What are wrapper classes?

Java Interview Questions and Answers (part 2)

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191. What are wrapper classes?

Wrapper classes are classes that allow primitive types to be accessed as objects.

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Other Interview Questions

• Which java.util classes and interfaces support event handling?
• Is sizeof a keyword?
• What are wrapper classes?
• Does garbage collection guarantee that a program will not run out of memory?
• What restrictions are placed on the location of a package statement within a source code file?
• Can an object's finalize() method be invoked while it is reachable?
• What is the immediate superclass of the Applet class?
• What is the difference between preemptive scheduling and time slicing?
• Name three Component subclasses that support painting.
• What value does readLine() return when it has reached the end of a file?
• What is the immediate superclass of the Dialog class?
• What is clipping?
• What is a native method?
• Can a for statement loop indefinitely?
• What are order of precedence and associativity, and how are they used?
• When a thread blocks on I/O, what state does it enter?
• To what value is a variable of the String type automatically initialized?
• What is the catch or declare rule for method declarations?
• What is the difference between a MenuItem and a CheckboxMenuItem?
• What is a task's priority and how is it used in scheduling?
• What class is the top of the AWT event hierarchy?
• When a thread is created and started, what is its initial state?
• Can an anonymous class be declared as implementing an interface and extending a class?
• What is the range of the short type?
• What is the range of the char type?
• In which package are most of the AWT events that support the event-delegation model defined?
• What is the immediate superclass of Menu?
• What is the purpose of finalization?
• Which class is the immediate superclass of the MenuComponent class.
• What invokes a thread's run() method?
• What is the difference between the Boolean & operator and the && operator?
• Name three subclasses of the Component class.
• What is the GregorianCalendar class?
• Which Container method is used to cause a container to be laid out and redisplayed?
• What is the purpose of the Runtime class?
• How many times may an object's finalize() method be invoked by the garbage collector?
• What is the purpose of the finally clause of a try-catch-finally statement? garbage collector?
• What is the argument type of a program's main() method?
• Which Java operator is right associative?
• What is the Locale class?
• Can a double value be cast to a byte?
• What is the difference between a break statement and a continue statement?
• What must a class do to implement an interface?
• What method is invoked to cause an object to begin executing as a separate thread?
• Name two subclasses of the TextComponent class.
• What is the advantage of the event-delegation model over the earlier event-inheritance model?
• Which containers may have a MenuBar?
• How are commas used in the initialization and iteration parts of a for statement?
• What is the purpose of the wait(), notify(), and notifyAll() methods?
• What is an abstract method?
• How are Java source code files named?
• What is the relationship between the Canvas class and the Graphics class?
• What are the high-level thread states?
• What value does read() return when it has reached the end of a file?
• Can a Byte object be cast to a double value?
• What is the difference between a static and a non-static inner class?
• What is the difference between the String and StringBuffer classes?
• If a variable is declared as private, where may the variable be accessed?
• What is an object's lock and which objects have locks?
• What is the Dictionary class?
• How are the elements of a BorderLayout organized?
• What is the % operator?
• When can an object reference be cast to an interface reference?
• What is the difference between a Window and a Frame?
• Which class is extended by all other classes?
• Can an object be garbage collected while it is still reachable?
• Is the ternary operator written x : y ? z or x ? y : z ?
• What is the difference between the Font and FontMetrics classes?
• How is rounding performed under integer division?
• What happens when a thread cannot acquire a lock on an object?
• 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?
• If a class is declared without any access modifiers, where may the class be accessed?
• What is the SimpleTimeZone class?
• What is the Map interface?
• Does a class inherit the constructors of its superclass?
• For which statements does it make sense to use a label?
• What is the purpose of the System class?
• Which TextComponent method is used to set a TextComponent to the read-only state?
• How are the elements of a CardLayout organized?
• Is &&= a valid Java operator?
• Name the eight primitive Java types.
• Which class should you use to obtain design information about an object?
• What is the relationship between clipping and repainting?
• Is "abc" a primitive value?
• What is the relationship between an event-listener interface and an event-adapter class?
• What restrictions are placed on the values of each case of a switch statement?
• What modifiers may be used with an interface declaration?
• Is a class a subclass of itself?
• What is the highest-level event class of the event-delegation model?
• What event results from the clicking of a button?
• How can a GUI component handle its own events?
• What is the difference between a while statement and a do statement?
• How are the elements of a GridBagLayout organized?
• What advantage do Java's layout managers provide over traditional windowing systems?
• What is the Collection interface?
• What modifiers can be used with a local inner class?
• 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?
• Can an exception be rethrown?
• Which Math method is used to calculate the absolute value of a number?
• How does multithreading take place on a computer with a single CPU?
• When does the compiler supply a default constructor for a class?
• When is the finally clause of a try-catch-finally statement executed?
• Which class is the immediate superclass of the Container class?
• If a method is declared as protected, where may the method be accessed?
• How can the Checkbox class be used to create a radio button?
• Which non-Unicode letter characters may be used as the first character of an identifier?
• What restrictions are placed on method overloading?
• What happens when you invoke a thread's interrupt method while it is sleeping or waiting?
• What is casting?
• What is the return type of a program's main() method?
• Name four Container classes.
• What is the difference between a Choice and a List?
• What class of exceptions are generated by the Java run-time system?
• What class allows you to read objects directly from a stream?
• What is the difference between a field variable and a local variable?
• Under what conditions is an object's finalize() method invoked by the garbage collector?
• How are this () and super () used with constructors?
• What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution?
• What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?
• How is it possible for two String objects with identical values not to be equal under the == operator?
• Why are the methods of the Math class static?
• What Checkbox method allows you to tell if a Checkbox is checked?
• What state is a thread in when it is executing?
• What are the legal operands of the instanceof operator?
• How are the elements of a GridLayout organized?
• What an I/O filter?
• If an object is garbage collected, can it become reachable again?
• What is the Set interface?
• What classes of exceptions may be thrown by a throw statement?
• What are E and PI?
• Are true and false keywords?
• What is a void return type?
• What is the purpose of the enableEvents() method?
• What is the difference between the File and RandomAccessFile classes?
• What happens when you add a double value to a String?
• What is your platform's default character encoding?
• Which package is always imported by default?
• What interface must an object implement before it can be written to a stream as an object?
• How are this and super used?
• What is the purpose of garbage collection?
• What is a compilation unit?
• What interface is extended by AWT event listeners?
• What restrictions are placed on method overriding?
• How can a dead thread be restarted?
• What happens if an exception is not caught?
• What is a layout manager?
• Which arithmetic operations can result in the throwing of an ArithmeticException?
• What are three ways in which a thread can enter the waiting state?
• Can an abstract class be final?
• What is the ResourceBundle class?
• What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
• What is numeric promotion?
• What is the difference between a Scrollbar and a ScrollPane?
• What is the difference between a public and a non-public class?
• To what value is a variable of the boolean type automatically initialized?
• Can try statements be nested?
• What is the difference between the prefix and postfix forms of the ++ operator?
• What is the purpose of a statement block?
• What is a Java package and how is it used?
• What modifiers may be used with a top-level class?
• What are the Object and Class classes used for?
• How does a try statement determine which catch clause should be used to handle an exception?
• Can an unreachable object become reachable again?
• When is an object subject to garbage collection?
• What method must be implemented by all threads?
• What methods are used to get and set the text label displayed by a Button object?
• Which Component subclass is used for drawing and painting?
• What are the two basic ways in which classes that can be run as threads may be defined?
• What are the problems faced by Java programmers who don't use layout managers?
• What is the difference between an if statement and a switch statement?
• Can there be an abstract class with no abstract methods in it?
• Can an Interface be final?
• Can an Interface have an inner class?
• Can we define private and protected modifiers for variables in interfaces?
• What is Externalizable?
• What modifiers are allowed for methods in an Interface?
• What is a local, member and a class variable?
• I made my class Cloneable but I still get 'Can't access protected method clone. Why?
• What are the different identifier states of a Thread?
• What are some alternatives to inheritance?