Lecture 7B

Question 1

When is type compatibility checked?

Answer 1

At compile time, before runtime. Java is statically typed. This check is called type checking.

Question 2

When is a variable polymorphic?

Answer 2

When it can store references of objects with several different types.

Question 3

Shape s = new Circle(); name the types

Answer 3

The static type of s is reference type Shape, the dynamic type of s is reference type Circle. Java determines dynamic type at runtime.

Question 4

When does dynamic dispatch work?

Answer 4

Only for overriding methods. Ie Shape s = new Circle(); s.circleMethod() doesn’t work.

Question 5

Interfaces - rules:

Answer 5

Only visibility modifier allowed is public, only has abstract methods, only has none or static final fields,

Question 6

What i relationship between class an interface?

Answer 6

A weak is-a relationship.

Question 7

How to derive an interce from another interface?

Answer 7

Interface1 extends interface2, interface3 {}

Question 8

When to use an interface?

Answer 8

To achieve a restricted form of multi-inheritance, to separate public aspects of a class from implementation (an interface is all you need to interact with a class), to demand functionality from several different classes.

Question 9

What are primitive wrapper classes?

Answer 9

Integer, Double etc Conversion between primitives and wrappers done automatically by boxing and unboxing.