Compile Time Type VS Runtime Type

Compile-time	Run-time
Left hand side of the assignment	Right hand side of the assignment
The type in which the variable is declared	The type the variable is assigned to
Restricts methods that can be called upon declaration/compilation	Actual methods to invoke are determined during runtime
Method overloading	Method overriding
Static/early binding	Dynamic/late binding
Return type of function	Actual type of object returned from function

For example:

Circle c = new FilledCircle(1.0, Color.BLUE);

Compile-time type of c is Circle and the runtime type is FilledCircle.
Remember that in Circle.java, we have these methods:

• Circle(double radius), the constructor
• double getArea()
• double getPerimeter()
• public String toString()

While in FilledCircle.java, we have these:

• FilledCircle(double radius, Color color), the constructor
• FilledCircle fillColor(Color color)
• @Override public String toString(), an overriding method!

Imagine you are in a room of doors with different materials and colors. Different method name means different material.

Therefore, since the compile-time type of c is Circle, we can only call the methods inside Circle.java.

Compile-time type selects which door materials you can have access into.

However, since the runtime type of c is FilledCircle, the actual method called for toString is the overriden method!

For each door material, there may be many doors, and runtime type will decide which color is the correct one.

Also, in PA1, we had to lock a question

public Markable lock() { // returns Markable
    return this; // of type MCQ/TFQ/FillInBlank
}

This would allow:

• Question::lock().mark() to work
• Question::mark() to fail
• Question::answer() to work
• Question::lock().answer() to fail