Lecture 5: The keyword `this`; modifiers

lecture video

The keyword this

• Within the body of a (nonstatic) method, this always refers to the instance upon which the method was invoked.
• Use to differentiate between local and instance variables
• Use to reference the instance
• Use to call constructors

Modifiers

Access control modifiers

• For classes, methods, and variables
• Controls visibility in the context of a larger program
• public, protected (subclasses and classes in same package), private. Default is called package-private.

The static modifier

• For variables and methods
• Value is associated with class as whole, not with an instance

The abstract modifier

• For methods
• We will learn about later

The final modifier

• For variables
• Makes it so the variable can never be changed after it is initialized
• If primitive, constant. If reference, a memory address.

Files used in class

• Counter2.java
• CounterDemo2.java
• MoreWithDataTypes.java

Additional exercises

1. Write a method averageTwo for the Counter class that takes in two instances of Counter and returns a new Counter whose count is the average (rounded down) of the two. Test it with the CounterDemo program.
2. If you didn’t already, make the averageTwo method a static method. Change your code in CounterDemo so that averageTwo is accessed in a static way. (That is, call the method from the class name, not from an instance of the class.)