Java Interview Questions

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  • What are synchronized methods and synchronized statements? Synchronized methods are methods that are used to control access to an object. For example, 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 different ways in which a thread can enter the waiting state? A thread can enter the waiting state by invoking its sleep() method, blocking on I/O, unsuccessfully attempting to acquire an object’s lock, or invoking an object’s wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
  • 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.
  • What’s new with the stop(), suspend() and resume() methods in new JDK 1.2? The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
  • 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. For example, setLayout(new FlowLayout()); will be set the layout as FlowLayout.
  • Which containers use a FlowLayout as their default layout? The Panel and Applet classes use the FlowLayout as their default layout.
  • What state does a thread enter when it terminates its processing? When a thread terminates its processing, it enters the dead state.
  • What is the Collections API? The Collections API is a set of classes and interfaces that support operations on collections of objects. One example of class in Collections API is Vector and Set and List are examples of interfaces in Collections API.
  • What is the List interface? The List interface provides support for ordered collections of objects. It may or may not allow duplicate elements but the elements must be ordered.
  • 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. The main visible advantage of this class is programmer needn’t to worry about the number of elements in the Vector.
  • 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, 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? Yielding means a thread returning to a ready state either from waiting, running or after creation, where as sleeping refers a thread going to a waiting state from running state. With reference to Java, when a task invokes its yield() method, it returns to the ready state and when a task invokes its sleep() method, it returns to the waiting state
  • What are wrapper classes? Wrapper classes are classes that allow primitive types to be accessed as objects. For example, Integer, Double. These classes contain many methods which can be used to manipulate basic data types
  • 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. The main purpose of Garbage Collector is recover the memory from the objects which are no longer required when more memory is needed.
  • Name Component subclasses that support painting? The following classes support painting: Canvas, Frame, Panel, and Applet.
  • What is a native method? A native method is a method that is implemented in a language other than Java. For example, one method may be written in C and can be called in Java.
  • How can you write a loop indefinitely?
  • for(;;) //for loop
    while(true); //always true

    1. 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.
    2. 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. For example, closing a opened file, closing a opened database Connection.
    3. What invokes a thread’s run() method? After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.
    4. What is the GregorianCalendar class? The GregorianCalendar provides support for traditional Western calendars.
    5. What is the SimpleTimeZone class? The SimpleTimeZone class provides support for a Gregorian calendar.
    6. 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.
    7. What is the purpose of the Runtime class? The purpose of the Runtime class is to provide access to the Java runtime system.
    8. What is the purpose of the System class? The purpose of the System class is to provide access to system resources.
    9. 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. For example,

    try
    {
    //some statements
    }
    catch
    {
    // statements when exception is cought
    }
    finally
    {
    //statements executed whether exception occurs or not
    }

    1. 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.
    2. 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.