Using Objects

• Mixing Objects and Static Data/Methods
  • Static (class) variables and methods in the class upon which an object is based are accessible by the objects (even if they are private).
  • Class variables may be used to store data that is common to all objects.
  • Class methods are often used to access and mutate class variables.
  • Example: BankAccount1.java and UseBankAccount1.java

• this() and this.
  • A constructor can call another constructor using this()
    • Example: MiamiWind.java and UseMiamiWind.java
  • An object can refer to itself using this.
    • Within an object, methods and instance variables are referred to directly. They belong to the current object, so it is not necessary to give the object reference and the dot.
    • this. is used for, e.g., allowing the same name for a method parameter and an instance variable (aaargh). Better reasons for using this. come later in life.
    • Example: TestResult2.java and UseTestResult2.java

• Objects with Objects
  • It is common for instance variables to be reference variables. They refer to other objects.
  • Such references can be to an object of another standard class.
  • Example: Species1.java
  • Such references can be to an object of the same class. Objects from the same class can access each others' private parts.
  • Example: Person1.java and UsePerson1.java
  • Such references can be to an object of another user class.
  • Example: Gun.java and Cop.java and CopWithGun.java

• Arrays of Objects
  • As objects represent things in reality, it is common to store collections of objects.
  • Arrays of object (reference) variables are one way to achieve this.
  • Example: Car.java and CarDealer.java and ShonkyCars.java

Exercises

Exam Style Questions

• Given the following class:

  //=============================================================================
  public class MyLeader {
  //-----------------------------------------------------------------------------
      private MyLeader leader;
  //-----------------------------------------------------------------------------
      public MyLeader() {
  
          leader = null;
      }
  //-----------------------------------------------------------------------------
      public void setLeader(MyLeader leader) {
  
          this.leader = leader
      }
  //-----------------------------------------------------------------------------
  }
  //=============================================================================
  

  write a program that creates an array of five MyLeader objects, makes each object's leader point to the first object in the array.
• Draw a picture of the object structure created in this program

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• Can a private static method/variable in a class be accessed from an instance of that class?