How to define an Abstract class?

Java Interview Questions and Answers (part 1)

(Continued from previous question...)

14. 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();
}

(Continued on next question...)

Other Interview Questions

• What's Java?
• What does a well-written OO program look like?
• Can you have virtual functions in Java?
• Jack developed a program by using a Map container to hold key/value pairs. He wanted to make a change to the map. He decided to make a clone of the map in order to save the original data on side. What do you think of it? ?
• What is more advisable to create a thread, by implementing a Runnable interface or by extending Thread class?
• What is NullPointerException and how to handle it?
• An application needs to load a library before it starts to run, how to code??
• How could Java classes direct program messages to the system console, but error messages, say to a file?
• What's the difference between an interface and an abstract class?
• Name the containers which uses Border Layout as their default layout?
• What do you understand by Synchronization?
• What is Collection API?
• Is Iterator a Class or Interface? What is its use?
• What is similarities/difference between an Abstract class and Interface?
• How to define an Abstract class?
• How to define an Interface?
• If a class is located in a package, what do you need to change in the OS environment to be able to use it?
• How many methods in the Serializable interface?
• How many methods in the Externalizable interface?
• What is the difference between Serializalble and Externalizable interface?
• What is a transient variable?
• Which containers use a border layout as their default layout?
• How are Observer and Observable used?
• What is synchronization and why is it important?
• What are synchronized methods and synchronized statements?
• What are three ways in which a thread can enter the waiting state?
• Can a lock be acquired on a class?
• What's new with the stop(), suspend() and resume() methods in JDK 1.2?
• What is the preferred size of a component?
• What's the difference between J2SDK 1.5 and J2SDK 5.0?
• What would you use to compare two String variables - the operator == or the method equals()?
• What is thread?
• What is multi-threading?
• How does multi-threading take place on a computer with a single CPU?
• How to create a thread in a program?
• Can Java object be locked down for exclusive use by a given thread?
• Can each Java object keep track of all the threads that want to exclusively access to it?
• Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
• What invokes a thread's run() method?
• What is the purpose of the wait(), notify(), and notifyAll() methods?
• What are the high-level thread states?
• What is the difference between yielding and sleeping?
• What happens when a thread cannot acquire a lock on an object?
• What is the difference between Process and Thread?
• Can an inner class declared inside of a method access local variables of this method?
• What can go wrong if you replace &emp;&emp; with &emp; in the following code: String a=null; if (a!=null && a.length()>10) {...}
• What is the Vector class?
• What modifiers may be used with an inner class that is a member of an outer class?
• If a method is declared as protected, where may the method be accessed?
• What is an Iterator interface?
• How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
• What's the main difference between a Vector and an ArrayList?
• What are wrapped classes?
• Does garbage collection guarantee that a program will not run out of memory?
• What is the difference between preemptive scheduling and time slicing?
• Name Component subclasses that support painting?
• What is a native method?
• How can you write a loop indefinitely?
• Can an anonymous class be declared as implementing an interface and extending a class?
• What is the purpose of finalization?
• When should the method invokeLater()be used?
• How many methods in Object class?
• How does Java handle integer overflows and underflows?
• What is the numeric promotion?
• Is the numeric promotion available in other plantform?
• What is the difference between the Boolean & operator and the && operator?
• When is the ArithmeticException throwQuestion: What is the GregorianCalendar class?
• What is the SimpleTimeZone class?
• How can a subclass call a method or a constructor defined in a superclass?
• What is the Properties class?
• What is the purpose of the Runtime class?
• What is the purpose of the System class?
• What is the purpose of the finally clause of a try-catch-finally statement?
• What is the Locale class?
• What is an abstract method?
• What is the difference between interface and abstract class?
• What is a static method?
• What is a protected method?
• What is the difference between a static and a non-static inner class?
• What is an object's lock and which object's have locks?
• When can an object reference be cast to an interface reference?
• What is the difference between a Window and a Frame?
• What is the difference between a Window and a Frame?
• Which package has light weight components?
• What are peerless components?
• What is the difference between the Font and FontMetrics classes?
• What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
• What classes of exceptions may be caught by a catch clause?
• What is the difference between throw and throws keywords?
• If a class is declared without any access modifiers, where may the class be accessed?
• What is the Map interface?
• Does a class inherit the constructors of its superclass?
• Name primitive Java types.
• Which class should you use to obtain design information about an object?
• How can a GUI component handle its own events?
• How are the elements of a GridBagLayout organized?
• What advantage do Java's layout managers provide over traditional windowing systems?
• What are the problems faced by Java programmers who don't use layout managers?
• What is the difference between static and non-static variables?
• What is the difference between the paint() and repaint() methods?
• What is the purpose of the File class?
• Why would you use a synchronized block vs. synchronized method?
• What restrictions are placed on method overriding?
• What is casting?
• Explain the usage of the keyword transient?
• What class allows you to read objects directly from a stream?
• How are this() and super() used with constructors?
• 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?
• What is an IO filter?
• What is the Set interface?
• How can you force garbage collection?
• What is the purpose of the enableEvents() method?
• What is the difference between the File and RandomAccessFile classes?
• What interface must an object implement before it can be written to a stream as an object?
• What is the ResourceBundle class?
• How do you know if an explicit object casting is needed?
• What is a Java package and how is it used?
• What are the Object and Class classes used for?
• What is Serialization and deserialization?
• Explain the usage of Java packages.
• Does the code in finally block get executed if there is an exception and a return statement in a catch block?
• How do you restrict a user to cut and paste from the html page?
• Is Java a super set of JavaScript?
• What is a Container in a GUI?
• How the object oriented approach helps us keep complexity of software development under control?
• What is polymorphism?
• What is design by contract?
• What are use cases?
• What is scalability and performance?
• What is the benefit of subclass?
• How to add menushortcut to menu item?
• Where and how can you use a private constuctor.
• In System.out.println(),what is System,out and println,pls explain?
• Can you write a Java class that could be used both as an applet as well as an application?
• Can you make an instance of an abstract class? For example - java.util.Calender is an abstract class with a method getInstance() which returns an instance of the Calender class.
• What is the output of x > y? a:b = p*q when x=1,y=2,p=3,q=4?
• What is the difference between Swing and AWT components?
• Why Java does not support pointers?
• Parsers? DOM vs SAX parser
• Can you declare a class as private?
• What is the difference between shallow copy and deep copy?
• Can one create a method which gets a String and modifies it?
• Why is multiple inheritance not possible in Java?
• What's the difference between constructors and other methods?
• What is the relationship between synchronized and volatile keyword?
• This class (IncrementImpl) will be used by various threads concurrently; can you see the inherent flaw(s)? How would you improve it?
• What are the drawbacks of inheritance?
• Is there any other way that you can achieve inheritance in Java?
• Two methods have key words static synchronized and synchronized separately. What is the difference between them?
• How do you create a read-only collection?
• Can a private method of a superclass be declared within a subclass?
• Why Java does not support multiple inheritance ?
• What is the difference between final, finally and finalize?
• What kind of security tools are available in J2SE 5.0?
• How to make an array copy from System?
• Can we use System.arraycopy() method to copy the same array?
• What is shallow copy or shallow clone in array cloning?
• When is the ArrayStoreException thrown?
• How to check two arrays to see if contents have the same types and contain the same elements?
• Can you call one constructor from another if a class has multiple constructors?
• What are the different types of inner classes?
• In which case would you choose a static inner class?
• What is the differnce between final, finally and finalize?
• What is weak reference in Java
• What's the difference between the methods sleep() and wait()
• The following statement prints true or false, why?
• Why do we need to use getSystemResource() and getSystemResources() method to load resources?
• ArithmeticException?
• What is a transient variable?
• Which containers use a border Layout as their default layout?
• Why do threads block on I/O?
• Wha is the output from System.out.println("Hello"+null);?
• What is synchronization and why is it important?
• Can a lock be acquired on a class?
• What's new with the stop(), suspend() and resume() methods in JDK 1.2??
• Is null a keyword?
• What is the preferred size of a component?
• What method is used to specify a container's layout?
• Which containers use a FlowLayout as their default layout?
• What state does a thread enter when it terminates its processing?
• What is the Collections API?
• which characters may be used as the second character of an identifier, but not as the first character of an identifier?
• What is the List interface?
• How does Java handle integer overflows and underflows?
• What is the Vector class?
• What modifiers may be used with an inner class that is a member of an outer class?
• What is an Iterator interface?
• What is the difference between the >> and >>> operators?
• Which method of the Component class is used to set the position and size of a component?
• How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
• What is the difference between yielding and sleeping?