Shreyas’ Notes

Introduction to Program Design

COMP 215

fall, sophomore year

waterfall method:

  1. requirements
  2. design
  3. implementation
  4. testing and debugging
  5. maintenance

design: planning how to build your software to meet functional metrics and software quality metrics


programming language:

programming paradigms:

Taxonomy of Programming Languages §

abstraction: hiding details to focus on essentials


when is type checking performed?

are type errors ever permitted? how strict are the rules?

Wrapper Classes §

For each primitive type, Java defines a “wrapper” class that allows primitives to be treated like objects.

Autuboxing and unboxing: automatic conversion from primitive types to the corresponding wrapper classes and vice-versa.

Casting: explicit type conversion.

float a = 5.0f;
int b = (float) a;
Object o = ...;
Foo a = (Foo) o;

conversion functions.

Object Design §

principles of OOP:

Inheritance §

Declaring that one class inherits features and functionalities from another class.

A class may only directly extend one other class.

Subclasses inherit protected, public fields and methods. If the subclass is defined in the same package as the superclass, package-protected fields and methods are also inherited. They inherit these things from all classes they’re descendants of, directly and indirectly.

public class Foo extends Bar {

subclasses can override inherited methods.

subclasses can shadow inherited fields.

Objects §

A class is a template for a custom data type. An object is an instance of a class.

constructor: special method used to create new objects

Java Project Structure §

One file: one class.

One directory: one package.

├── .idea
├── out
└── src

Each package has its own namespace. Classes in the same namespace can refer to each other without requiring imports.

Parallel package hierarchies for source and tests.

import [package].[class];
import [package].*;
import [package].[class].[staticMethod];
public static void main(String[] args) { ... }

Testing §

  1. Requirements
  2. Design
  3. Implementation
  4. Testing and Debugging
  5. Maintenance

JUnit §

import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class FooTest {
    static void setup() {

    void testSomething() {
        assertEquals(expected, actual);


Abstract Classes and Polymorphism §

Abstract Classes §

As opposed to concrete classes.

public abstract Foo { ... }

Abstract methods §

public abstract int exampleMethod();

Abstract methods can’t be private.

All concrete subclasses must implement all abstract methods. In fact, they must implement all abstract methods that were not implemented in a higher level of the hierarchy.

Polymorphism §

polymorphism: the ability for one method to take on multiple forms.

covariance: the ability to use a more specific type than declared

Interfaces §

Encapsulate behaviours.

All methods must be public. Explicit public and abstract designators are optional.

To implement a method, the default keyword must be used.

default int exampleMethod() {
public interface Bar {

public class Foo implements Bar {

When a concrete class implements an interface, it must implement all unimplemented methods.

When an abstract class implements an interface, implementing the interface’s methods is optional.

Interfaces vs Abstract classes: Use an abstract class if:

public class Foo extends Baz implements Bar {

If a method in a superclass has the same signature as a method in an implemented interface, the superclass hierarchy takes precedence.

Interfaces may implement other interfaces. However, they may not extend classes.

Object class §

A built-in class. Every class that doesn’t explicitly extend another class directly extends Object. Object is at the top of all class hierarchies.

public boolean equals(Object o) §

Returns true iff this and o are equal.

Compares by value if defined appropriately.

The instanceof operator allows checking if an object is an instance of a certain class.

public int hashCode() §

Returns a hash code representing this.

public String toString() §

Returns a string representation of this.

Containers §

interfaces defined in java.util:

Generics §

generic type

type parameter

parameterized type

raw type

Both generic and non-generic types can extend generic types. Generic types can also extend non-generic types.

Generic methods §

public <T> List<T> foo(List<T> bar) {
    List<T> baz = new ArrayList<T>();

generic methods: flexibility. good for pure functions.

genertic types: consistency, type safety. better when you want consistent types across methods that share state.

Bounded Type Parameters §

Wildcards §


Bounded wildcards:

Maps §

Map<K, V>, collection of key-value pairs


Type Erasure and Invariance §

Type erasure: compiler replaces parameteterized types with raw types and inserts any necessary casts

Because of type erasure, type parameters are not known at runtime.

Type Inference §

Type inference: the ability of a compiler to infer the type of a variable when it’s not explicitly defined.

Performed at compile time.

var x = 5;



List<String> list = new ArrayList<>();
List<List<String>> list = new ArrayList<>();

Sets §

Set<E>: unordered collection of unique elements of type E