SCJD certificate Notes

Since the sole purpose of this blog is to “preserve what is know”, this topic will be assigned for the pin points of SCJD certificate that I'm trying “fingers crossed” to take.

legal and illegal identifiers

Default Access

• Think of default access as package-level access, because a class with default access can be seen only by classes within the same package

Public Access

• A class declaration with the public keyword gives all classes
  from all packages access to the public class. In other words, all classes in the Java
  Universe (JU) have access to a public class

Other (Nonaccess) Class Modifiers

Final Classes

• When used in a class declaration, the final keyword means the class can't be subclassed

Abstract Classes

• An abstract class can never be instantiated. Its sole purpose, mission in life, raison d'être, is to be extended (subclassed)
• but always remember that if even a single method is abstract, the whole class must be declared abstract. One abstract method spoils the whole bunch. You can, however, put none-abstract methods in an abstract class.

Declaring an Interface

• When you create an interface, you're defining a contract for what a class can do, without saying anything about how the class will do it. An interface is a contract
• For Example
  you might want both a Ball and a Tire to have bounce behavior, but
  Ball and Tire don't share any inheritance relationship; Ball extends Toy while Tire
  extends only java.lang.Object. But by making both Ball and Tire implement
  Bounceable, you're saying that Ball and Tire can be treated as, "Things that
  can bounce,
  
• All interface methods are implicitly public and abstract. In other words,
  you do not need to actually type the public or abstract modifiers in the
  method declaration, but the method is still always public and abstract.
• All variables defined in an interface must be public, static, and final—in other words, interfaces can declare only constants, not instance variables.
• Interface methods must not be static.
• Because interface methods are abstract, they cannot be marked final, strictfp, or native. (More on these modifiers later.)
• An interface can extend one or more other interfaces.
• An interface cannot extend anything but another interface.
• An interface cannot implement another interface or class.
• An interface must be declared with the keyword interface.
• Interface types can be used polymorphically.
• You just need to know that both of these declarations are legal, and functionally identical:
  
  
• the following five method declarations, if declared within their own interfaces, are legal and identical!
  
  
  
• the following five method declarations, if declared within their own interfaces, are legal and identical!
  
  
  
• The following interface method declarations won't compile:
  
  
  

Declaring Interface Constants

You're allowed to put constants in an interface. By doing so, you guarantee that any class implementing the interface will have access to the same constant.They must always be



Look for interface definitions that define constants, but without explicitly using the required modifiers. For example, the following are all identical: