Core Java Interview questions : : java.garnaik.com

CoreJava Interview Questions

Question : How you can force the garbage collection ?

Answer : Garbage collection automatic process and can't be forced. We can call garbage collector in Java by calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.


Question : What are the field/method access levels (specifiers) and class access levels ?

Answer : Each field and method has an access level:
private: accessible only in this class
(package): accessible only in this package
protected: accessible only in this package and in all subclasses of this class
public: accessible everywhere this class is available
Similarly, each class has one of two possible access levels:
(package): class objects can only be declared and manipulated by code in this package
public: class objects can be declared and manipulated by code in any package
For both fields and classes, package access is the default, and is used when no access is specified


Question : What are the static fields & static Methods ?

Answer : If a field or method defined as a static, there is only one copy for entire class, rather than one copy for each instance of class. static method cannot accecss non-static field or call non-static method

Example Java Code

static int counter = 0;

A public static field or method can be accessed from outside the class using either the usual notation:

Java-class-object.field-or-method-name

or using the class name instead of the name of the class object:

Java- class-name.field-or-method-name

Question : What are the Final fields & Final Methods ?

Answer : Fields and methods can also be declared final. A final method cannot be overridden in a subclass. A final field is like a constant: once it has been given a value, it cannot be assigned to again.

Java Code

private static final int MAXATTEMPTS = 10;

Question : Describe the wrapper classes in Java ?

Answer : Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive                       Wrapper
boolean                        java.lang.Boolean
byte                              java.lang.Byte
char                              java.lang.Character
double                          java.lang.Double
float                              java.lang.Float
int                                 java.lang.Integer
long                              java.lang.Long
short                             java.lang.Short
void                              java.lang.Void



Question : What are different types of inner classes ?

Answer : Inner classes nest within other classes. A normal class is a direct member of a package. Inner classes, which became available with Java 1.1, are four types

Static member classes
Member classes
Local classes
Anonymous classes
Static member classes - a static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class.

Member Classes - a member class is also defined as a member of a class. Unlike the static variety, the member class is instance specific and has access to any and all methods and members, even the parent's this reference.

Local Classes - Local Classes declared within a block of code and these classes are visible only within the block.

Anonymous Classes - These type of classes does not have any name and its like a local class

Java Anonymous Class Example

public class SomeGUI extends JFrame
{
... button member declarations ...

   protected void buildGUI()
   {
      button1 = new JButton();
      button2 = new JButton();
      ...

      button1.addActionListener(
         new java.awt.event.ActionListener()                               <------ Anonymous Class
         {
            public void actionPerformed(java.awt.event.ActionEvent e)
            {
               // do something
            }
         }
      );