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.
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:
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.
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.
Yes. the main() method is a special method for a program entry. You can overload main() method in any ways. But if you change the signature of the main method, the entry point for the program will be gone. For example:
//the following are legal methods for entry point public static void main(String[] args) {} // standard static public void main(String[] args) {} // unconventional private static void main(String[] args) {} // unconventional //the following are legal overloaded methods public static int main(String arg) {} public void main(String str1, string str2) {} public String main(String arg, int i) {} ....
For a good practice, don't overload main method or use a similar main method in a class which is not served as an entry point for your program.
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.
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.
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.
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().
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.
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.
The Window, Frame and Dialog classes use a border layout as their default layout.
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.
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.
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. For example:
//a typical synchronized method public synchronized void deposit(Amount amt) { .... } // a typical synchronized statement synchronized ( objReference ) { ..... }
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.
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
The preferred size of a component is the minimum component size that will allow the component to display normally.
The setLayout() method is used to specify a container's 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.
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.
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.
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.
Yes.
When a thread terminates its processing, it enters the dead state.
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.
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.
The high-level thread states are ready, running, waiting, and dead.
The Collections API is a set of classes and interfaces that support operations on collections of objects.
The List interface provides support for ordered collections of objects.
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
The Vector class provides the capability to implement a growable array of objects
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
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.
The Iterator interface is used to step through the elements of a Collection.
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.
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 sizeof operator is not a keyword in Java.
Wrapped classes are classes that allow primitive types to be accessed as objects.
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
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.
The Canvas, Frame, Panel, and Applet classes support painting.
A native method is a method that is implemented in a language other than Java.
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
Object
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.
The GregorianCalendar provides support for traditional Western calendars.
The SimpleTimeZone class provides support for a Gregorian calendar.
validate()
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.
The purpose of the Runtime class is to provide access to the Java runtime system.
The purpose of the System class is to provide access to system resources.
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
It must provide all of the methods in the interface and identify the interface in its implements clause.
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.
A protected method is a method that can be accessed by any method in its package and inherited by any subclass of its 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.
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.
An object reference be cast to an interface reference when the object implements the referenced interface.
The Frame class extends Window to define a main application window that can have a menu bar.
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.
javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.
The peerless components are called light weight components.
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font 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.
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.
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.
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.
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
A class does not inherit constructors from any of its superclasses.
The primitive types are byte, char, short, int, long, float, double, and boolean.
The Class class is used to obtain information about an object's design.
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.
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.
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.
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.
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.
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.
The File class is used to create objects that provide access to the files and directories of a local file system.
Two methods may not have the same name and argument list but different return types.
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.
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.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
The ObjectInputStream class supports the reading of objects from input streams.
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.
An I/O 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.
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
The List interface provides support for ordered collections of objects.
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.
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.
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
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.
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
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.
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.
Serialization is the process of writing the state of an object to a byte stream.
Deserialization is the process of restoring these objects.
Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data.
If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block.
Using javaScript to lock keyboard keys. It is one of solutions.
No. They are completely different. Some syntax may be similar.
A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container.
We can discuss such issue from the following aspects:
Polymorphism means "having many forms". It allows methods (may be variables) to be written that needn't be concerned about the specifics of the objects they will be applied to. That is, the method can be specified at a higher level of abstraction and can be counted on to work even on objects of un-conceived classes.
The design by contract specifies the obligations of a method to any other methods that may use its services and also theirs to it. For example, the preconditions specify what the method required to be true when the method is called. Hence making sure that preconditions are. Similarly, postconditions specify what must be true when the method is finished, thus the called method has the responsibility of satisfying the post conditions.
In Java, the exception handling facilities support the use of design by contract, especially in the case of checked exceptions. The assert keyword can be used to make such contracts.
A use case describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance. It is part of the analysis of a program. The collection of use cases should, ideally, anticipate all the standard circumstances and many of the extraordinary circumstances possible so that the program will be robust.
Performance is a measure of "how fast can you perform this task." and scalability describes how an application behaves as its workload and available computing resources increase.
Generally:
If you have a button instance called aboutButton, you may add menu short cut by calling aboutButton.setMnemonic('A'), so the user may be able to use Alt+A to click the button.
Private constructor can be used if you do not want any other class to instanstiate it by using new. The instantiation is done from a static public method, which is used when dealing with the factory method pattern.
System is a predefined final class,out is a PrintStream object acting as a field member and println is a built-in overloaded method in the out object.
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.
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.
When this kind of question has been asked, find the problems you think is necessary to ask back 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.
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.
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 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.
DOM | SAX |
1. Tree of nodes | 1. Sequence of events |
2. Memory: Occupies more memory, preffered for small XML documents | 2. Doesn't use any memory preferred for large documents |
3. Slower at runtime | 3. Faster at runtime |
4. Stored as objects | 4. Objects are to be created |
5. Programmatically easy, since objects | 5. Need to write code for creating objects are to reffered |
6. Ease of navigation | 6. Backward navigation is not possible as it sequentially processes the document |
This question is not asked to test your memory. It tests you how well you know Java. Ten in total.