Wednesday, August 17, 2011

Java Interview Notes Lesson 1

These notes below are aimed to be a quick reference to any one gearing for a Java Interview. I have broken the series in parts, and this is part one of the tutorial lesson.

Classes
  • A source code file can have only one public class.
  • If the source file contains a public class, the filename must match the public class name.
  • There can be more than one nonpublic class in a file.
  • Files with no public classes have no naming restrictions.
  • Classes can also be final or abstract, but not both at the same time.
  • Classes can have only public or default access.
    • A class with default access can be seen only by classes within the same package.
    • A class with public access can be seen by all classes from all packages

Local Variables
  •  Local variables can not have access modifiers, and final is the only modifier applicable for them.

Interface
  • Interface methods are by default public and abstract. Explicit declaration of these modifiers is optional. 
  • Interfaces can have variables declared which are by implicitly constants as they are always implicitly  public, static, and final.
Polymorphism
  • The reference variable's type (not the object's type), determines which methods can be called.
  • Object type (not the reference variable's type), determines which overridden method is used at runtime.

Some Pointer Notes:
  • Abstract class cannot be instantiated.
  • Final class cannot be subclassed.
  • Private instance variables and methods are not visible to subclass, so they cannot be inherited.
  • Final methods cannot be overridden in a subclass. 
  • Abstract classes have constructors that are called when a concrete subclass is instantiated.
  • Interfaces do not have constructors.

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