Prepare for OCAJP, (5)

Design classes.

Class Inheritance

  1. Extending a class using keyword extends
  2. Private variable in parent class
    1. Subclass cannot access parent class’s private
    2. If we have an instance of a subclass, that private variable from parent class exists
  3. Class Access Modifiers
    • public can be referenced and used in any class
    • default package private by subclass or class within same package.
  4. Java Objects
    if you look at the inheritance structure of any class, it will always end with java.lang.Object on the top of the tree
  5. Compiler Enhancements Constructor
    • compiler inserts a no-argument constructor super()if the first statement is not a call to parent constructor.
      //following definitions are equivalent
      public class Moss {                           }
      public class Moss { public Moss(){          } }
      public class Moss { public Moss(){ super(); } }
    • if parent class doesn’t have a no-argument constructor, you must create one constructor in clild class, explicitly call a parent constructor via the super()
      public class Mammal { public Mammal(int age) { } }
      public class Elephant extends Mammal { 
        public Elephant() {  }// DOES NOT COMPILE   
      public class Elephant extends Mammal {
        public Elephant() { super(10); } //WILL COMPILE, an explicit call to a parent constructor.
  6. Constructor Definition Rules
    • First statement of every constructor is a call to another constructor. Within the class using this(), or to parent using super().
    • The super() call may not be used after the first statement of the constructor.
    • If no super() exists, compiler will insert one at the beginning of constructor.
    • If the parent doesn’t have a no-argument constructor and the child doesn’t define any constructors, the compiler will throw an error
    • If the parent doesn’t have a no-argument constructor, the compiler requires an explicit call to a parent constructor in each child constructor.
     class Primate { public Primate() { System.out.println("Primate"); } }
     class Ape extends Primate { public Ape() { System.out.println("Ape"); } }
     public class Chimpanzee extends Ape { public static void main(String[] args) { new Chimpanzee(); } }
     //Primate Ape
  7. Calling Inherited Class Members
    • If the parent class and child class are part of the same package, the child class may also use any default members defined in the parent class.
    • A child class may never access a private member of the parent class
    • Can explicitly reference a member of the parent class by using the super keyword
    • Inheriting a class grants us access to the public and protected members of the parent class.
  8. Overriding a Method:
    • there is a method defined in both the parent and child class, you want to define a new version of an existing method in a child class.
    • You declare a new method with same signature and return type.
    • signiature includes the name and lsit of input parameters
    • use super to reference parent version method
    • clild class method must be at least as accessible or more accessbile than parent class
    • child class may not throw a checked exception newer or broader. (no bigger exception)
    • return value must be the same or a subclass
  9. Overloading
    • an overloaded method will use a different signature than an overridden method.
    • Check: access modifiers, return types and exceptions
      public class Bird {
        public void fly() { System.out.println("Bird is flying");} 
        public void eat(int food) { System.out.println("Bird is eating "+food+" units of food"); }
      public class Eagle extends Bird {
        public int fly(int height) { 
          System.out.println("Bird is flying at "+height+" meters"); return height; 
        public int eat(int food) { // DOES NOT COMPILE, return type not right
          System.out.println("Bird is eating "+food+" units of food");
          return food; 
      //fly() is overloaded, since signature changes from a no-argument constructor to one int argument
      //eat() is overridden, since signature is the same. 
  10. Hiding Static Methods
    • A hidden method occurs when a static method with same name and signature in both parent and child classes.
    • Rules for overriding apply for hidden method.
    • New rule: stay static, or stay not static, remain the same
    • if it has static, in method it will call parent variable
    • if don’t have static, in will call instance variable
  11. Inheriting Variables
    • Hiding Variables: you define a variable with the same name as a variable in a parent class.
    • explicit use of the super keyword to reference a hidden variable

Abstract Classes

abstract class marked with the abstract keyword and cannot be instantiated. abstract method marked with the abstract keyword defined in an abstract class, no implementation is provided in the class

  1. abstract class vs. abstract method
    • An abstract class is not required to include any abstract methods.
    • An abstract class may include nonabstract methods and variables.
    • abstract class cannot be marked as final, because it must be extended.
    • An abstract method may only be defined in an abstract class.
    • An abstract method defined within an nonabstract class will cause complie error.
    • A method may not be marked as both abstract and private. Private method cannot be accessed by child.
  2. Concreate Class
    A concrete class is the first nonabstract subclass that extends an abstract class. A concrete class is required to implement all inherited abstract methods.

  3. Extending an Abstract Class with Another Abstract
    • Abstract clases can extend other abstract classes are not required to implement any abstract method.
    • A concrete class that extends an abstract must implement all inherited abstract methods.
    • A concreate class is not required to implement an abstract method, if an intermediate abstract class provides implementation.
     public abstract class Animal { public abstract String getName(); }
     public abstract class BigCat extends Animal { 
       public String getName() {return "BigCat";}
       public abstract void roar();
     public class Lion extends BigCat { 
       //getName() is no longer abstract by the time it reaches here
       public void roar() { System.out.println("The Lion lets out a loud ROAR!"); } 


  1. Definating an interface
    • An interface is defined with interface keyword.
    • Classes invokes it using implements keyword.
    • Interfaces cannot be instantiated directly.
    • An interface is not required to have any methods.
    • Marking an interface as private, protected, or final will trigger a compiler error
    • Marking a interface method as private, protected, or final will trigger compiler errors
  2. Inheriting an Interface
    1. An interface extends another interface
    2. An abstract class implements an interface
    3. An abstract class implements interface will inherits all abstracts method as its own abstract method
    4. The first concrete class that implements an interface, or extends that abstract class, must provide an implementation for all inherited abstract methods.
  3. Multiple Inheritance
    • If two methods with same signature appear in two interfaces and implemented by a class, only need to implement once.
    • If two methods have different signature but input parameters are different, just implement seperately. It’s method overloading.
    • Unfortunately, if the method name and input parameters are the same but the return types are different, won’t compile.
  4. Interface Variables
    • interface variables are constant, assumed to be public satic final.
    • Making it private or protected will trigger a compiler error.
    • The value of an interface variable must be set when it is declared since it is marked as final.
  5. Default Interface Methods
    • A default method is defined within an interface with the default keyword.
    • A default method may only be declared within an interface and not within a class or abstract class.
    • If a method is marked as default, it must provide a method body.
    • A default method is not assumed to be static, final, or abstract, as it may be used or overridden by a class that implements the interface
    • Classes have the option to override the default method if they need to, but they are not required to.
    • If the class doesn’t override the method, the default implementation will be used.
  6. Static Interface Methods
    • A static method defined in an interface is not inherited in any classes that implement the interface.
    • Use interface name to refer this method


  1. Definition
    • Polymorphism An object can be accessed using a reference of itself, or a super class, or an interface.
    • A cast is not required if object is being reassigned to a super type or interface of the object.
    • In polymorphism, only one object is created.
    • Once this object is assigned a new type, it only can access members of that reference type.
    • Depending on the type of the reference, we may only have access to certain methods.
    • Changing reference type could allow you access new properties
  2. Casting Objects
    • Once we changed the reference type, we lost access to more specific methods in that object
    • We reclaim reference by casting object back
    • We explicitly cast object to a subclass
    • Doesn’t require explicit cast if it’s from subclass to a superclass
    • Compile error if casts to unrelated types
    • Runtime fail ClassCastException if casts to related types, but object isn’t an instance of that class
     public class Bird {}
     public class Animal {}
     public class Fish extents Animal { 
       public static void main(String[] args) { 
         Fish fish = new Fish(); 
         Bird bird = (Bird)fish; // DOES NOT COMPILE, not related
         Animal animal = new Animal();
         Bird bird = (Bird) bird; //WILL COMPILE, throw ClassCastException.
         //reason is: basicly, this object is an Animal, nothing to do with a bird

    we should perform cast only if the isntanceof operator returns true

  3. Virtual Methods
    • Most important feature of polymorphism is to support virtual methods.
    • A virtual method is a method in which the specific implementation is not determined until runtime.
    • All non-final, non-static, and non-private Java methods are considered virtual methods, since any of them can be overridden at runtime.
    • It’s special! If you call a method on an object taht overrides a method, you get the overriden method. Even if the call to the method is on a parent reference, or within the parent class.
     public class Bird {
       public String getName() { 
         return "Unknown"; 
       public void displayInformation() {
         System.out.println("The bird name is: "+getName()); 
     public class Peacock extends Bird {
       public String getName() { return "Peacock"; } 
       public static void main(String[] args) {
         Bird bird = new Peacock();
         bird.displayInformation(); //The bird name is: Peacock
    1. new Peacock(),means it’s a Peacock object. It has members: getName() from parent, displayInfomation() from parent and getName() from it self.
    2. It can reach parent’s getName(), by super.getName().
    3. Bird bird = new Peacock(); means the reference is a bird type
    4. bird.displayInfomation() will call this method in parent. However, it also triggers getName()
    5. this object is a Peacock, it has it’s own getName(), so this one is being used
    6. what if peacock doesn’t have this method? it will print: The bird name is: Unknown
  4. Polymorphic Parameters
    One of the most useful applications of polymorphism is the ability to pass instances of a subclass or interface to a method
     public class Reptile                    { public String getName() { return "Reptile"; } }
     public class Alligator extends Reptile  { public String getName() { return "Alligator"; } }
     public class Crocodile extends Reptile { public String getName() { return "Crocodile"; } }
     public class ZooWorker {
       public static void feed(Reptile reptile) { 
         System.out.println("Feeding reptile "+reptile.getName()); 
       public static void main(String[] args) { 
         feed(new Alligator());  //Feeding: Alligator 
         feed(new Crocodile());  //Feeding: Crocodile 
         feed(new Reptile());    //Feeding: Reptile
  5. Method Overriding

     public class Animal { public String getName() { return "Animal"; } }
     // DOES NOT COMPILE, beacause weaker access privileges
     public class Gorilla extends Animal { protected String getName() { return "Gorilla"; } }

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