- What’s the difference between Transient and Volatile?
Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. These variables are not serialized. Transient instance fields are neither saved nor restored by the standard serialization. You have to handle restoring them yourself.
Volatile: Volatile modifier tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program. For example a Variable might be read from Cache and not update the content if it has been changed by another thread. Specifying a variable as volatile tells the JVM that any threads using that variable are not allowed to cache that value at all. Making the Variable Volatile will ensure that the compiler will get the content of the variable every time it is used and not cache its content. If not used Carefully this modifier might introduce bugs.
- Is JVM platform dependent?
Although java is platform independent but still JVM IS platform dependent one of the feature called byte code makes JVM platform dependent. Byte code is an intermediate machine code of compiled source code. The byte code can run on all machines, however the JVM must be installed in each machine.
- What are wrapper classes? Why do we need wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are Boolean, Byte, Character, Double, Float, Integer, Long, and Short.
We can create instances of these classes hence we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.
- What are Checked and Unchecked Exception?
A checked exception is generally known as Compiletime Exception. Checked exception forces client programmers to deal with the possibility that the exception will be thrown hence the programmer has to handle these types of exceptions. e.g., IOException thrown by java.io.FileInputStream’s read( ) method.
Where as Unchecked exceptions are Runtime Exception. 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. e.g., StringIndexOutOfBounds Exception thrown by String’s charAt () method· Checked exceptions must be caught at compile time but Runtime exceptions do not need to be.
- What is deadlock and how it can be avoid?
Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource, which is held by the other waiting thread. In Java, this resource is usually the object lock obtained by the synchronized keyword.
- What is the difference between interface and abstract class?
Difference between interface and abstract:
All the methods declared inside interface should be abstract, and there is no need to use the key word “abstract” for those method, but in case of abstract class at least one method should be abstract, most importantly u have to use the “abstract” key word for that method, besides that it may contain concrete methods. Abstract class must be extended abstract () methods generally contain only declaration part without definition part.
- Why are there no global variables in Java?
Global variables are considered bad form for a variety of reasons:
Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables).
State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state.
When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once.
- What are the steps in the JDBC connection?
For making a JDBC connection we need to go through the following steps:
Step 1: Register the database driver by using:
Class.forName (” driver class for that specific database”);
Step 2 : Create a database connection using:
Connection con = DriverManager.getConnection (url, username, password);
Step 3: Create a query using:
Statement stmt = Connection.Statement (”select * from TABLE NAME”);
Step 4: Exceute the query:
Stmt.exceuteUpdate ();
An enumeration is an interface which containing methods for accessing the underlying data structure. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.
- How Application is differ from Applet?
Applications:
Applications are Stand Alone and the doesn’t need web-browser.
Execution of Applications starts with main ().
Applets:
Needs no explicit installation on local machine. Can be transferred through Internet on to the local machine and may run as part of web-browser.
Execution Applets starts with init () method and it must run within a GUI, it may be AWT / Swing.
The Connected Device Configuration (CDC) is a specification for a J2ME configuration. Conceptually, CDC deals with devices with more memory and processing power than CLDC; it is for devices with an always-on network connection and a minimum of 2 MB of memory available for the Java system.
The MIDP defines a set of APIs for mobile devices, such as cell phones and low-end PDAs
- What is variable typing in javascript?
It is perfectly legal to assign a number to a variable and then assign a string to the same variable as follows
Example: i = 10;
i = “string”;
- What does a well-written java program look like?
A well-written java program exhibits recurring structures that promote abstraction, flexibility, modularity and elegance.
- Can a private method of a superclass be declared within a subclass?
Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.
- Why Java does not support multiple inheritence ?
Java DOES support multiple inheritance via interface implementation.
- What is the difference between final, finally and finalize?
- final - declare constant
- finally - handles exception
- finalize - helps in garbage collection
- Where and how can you use a private constructor?
Private constructor can be used if you do not want any other class to instanstiate the object , the instantiation is done from a static public method, this method is used when dealing with the factory method pattern when the designer wants only one controller (fatory method ) to create the object.
- In System.out.println(),what is System,out and println,pls explain?
System is a predefined final class,out is a PrintStream object and println is a built-in overloaded method in the out object.
- What is meant by "Abstract Interface"?
First, an interface is abstract. That means you cannot have any implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods.
- Can you make an instance of an abstract class? For example - java.util. Calendar is an abstract class with a method getInstance() which returns an instance of the Calendar class.
No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed. If you have an abstract class and you want to use a method which has been implemented, you may need to subclass that abstract class, instantiate your subclass and then call that method.
- What is the output of x<y? a:b = p*q when x=1,y=2,p=3,q=4?
When this kind of question has been asked, find the problems you think is necessary to ask before you give an answer. Ask if variables a and b have been declared or initialized. If the answer is yes. You can say that the syntax is wrong. If the statement is rewritten as: x<y? a:(b=p*q); the return value would be variable a because the x is 1 and less than y = 2; the x < y statement return true and variable a is returned.
- What is the difference between Swing and AWT components?
AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components 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.
- Why Java does not support pointers?
Because pointers are unsafe. Java uses reference types to hide pointers and programmers feel easier to deal with reference types without pointers. This is why Java and C# shine.
- Parsers? DOM vs SAX parser
parsers are fundamental xml components, a bridge between XML documents and applications that process that XML. The parser is responsible for handling xml syntax, checking the contents of the document against constraints established in a DTD or Schema.
A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000/XP, Linux, Solaris, and MacOS.
- What is the main difference between Java platform and other platforms?
The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms. The Java platform has two components:
1. The Java Virtual Machine (Java VM)
2. The Java Application Programming Interface (Java API)
- What is the Java Virtual Machine?
The Java Virtual Machine is a software that can be ported onto various hardware-based platforms.
The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets
The package is a Java namespace or part of Java libraries. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
The native code is code that after you compile it, the compiled code runs on a specific hardware platform.
- Is Java code slower than native code?
Not really. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.
- What is the serialization?
The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage.
- How to make a class or a bean serializable?
By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable
- 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.
- 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().
- 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.
- What is a transient variable?
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.
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- Which containers use a border layout as their default layout?
The Window, Frame and Dialog classes use a border layout as their default layout.
- 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.
- 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.
- What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to 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.
- 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.
- 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.
- What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to 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.
- 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 I/O, 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.
- 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.
- What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.
- Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
A thread is an independent path of execution in a system.
Multithreading means various threads that run in a system.
- How does multithreading 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.
- How to create multithread 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.
- 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
- Can each Java object keep track of all the threads that want to exclusively access to it?
Yes
- What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
- 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.
- 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.
- What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
- What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
- What is the List interface?
The List interface provides support for ordered collections of objects.
- 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
- What is the Vector class?
The Vector class provides the capability to implement a growable array of objects 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.
- 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.
- What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
- 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.
- 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.
The size of operator is not a keyword.
- What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
- 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
- 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.
- Name Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
A native method is a method that is implemented in a language other than Java.
- How can you write a loop indefinitely?
for(;;)--for loop; while(true)--always true, etc.
- Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a super class, but may not be declared to do both.
- 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.
- Which class is the super class for every class.
Object.
- 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.
- What is the Gregorian Calendar class?
The Gregorian Calendar provides support for traditional Western calendars.
- What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
- Which Container method is used to cause a container to be laid out and redisplayed?
validate()
- What is the Properties class?
The properties class is a subclass of Hash table 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.
- What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
- What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
- 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.
- 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.
- What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.
- 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 (an interface of a 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.
- 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.
- 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.
- 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 be cast to an interface reference when the object implements the referenced interface.
- 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.
- What do heavy weight components mean?
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.
- Which package has light weight components?
javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.
- What are peerless components?
The peerless components are called light weight components.
- 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.
- 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.
- 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.
- 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.
- 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 mehtod, 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.
- 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.
- What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
- Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.
- Name primitive Java types.
The primitive types are byte, char, short, int, long, float, double, and boolean.
- 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.
- 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.
- How are the elements of a GridBag Layout organized?
The elements of a GridBag Layout are organized according to a grid.
