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.