Integration testing

Introduction to Testing in Java

Maria Milusheva

Senior Software Engineer

Defining integration testing

Integration - the process of combining individual software components or subsystems into a unified system that functions cohesively.

Integration test - a test that verifies the interaction between multiple components or modules of an application to ensure they work together correctly. It often involves testing with external dependencies.

An image of Ikea furniture right before assembly

Dependencies

Software dependency - a relationship between software components where one component relies on the other to work properly

Chart of dependency relations between classes

¹ Chart made using Mermaid: https://mermaid.live

Integration testing vs. unit testing

Unit tests:

Test individual components in isolation
Failures pinpoint issues with a specific part of the code
Faster to write and run
Use various tools to avoid testing dependencies
Assert on exact results and outcomes

Integration tests:

Test multiple components and their interactions
Failures require deeper analysis to trace
Can be heavy and take longer to set up and run
Tests real interactions with real dependencies
May assert on general behaviors instead of exact results

Example: Foreign exchange

Suppose we are building a currency exchange app:

public class ExchangeApp {
    private EuropeanCentralBankServer bank; // Enables ExchangeApp to use the bank server

    public ExchangeApp(EuropeanCentralBankServer bank) {
        this.bank = bank; // Save the object passed through the constructor
    }


    public double convertEuroTo(String currency, double amount) {
        double rate = this.bank.getRateEuroTo(currency); 
        return amount * rate; // Use the return values of the bank method in calculations
    }
}

Integration testing foreign exchange

Integration testing verifies both the convertEuroTo method and EuropeanCentralBankServer:

@Test
void convert_convertsWithoutError() {
    EuropeanCentralBankServer bank = new EuropeanCentralBankServer();
    ExchangeApp exchangeApp = new ExchangeApp(bank); // Pass bank object to constructor

    double amount = 1000.0;
    String currency = "USD";

    // convertEuroTo calls getRateEuroTo from the bank object
    double result = exchangeApp.convertEuroTo(currency, amount);

    assertTrue(result > 0); // Can't predict exact value, can only sanity test
}

Let's practice!

Introduction to Testing in Java