and Externalizable interface?
When you use Serializable interface, your class is serialized automatically

by default. But you can override writeObject() and readObject() two

methods to control more complex object serailization process. When you

use Externalizable interface, you have a complete control over your class's

serialization process.

9.What is a transient variable in Java?
A transient variable is a variable that may not be serialized. If you don't

want some field to be serialized, you can mark that field transient or static.

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


11.How are 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


12.You can create an abstract class that contains only abstract methods.

On the other hand, you can create an interface that declares the same

methods. So can you use abstract classes instead of interfaces?
Sometimes. But your class may be a descendent of another class and in

this case the interface is your only option.

13.How to define an Abstract class?
A class containing abstract method is called Abstract class. An Abstract

class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
public abstract string anyAbstractFunction();

14.How to define an Interface in Java ?
In Java Interface defines the methods but does not implement them.

Interface can include constants. A class that implements the interfaces is

bound to implement all the methods defined in Interface.
Emaple of Interface:

public interface sampleInterface {
public void functionOne();

public long CONSTANT_ONE = 1000;

15.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; and is located in the file

c:\dev\com\xyz\hr\ 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:

16.What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default

behavior. An Interface can only declare constants and instance methods,

but cannot implement default behavior and all methods are implicitly

abstract. An interface has all public members and no implementation. An

abstract class is a class which may have the usual flavors of class members

(private, protected, etc.), but has some abstract methods.

17.What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that

are no longer needed by a program so that their resources can be

reclaimed and reused. A Java object is subject to garbage collection when

it becomes unreachable to the program in which it is used.

18.How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().

19.How can you minimize the need of garbage collection and make the

memory use more effective?
Use object pooling and weak object references.

20.There are two classes: A and B. The class B need to inform a class A

when some important event has happened. What Java technique would you

use to implement it?
If these classes are threads I'd consider notify() or notifyAll(). For regular

classes you can use the Observer interface.

21.What access level do you need to specify in the class declaration to

ensure that only classes from the same directory can access it?
You do not need to specify any access level, and Java will use a default

package access level.

22.What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.

23.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 doesnt allow). HashMap does not guarantee that

the order of the map will remain constant over time. HashMap is

unsynchronized and Hashtable is synchronized.

24.Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control

the access of multiple threads to shared resources. Without synchonization,

it is possible for one thread to modify a shared variable while another

thread is in the process of using or updating same shared variable. This

usually leads to significant errors.

25.Explain 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, 'cause

when you are going for multiple inheritance..the only interface can help.

26.What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing

the value. Passby Value means passing a copy of the value to be passed.

27.Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.

28.Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's

Class object.

29.What's new with the stop(), suspend() and resume() methods in JDK

The stop(), suspend() and resume() methods have been deprecated in JDK


30.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.

31.What's the difference between J2SDK 1.5 and J2SDK 5.0?
There's no difference, Sun Microsystems just re-branded this version.

32.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.

33.What is thread?
A thread is an independent path of execution in a system.

34.What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
How can a subclass call a method or a constructor defined in a superclass?
Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

35.How does multi-threading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

36.What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to a method or an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

37.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 causes dirty data and leads to significant errors.

38.Can Java object be locked down for exclusive use by a given thread?
Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.

39.Can each Java object keep track of all the threads that want to exclusively access to it?
Yes. Use Thread.currentThread() method to track the accessing thread.

40.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.

41.How to create a thread in a program?
You have two ways to do so. First, making your class "extends" Thread class. Second, making your class "implements" Runnable interface. Put jobs in a run() method and call start() method to start the thread.

42.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.

43.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.

44.What is the difference between Process and Thread?
A process can contain multiple threads. In most multithreading operating systems, a process gets its own memory address space; a thread doesn't. Threads typically share the heap belonging to their parent process. For instance, a JVM runs in a single process in the host O/S. Threads in the JVM share the heap belonging to that process; that's why several threads may access the same object. Typically, even though they share a common heap, threads have their own stack space. This is how one thread's invocation of a method is kept separate from another's. This is all a gross oversimplification, but it's accurate enough at a high level. Lots of details differ between operating systems. Process vs. Thread A program vs. similar to a sequential program an run on its own vs. Cannot run on its own Unit of allocation vs. Unit of execution Have its own memory space vs. Share with others Each process has one or more threads vs. Each thread belongs to one process Expensive, need to context switch vs. Cheap, can use process memory and may not need to context switch More secure. One process cannot corrupt another process vs. Less secure. A thread can write the memory used by another thread

45.Can an inner class declared inside of a method access local variables of this method?
It's possible if these variables are final.

46.What can go wrong if you replace &emp;&emp; with &emp; in the following code: String a=null; if (a!=null && a.length()>10) {...}
A single ampersand here would lead to a NullPointerException.

47.What is the Vector class?
The Vector class provides the capability to implement a growable array of objects

48.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.

49.What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

50.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.

51.What's the main difference between a Vector and an ArrayList?
Java Vector class is internally synchronized and ArrayList is not.

52.What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.

53.Does garbage collection guarantee that a program will not run out of memory?
No, it doesn't. 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.

54.What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

55.Name Component subclasses that support painting ?
The Canvas, Frame, Panel, and Applet classes support painting.

56.What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.

57.What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.

58.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.

59.What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.

60.What is a native method?
A native method is a method that is implemented in a language other than Java.

61.What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

62.When should the method invokeLater()be used?
This method is used to ensure that Swing components are updated through the event-dispatching thread.

63.How many methods in Object class?
This question is not asked to test your memory. It tests you how well you know Java. Ten in total.
equals() & hashcode()
wait() & notify()

64.How can you write a loop indefinitely?
for(;;)--for loop; while(true)--always true, etc.

65.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.

66.How does Java handle integer overflows and underflows?
It uses low order bytes of the result that can fit into the size of the type allowed by the operation.
What is the numeric promotion?
Numeric promotion is used with both unary and binary bitwise operators. This means that byte, char, and short values are converted to int values before a bitwise operator is applied.
If a binary bitwise operator has one long operand, the other operand is converted to a long value.
The type of the result of a bitwise operation is the type to which the operands have been promoted. For example:
short a = 5;
byte b = 10;
long c = 15;
The type of the result of (a+b) is int, not short or byte. The type of the result of (a+c) or (b+c) is long.

67.Is the numeric promotion available in other platform?
Yes. Because Java is implemented using a platform-independent virtual machine, bitwise operations always yield the same result, even when run on machines that use radically different CPUs.
What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.

68.What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.

69.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.
Operator & has no chance to skip both sides evaluation and && operator does. If asked why, give details as above.

70.When is the ArithmeticException throwQuestion: What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.

71.What is multi-threading?
Multi-threading means various threads that run in a system.

72.What invokes a thread's run() method?
After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

73.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.
When can an object reference be cast to an interface reference?
An object reference can be cast to an interface reference when the object implements the referenced interface.

74.What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.

75.What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.

76.What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

77.Which package has light weight components?
javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.

78.What are peerless components?
The peerless components are called light weight components.

79.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

80.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.

81.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.

82.What is the difference between throw and throws keywords?
The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as argument. The exception will be caught by an immediately encompassing try-catch construction or propagated further up the calling hierarchy.
The throws keyword is a modifier of a method that designates that exceptions may come out of the method, either by virtue of the method throwing the exception itself or because it fails to catch such exceptions that a method it calls may throw.

83.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.

84.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.

85.What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass. Or, a method that has no implementation.

86.What is the difference between interface and abstract class?
interface contains methods that must be abstract; abstract class may contain concrete methods. interface contains variables that must be static and final; abstract class may contain non-final and final variables. members in an interface are public by default, abstract class may contain non-public members. interface is used to "implements"; whereas abstract class is used to "extends". interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance. interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces. interface is absolutely abstract; abstract class can be invoked if a main() exists. interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces. If given a choice, use interface instead of abstract class.

87.What is a static method?
A static method is a method that belongs to the class rather than any object of the class and doesn't apply to an object or even require that any objects of the class have been instantiated.

88.What is a protected method?
A protected method is a method that can be accessed by any method in its package and inherited by any subclass of its class.

89.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.

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

91.What is the difference between a Window and a Frame?
Heavy weight components like Abstract Window Toolkit (AWT), depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button. In this relationship, the Motif button is called the peer to the java.awt.Button. If you create two Buttons, two peers and hence two Motif Buttons are also created. The Java platform communicates with the Motif Buttons using the Java Native Interface. For each and every component added to the application, there is an additional overhead tied to the local windowing system, which is why these components are called heavy weight.

92.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 or friendly access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

93.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.

94.How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.

95.How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

96.What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.

97.What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.

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

99.Name primitive Java types.
The primitive types are byte, char, short, int, long, float, double, and Boolean.

100.What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.

101.Why would you use a synchronized block vs. synchronized method?
Synchronized blocks place locks for shorter periods than synchronized methods.

102.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.

103.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.

104.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).

105.What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.

106.What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.

107.What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

108.What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

109.What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

110.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.

111.What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

112.What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

113.What interface must an object implement before it can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

114.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.

115.How is it possible for two String objects with identical values not to be equal under the == operator? How are this() and super() used with constructors?
The == operator compares two objects to determine if they are the same objects in memory. It is possible for two String objects to have the same value, but located in different areas of memory.
What is an IO filter?
An IO filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

116.What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

117.How do you restrict a user to cut and paste from the html page?

Using Servlet or client side scripts to lock keyboard keys. It is one of solutions.

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