Notes on CS2040S from Week 1 Lecture 2

CS2040S_w1l2.md

CS2040S Week 1 Lecture 2

Notes from Week 1 on Data Structures and Algorithms.

Not covered

• Lambda expressions
• Type inference
• Packages

Basics of OOP

Programming paradigms

• Imperative
• Functional
• Declarative
• Object Oriented

Principles behind OOP

• Abstraction – separate interface from implementation (don't worry about details)
• Encapsulation - hide implementation from user-facing interface
• Inheritance - build nw classes by entending functionality
• Polymorphism - Change functionality via implementation

Principles of Java

• Everything is an object.
• Everything has a type

An object has a state (data) and behaviour (methods).

• A class is a template for an object (name of class == name of file)
• Each class has member variables (attributes / fields)
• Each class also has member methods (functions)

Class vs. Object

• Class is a template for how to make an object
• Object is an instantiation of a class ("using the template")
• Regular variables/functions are per object
• Static variables/functions are per class

Interface How to manipulate an object. Provides methods to interact with a class. Only contains method names with no real code inside. Using implements in your code promises to implement and use the Interface. Also, write your interface names with I in front of it (eg: IFile)

To use an interface,

// Do not need to worry about implementation of IFile or File
IFile copyFile(IFile oldFile) {
    File newFile = new File();
    FileData data = oldFile.getData();
    newFile.setData(data);
    
    return newFile;
}

Access Control

Behaviour is public while data is private. Always specify the access you want.

• private: used within the class (attributes and methods)
• public: used by anyone (attributes and methods)
• protected: within the same package, and by subclasses

If nothing specified, it can be used within the package.

public class A {
    private int secretFunction();
}

public class B {
    public int secretFunction(A example) {
        int readMe = example.secretFunction(); // crashes because secretFunction is private
    }
}

• Interfaces have completely public methods
• Classes can have both public and private methods

Initializing an object

• You need to use constructors with attributes that need to be passed in during instantiation

Common Problems

• You need a main method
• Ensure editor is running the right main method
• Ensure class name == file name For solutions to most problems, check out the Coursemology Forum.

Extra Links

• Java Basics
• Java OOP