Week 7 [Mon, Feb 27th] - Topics

Java does not directly support constants. The convention is to use a static final variable where a constant is needed. The static modifier causes the variable to be available without instantiating an object. The final modifier causes the variable to be unchangeable. Java constants are normally declared in ALL CAPS separated by underscores.

public class Account{

  public static final double MAX_BALANCE = 1000000.0;

}

    

    


    
    

An Enumeration is a fixed set of values that can be considered as a data type. An enumeration is often useful when using a regular data type such as int or String would allow invalid values to be assigned to a variable.

You can define an enum type by using the enum keyword. Because they are constants, the names of an enum type's fields are in uppercase letters e.g., FLAG_SUCCESS by convention.

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY
}

    

    


    
    

Day today = Day.MONDAY;
Day[] holidays = new Day[]{Day.SATURDAY, Day.SUNDAY};

switch (today) {
case SATURDAY:
case SUNDAY:
    System.out.println("It's the weekend");
    break;
default:
    System.out.println("It's a week day");
}

    

    


    
    

Note that while enumerations are usually a simple set of fixed values, Java enumerations can have behaviors too, as explained in this tutorial from -- Java Tutorial

You can use the java.io.File class to represent a file object. It can be used to access properties of the file object.

import java.io.File;

public class FileClassDemo {

    public static void main(String[] args) {
        File f = new File("data/fruits.txt");
        System.out.println("full path: " + f.getAbsolutePath());
        System.out.println("file exists?: " + f.exists());
        System.out.println("is Directory?: " + f.isDirectory());
    }

}

    

    


    
    

full path: C:\sample-code\data\fruits.txt
file exists?: true
is Directory?: false

    

    


    
    

If you use backslash to specify the file path in a Windows computer, you need to use an additional backslash as an escape character because the backslash by itself has a special meaning. e.g., use "data\\fruits.txt", not "data\fruits.txt". Alternatively, you can use forward slash "data/fruits.txt" (even on Windows).

You can read from a file using a Scanner object that uses a File object as the source of data.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class FileReadingDemo {

    private static void printFileContents(String filePath) throws FileNotFoundException {
        File f = new File(filePath); // create a File for the given file path
        Scanner s = new Scanner(f); // create a Scanner using the File as the source
        while (s.hasNext()) {
            System.out.println(s.nextLine());
        }
    }

    public static void main(String[] args) {
        try {
            printFileContents("data/fruits.txt");
        } catch (FileNotFoundException e) {
            System.out.println("File not found");
        }
    }

}

    

    


    
    

5 Apples
3 Bananas
6 Cherries

    

    


    
    

You can use a java.io.FileWriter object to write to a file.

import java.io.FileWriter;
import java.io.IOException;

public class FileWritingDemo {

    private static void writeToFile(String filePath, String textToAdd) throws IOException {
        FileWriter fw = new FileWriter(filePath);
        fw.write(textToAdd);
        fw.close();
    }

    public static void main(String[] args) {
        String file2 = "temp/lines.txt";
        try {
            writeToFile(file2, "first line" + System.lineSeparator() + "second line");
        } catch (IOException e) {
            System.out.println("Something went wrong: " + e.getMessage());
        }
    }

}

    

    


    
    

first line
second line

    

    


    
    

Note that you need to call the close() method of the FileWriter object for the writing operation to be completed.

You can create a FileWriter object that appends to the file (instead of overwriting the current content) by specifying an additional boolean parameter to the constructor.

private static void appendToFile(String filePath, String textToAppend) throws IOException {
    FileWriter fw = new FileWriter(filePath, true); // create a FileWriter in append mode
    fw.write(textToAppend);
    fw.close();
}

    

    


    
    

The java.nio.file.Files is a utility class that provides several useful file operations. It relies on the java.nio.file.Paths file to generate Path objects that represent file paths.

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class FilesClassDemo {

    public static void main(String[] args) throws IOException{
        Files.copy(Paths.get("data/fruits.txt"), Paths.get("temp/fruits2.txt"));
        Files.delete(Paths.get("temp/fruits2.txt"));
    }

}

    

    


    
    

The techniques above are good enough to manipulate simple text files. Note that it is also possible to perform file I/O operations using other classes.

UML notes can augment UML diagrams with additional information. These notes can be shown connected to a particular element in the diagram or can be shown without a connection. The diagram below shows examples of both.

Example:

A class diagram can also show different types of relationships between classes: inheritance, compositions, aggregations, dependencies.

A class diagram can also show different types of class-like entities:

Software design has two main aspects:

• Product/external design: designing the external behavior of the product to meet the users' requirements. This is usually done by product designers with input from business analysts, user experience experts, user representatives, etc.
• Implementation/internal design: designing how the product will be implemented to meet the required external behavior. This is usually done by software architects and software engineers.

The guiding principle of abstraction is that only details that are relevant to the current perspective or the task at hand need to be considered. As most programs are written to solve complex problems involving large amounts of intricate details, it is impossible to deal with all these details at the same time. That is where abstraction can help.

Data abstraction: abstracting away the lower level data items and thinking in terms of bigger entities

Control abstraction: abstracting away details of the actual control flow to focus on tasks at a higher level

Abstraction can be applied repeatedly to obtain progressively higher levels of abstraction.

Abstraction is a general concept that is not limited to just data or control abstractions.

Coupling is a measure of the degree of dependence between components, classes, methods, etc. Low coupling indicates that a component is less dependent on other components. High coupling (aka tight coupling or strong coupling) is discouraged due to the following disadvantages:

• Maintenance is harder because a change in one module could cause changes in other modules coupled to it (i.e. a ripple effect).
• Integration is harder because multiple components coupled with each other have to be integrated at the same time.
• Testing and reuse of the module is harder due to its dependence on other modules.

In the example below, design A appears to have more coupling between the components than design B.

X is coupled to Y if a change to Y can potentially require a change in X.

Cohesion is a measure of how strongly-related and focused the various responsibilities of a component are. A highly-cohesive component keeps related functionalities together while keeping out all other unrelated things.

Higher cohesion is better. Disadvantages of low cohesion (aka weak cohesion):

• Lowers the understandability of modules as it is difficult to express module functionalities at a higher level.
• Lowers maintainability because a module can be modified due to unrelated causes (reason: the module contains code unrelated to each other) or many modules may need to be modified to achieve a small change in behavior (reason: because the code related to that change is not localized to a single module).
• Lowers reusability of modules because they do not represent logical units of functionality.

Cohesion can be present in many forms. Some examples:

• Code related to a single concept is kept together, e.g. the Student component handles everything related to students.
• Code that is invoked close together in time is kept together, e.g. all code related to initializing the system is kept together.
• Code that manipulates the same data structure is kept together, e.g. the GameArchive component handles everything related to the storage and retrieval of game sessions.

Software development goes through different stages such as requirements, analysis, design, implementation and testing. These stages are collectively known as the software development life cycle (SDLC). There are several approaches, known as software development life cycle models (also called software process models), that describe different ways to go through the SDLC. Each process model prescribes a "roadmap" for the software developers to manage the development effort. The roadmap describes the aims of the development stage(s), the artifacts or outcome of each stage, as well as the workflow i.e. the relationship between stages.

The sequential model, also called the waterfall model, models software development as a linear process, in which the project is seen as progressing steadily in one direction through the development stages. The name waterfall stems from how the model is drawn to look like a waterfall (see below).

When one stage of the process is completed, it should produce some artifacts to be used in the next stage. For example, upon completion of the requirements stage, a comprehensive list of requirements is produced that will see no further modifications. A strict application of the sequential model would require each stage to be completed before starting the next.

This could be a useful model when the problem statement is well-understood and stable. In such cases, using the sequential model should result in a timely and systematic development effort, provided that all goes well. As each stage has a well-defined outcome, the progress of the project can be tracked with relative ease.

The major problem with this model is that the requirements of a real-world project are rarely well-understood at the beginning and keep changing over time. One reason for this is that users are generally not aware of how a software application can be used without prior experience in using a similar application.

The iterative model (sometimes called iterative and incremental) advocates having several iterations of SDLC. Each of the iterations could potentially go through all the development stages, from requirements gathering to testing & deployment. Roughly, it appears to be similar to several cycles of the sequential model.

In this model, each of the iterations produces a new version of the product. Feedback on the new version can then be fed to the next iteration. Taking the Minesweeper game as an example, the iterative model will deliver a fully playable version from the early iterations. However, the first iteration will have primitive functionality, for example, a clumsy text based UI, fixed board size, limited randomization, etc. These functionalities will then be improved in later releases.

The iterative model can take a breadth-first or a depth-first approach to iteration planning.

• breadth-first: an iteration evolves all major components in parallel e.g., add a new feature fully, or enhance an existing feature.
• depth-first: an iteration focuses on fleshing out only some components e.g., update the backend to support a new feature that will be added in a future iteration.

Most projects use a mixture of breadth-first and depth-first iterations i.e., an iteration can contain some breadth-first work as well as some depth-first work.

In 2001, a group of prominent software engineering practitioners met and brainstormed for an alternative to documentation-driven, heavyweight software development processes that were used in most large projects at the time. This resulted in something called the agile manifesto (a vision statement of what they were looking to do).

You are uncovering better ways of developing software by doing it and helping others do it.

Through this work you have come to value:

• Individuals and interactions over processes and tools
• Working software over comprehensive documentation
• Customer collaboration over contract negotiation
• Responding to change over following a plan

That is, while there is value in the items on the right, you value the items on the left more.
_{-- Extract from the Agile Manifesto}

Subsequently, some of the signatories of the manifesto went on to create process models that try to follow it. These processes are collectively called agile processes. Some of the key features of agile approaches are:

• Requirements are prioritized based on the needs of the user, are clarified regularly (at times almost on a daily basis) with the entire project team, and are factored into the development schedule as appropriate.
• Instead of doing a very elaborate and detailed design and a project plan for the whole project, the team works based on a rough project plan and a high level design that evolves as the project goes on.
• There is a strong emphasis on complete transparency and responsibility sharing among the team members. The team is responsible together for the delivery of the product. Team members are accountable, and regularly and openly share progress with each other and with the user.

There are a number of agile processes in the development world today. eXtreme Programming (XP) and Scrum are two of the well-known ones.