Java SE: Programming Complete - 6

https://learn.oracle.com/ols/course/java-se-programming-complete/82508/85200

Inheritance

Extend Classes

• java.lang.Object is an ultimate paraent of any other class in Java
• A class can have only one immediate parent, as multiple inheritance is not allow in Java

• Object defines common, generic operations that all other Java classes inherit and reuse:

• The toString method creates teext value for the object
• The equals method compares a pair of objects
• The hashCode method generates int hash value for the object.
• The clone method produces a replica of the object.
• wait, notify and notifyAll methods control threads.

Reuse Parent Class Code Through Inheritance

• The purpose of inheritance is to reuse generic superclass behaviors and state in subclass

Instantiating Classes and Accessing Objects

• Heap memory allocated to store object (class instance) that contians

• Code of the specific subtype

• All of the parents up the class hierarchy

• Object reference can be of generic or specific type

  public class Product {}
  public class Food extends Product {}
  
  Object x1 = new Food();
  Product x2 = new Food();
  Food x3 = new Food();
  

  In this example,

  • x1 can only access code declared on the Object class
  • x2 can access Object and Product
  • x3 can access Object, Product and Food.

Rules of Reference Type Casting

• Casting is required to assign parent to child reference type.
• No casting is required to assign child to parent reference type
• Use instanceof operator before casting reference from generic to specific type.

Reference Code Within the Current or Parent Object

You can have same name variable in parent and subclass!!! Well-encapsulated code should only expose the method, so all variables should be private. In this case, no issues. BUT if the class have public/protected variable, then the same name variable in subclass will hide the parenet one. use this and super to distinguish them.

Define Subclass Contructors.

• The subclass contructor must invoke the superclass constructor.
• If superclass contains the no-arg constructor (default or explicitly defined), then the subclass can implicity invoke the superclass constructor.
• If superclass doesn’t provide the no-arg constructor, subclass constructor must explicitly invoke superclass constructor.
• Invocation of superclass constructor must be the first line of code in the subclass constructor.

Class and Object Initialization Summary

public class Shop {
  static {}
  public static void main(Stringp[] args) {
    Product p1 = new Food();
  }
}

public class Object {
  static {}
  public Object() {}
}

public class Product {
  static {}
  {}
  public Production() {}
}

public class Food extends Product {
  static {}
  {}
  public Food() {}
}

• Class loading and initialization execution order: (The following code is executed only once.)

1. Object class static initializer. (since Object is the root class)
2. Shop class static initializer
3. Product class static initializer
4. Food class static iniializer

   • All code of the class must be loaded into memory first.
   • It needs to be loaded only once per class

• Ojbect instantiation execution order:

1. Object class onstructor
2. Product instance initializer
3. Product constructor
4. Food instance initializer
5. Food constructor.

   • Each object instance must be initialized together with all of it’s parents.
   • Each object instance memory contains object data and references to the rest of the class code (shared between all instances).

Override Methods and Use Polymorphism

• The subclass can override parent class methods.
• The subclass can widen but can’t narrow access of methods it overrides.
• Polymorphism in Java means when a method is declared in a superclass and is overridden in a subclass, the subclass method takes precedence without casting reference to specific subclass type.
• Override annotation is optional, it’s used to ensure that subclass method signature matches the superclass method.

Define Abstract Classes and Methods

• Class can’t be directly instantiated if it is marked with the abstract keyword. (Even there is no abstract method inside)
• The abstract class purpose is to be extended by one or more concrete subclasses.
• It may also contain abstract methods that describe method signatures, without a method body.

Define Final Classes and Methods

• The final keyword can be used to limit class extensibility
• Class can’t extend a class that is marked with the final keyword
• The subclass can’t override a superclass method that is marked with the final keyword.

Override Object Class Operations

• toString(), equals(), hashCode
• The equals comare objects, the == operator compares values in the stack, so either primitive value, or two references. not real data inside the object. Java classes such as String, Number, LocalDate override the equals method.
• The hashCode methods should return same hash code if equals return true for two object.
• The hashCode should be immutable.
• A java.security.MessageDigest class should be used to generate secure hash values.