SOLID Java Code

SOLID Java Code
Omar Bashir
https://uk.linkedin.com/in/obprofile
@OmarBashir_40
obashir@yahoo.com

Coding Humanely
● What if our code had feelings ?
● What if we treated people like we treat code ?
– e.g., Sir, you will need a foot transplant to be able to
use our latest range of trainers.

Desirable Code Qualities
● Simplicity
– Ease of understanding.
● Extensibility
– Ease of extending functionality.
● Flexibility
– Ease of adapting to the operating environment.
● Testability
– Ease of testing from units of code to the entire
systems

Challenges: Size and Complexity
● Problems
– Most real world problems requiring automation are
complex.
● Requirements
– Increasing demand for comprehensive automation.
● Solutions
– Resulting solutions are large and sophisticated.

Modularisation
● Decomposing large systems into smaller and
simpler parts.
– Functions and procedures.
– Classes and modules.
● Building the parts.
● Assembling the parts to complete the system.
● To achieve desirable code qualities parts should
be interchangeable.

Keys to Interchangeability
● Coupling
– Interdependence between different software modules.
– Low coupling is preferred as it allows flexibility and
extensibility.
– Low coupling via simple and stable interfaces.
● Cohesion
– Degree to which elements of a module functionally
belong together.
– Higher cohesion reduces complexity of components.
● Higher cohesion and low coupling enables
interchangeability.

Spot Issues ?
package app;
import calc.Calculator;
public class App {
public static void main(String[] args) {
String line = System.console().readLine();
String[] parts = line.split(" ");
Calculator.operand1 = Double.parseDouble(parts[0]);
Calculator.operator = parts[1].charAt(0);
Calculator.calculate();
}
}

Spot Issues ?
package app;
import calc.Calculator;
public class App {
String line = System.console().readLine();
String[] parts = line.split(" ");
Calculator.operator = parts[1].charAt(0);
Calculator.calculate();
}
}
High Coupling

Spot Issues ?
package calc;
public class Calculator {
public static double operand1;
public static char operator;
public static void calculate() {
double result = Double.NaN;
switch(operator) {
case '+':
result = operand1 + operand2;
break;
case '-':
result = operand1 - operand2;
break;
case '*':
result = operand1 * operand2;
break;
case '/':
result = operand1 / operand2;
break;
default:
System.out.printf("Unrecognised operator %cn", operator);
}
System.out.printf("%f %c %f = %fn",
operand1, operator, operand2, result);
}
}

Spot Issues
package calc;
public class Calculator {
public static char operator;
public static void calculate() {
double result = Double.NaN;
switch(operator) {
case '+':
result = operand1 + operand2;
break;
case '-':
result = operand1 - operand2;
break;
case '*':
result = operand1 * operand2;
break;
case '/':
result = operand1 / operand2;
break;
default:
System.out.printf("Unrecognised operator %cn", operator);
}
System.out.printf("%f %c %f = %fn",
operand1, operator, operand2, result);
}
}
High Coupling
Low Cohesion

Simplifying with SOLID
● Types of coupling
– Content coupling, Common coupling, Stamp coupling,
Control coupling, Data coupling.
● Types of cohesion
– Co-incidental cohesion, Logical cohesion, Temporal
cohesion, Procedural cohesion, Communicational
cohesion, Sequential cohesion, Functional cohesion.
● SOLID
– Five principles of object oriented programming.
– Aims to deliver extensible and maintainable software.

SOLID
● Single Responsibility Principle (SRP)
– A class should have a single responsibility.
● Open Close Principle (OCP)
– Open for extension but closed for modification.
● Liskov Substitution Principle (LSP)
– Substitution of objects by instances of sub-classes.
● Interface Segregation Principle (ISP)
– Many client specific interfaces instead of one big one.
● Dependency Inversion Principle (DIP)
– Depend on abstractions instead of implementations.

1
Single Responsibility Principle

● A class should have one and only one reason
to change.
– A class should have only one responsibility.
● Promotes high cohesion and low coupling.
– High cohesion because of focused classes.
– Low coupling because of dependency on other
cohesive classes.

Example
public class MeanCalculator {
public double calculate(final String data) {
double sum = 0.0;
final String[] parsedData = data.split(",");
for (String item : parsedData) {
sum += Double.parseDouble(item);
}
return sum/parsedData.length;
}
}

Example
public double calculate(final String data) {
double sum = 0.0;
final String[] parsedData = data.split(",");
for (String item : parsedData) {
sum += Double.parseDouble(item);
}
return sum/parsedData.length;
}
}
Two reasons to change
1. Parsing
2. Computation

Example
Functional (Java 8) Implementation
public double calculate(final List<Double> data) {
double sum = 0.0;
for (Double item : data) {
sum += item;
}
return sum/data.size();
}
}
public double calculate(final List<Double> data) {
return data.
stream().
collect(Collectors.
averagingDouble(item -> item));
}
}

Open/Close Principle
● Software entities (Classes, methods, modules
etc.) should be open for extension but closed
for modification.
– Changing functionality need not require modifying
existing code.
● Preferred approaches,
– Inheritance,
– Composition of abstractions (plugins).

Example
public class Parser {
List<Double> parse(final String data) {
final String[] parsedCsv = data.split(",");
return toDoubleList(parsedCsv);
}
private List<Double> toDoubleList(final String[] data) {
final List<Double> list = new ArrayList<>();
for(String item: data) {
list.add(Double.parseDouble(item));
}
return list;
}
}

Example:Extendinga
ClosedParser
public class Parser {
public List<Double> parse(final String data, final Format format) {
String[] parsedData = null;
switch(format){
case CSV:
parsedData = data.split(",");
break;
case XML:
try {
parsedData = parseXml(data);
} catch (Exception exp) {
throw new IllegalArgumentException(exp);
}
break;
default:
throw new IllegalArgumentException(String.format("Unknown format %s", format));
}
return toDoubleList(parsedData);
}
/**
* Format: <DataList><Item>3</Item><Item>4.5</Item><Item>7.5</Item></DataList>
*/
private String[] parseXml(final String xmlString) throws Exception {
final Document xmlDoc = DocumentBuilderFactory.
newInstance().
newDocumentBuilder().
parse(new ByteArrayInputStream(xmlString.getBytes()));
final NodeList nodes = xmlDoc.getElementsByTagName("Item");
final String[] textList = new String[nodes.getLength()];
for (int i = 0; i < nodes.getLength(); i++) {
textList[i] = nodes.item(i).getTextContent().trim();
}
return textList;
}
}
return list;
}
}

public List<Double> parse(final String data,
final Format format) {
String[] parsedData = null;
switch(format){
case CSV:
parsedData = data.split(",");
break;
case XML:
try {
parsedData = parseXml(data);
}
break;
default:
throw new IllegalArgumentException(
String.format("Unknown format %s", format));
}

Example: Opening With Template
Method
public abstract class Parser {
public List<Double> parse(final String data) {
final String[] parsedData = split(data);
return toDoubleList(parsedData);
}
protected abstract String[] split(final String data);
}
return list;
}
}

Example: Parser Using Streams
public abstract class Parser {
return Arrays.asList(parsedData).stream().
map(item -> Double.parseDouble(item)).
collect(Collectors.toList());
}
}

Example: Opening With Template
Method
public class CsvParser extends Parser {
@Override
protected String[] split(String data) {
return data.split(",");
}
}

public class XmlParser extends Parser {
/**
* Format:
* <DataList><Item>3</Item><Item>4.5</Item><Item>7.5</Item></DataList>
*/
@Override
protected String[] split(String data) {
String[] textList = null;
try {
final Document xmlDoc = DocumentBuilderFactory.
newInstance().
newDocumentBuilder().
parse(new ByteArrayInputStream(data.getBytes()));
final NodeList nodes = xmlDoc.getElementsByTagName("Item");
textList = new String[nodes.getLength()];
for (int i = 0; i < nodes.getLength(); i++) {
textList[i] = nodes.item(i).getTextContent().trim();
}
}
return textList;
}
}

3
Liskov Substitution Principle

● Named after MIT professor Barbara Liskov.
● Functions that use references to base classes
must be able to use objects of the derived class
without knowing it.
– Derived classes must be substitutable for base class.
● Caution,
– Should not alter the behaviour of the application.
– Inheritance hierarchies should not violate domain
concepts.
● E.g., a square is a rectangle instead of both are shapes.

public class AvergerApp {
final List<String> params = Arrays.asList(args);
if ((params.size() != 2) ||
(params.stream().filter(i -> i.contains("=")).count() != 2)) {
System.err.println("Parameters: f=<CSV/XML> m=<message>");
} else {
final String format = params.stream().
filter(str -> str.contains("f")).findFirst().get().split("=")[1];
final String message = params.stream().
filter(str -> str.contains("m")).findFirst().get().split("=")[1];
final Parser parser = getParser(format);
final List<Double> data = parser.parse(message);
final double avg = new MeanCalculator().calculate(data);
System.out.printf("Average(%s) = %f", data, avg);
}
}
private static Parser getParser(final String format) {
Parser parser = null;
switch (format) {
case "CSV":
parser = new CsvParser();
break;
case "XML":
parser = new XmlParser();
break;
default:
throw new IllegalArgumentException("Unknown format " + format));
}
return parser;
}
}

4
Interface Segregation Principle

Interface Segragation Principle
● Multiple fine grained client or domain specific
interfaces are better than few coarse grained
interfaces.
● Consequences
– Classes only implement interfaces that are
requirement specific.
– Client classes are exposed only to the functionality
that they need.

Example
public interface AuthorisationService {
boolean isAuthorised(String userId,
Resource resource,
Action action);
List<Entitlement> entitlementReport(String userId);
List<Group> membershipReport(String userId);
boolean entitle(Group group, Entitlement entitlement);
boolean entitle(String userId, Entitlement entitlement);
boolean add(String userId, Group group);
boolean remove(String userId, Group group);
boolean remove(String userId, Entitlement entitlement);
boolean remove(Group group, Entitlement entitlement);
}

Example: Segregating Interfaces
public interface EntitlementsModifier {
boolean entitle(Group group, Entitlement entitlement);
boolean entitle(String userId, Entitlement entitlement);
boolean add(String userId, Group group);
boolean remove(String userId, Group group);
boolean remove(String userId, Entitlement entitlement);
boolean remove(Group group, Entitlement entitlement);
}
public interface EntitlementsChecker {
boolean isAuthorised(String userId,
Resource resource,
Action action);
List<Entitlement> entitlementReport(String userId);
List<Group> membershipReport(String userId);
}

5
Dependency Inversion Principle

● High-level modules should not depend on low-
level modules.
– Both should depend on abstractions.
● Abstractions should not depend on details.
– Details should depend on abstractions.
● Dependencies are a risk,
– Details bind dependants to concrete implementations.
● This limits flexibility and extensibility.
– Abstractions provides independence to dependants.
● Dependants are able to select most suitable
implementations.

Example: Averaging Calculator
● A class that uses
– An averaging algorithm to process input.
– A parser to parse text input to a collection of doubles.
– Input and output classes.
● Input formats can change.
– Binding the calculator to a concrete parser and IO
makes it inflexible.
– Averaging calculator references a parser, an input and
an output interface.
– The application specifies the parser and IO needed.

Example: Parser
public interface IParser {
List<Double> parse(final String data);
}
public abstract class Parser implements IParser {
return Arrays.asList(parsedData).stream().
map(item -> Double.parseDouble(item)).
collect(Collectors.toList());
}
}

Example: Input
public interface IInput {
String read();
}
public class FixedInput implements IInput {
private final String data;
public FixedInput(final String data) {
this.data = data;
}
@Override
public String read() {
return data;
}
}

Example: Output
public interface IOutput {
void write(final List<Double> data,
final double average);
}
public class ConsoleOutput implements IOutput {
@Override
public void write(final List<Double> data,
final double average) {
System.out.printf("Average%s = %fn",
data, average);
}
}

Example: Mean Processor
public interface IProcessor {
double process(List<Double> data);
}
public class MeanProcessor implements IProcessor {
public double process(final List<Double> data) {
return data.stream().
collect(Collectors.
averagingDouble(item -> item));
}
}

Example: Averaging Calculator
public class AveragingCalculator {
final private IInput input;
final private IOutput output;
final private IProcessor algo;
final private IParser parser;
public AveragingCalculator(final IProcessor algo,
final IParser parser,
final IInput input,
final IOutput output) {
this.algo = algo;
this.parser = parser;
this.input = input;
this.output = output;
}
public void run() {
final String inputData = input.read();
final List<Double> data = parser.parse(inputData);
final double average = algo.process(data);
output.write(data, average);
}
}

Example: CSV Averaging App
public class CsvAveragingApp {
final IInput input = new FixedInput(args[0]);
final IOutput output = new ConsoleOutput();
final IParser parser = new CsvParser();
final IProcessor algo = new MeanProcessor();
final AveragingCalculator calc =
new AveragingCalculator(algo,
parser,
input,
Output);
calc.run();
}
}

Example: XML Averaging App
public class XmlAveragingApp {
final IInput input = new FixedInput(args[0]);
final IOutput output = new ConsoleOutput();
final IParser parser = new XmlParser();
final IProcessor algo = new MeanProcessor();
final AveragingCalculator calc =
new AveragingCalculator(algo,
parser,
input,
output);
calc.run();
}
}

Opposite of SOLID
Open Close Principle

Opposite of SOLID
S
T
U
P
I
D

Opposite of SOLID
Singletons
Tight Coupling
Untestability
Premature Optimisation
Indescriptive Naming
Duplication

SOLID Java Code

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