How are Observer and Observable used?

Java Interview Questions and Answers (part 1)

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22. 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 objects.

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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?