Java Mockito example

Main.java

// Let's consider an example. UsdConverter class is responsible for converting local currency to USD (United States dollar). 
// The test should check the behavior of the tested unit in isolation from its dependencies. 
// As the exchange rate varies with time, UsdConverter uses ExchangeRateService to get the latest updates. 
// Moreover, getUsd method of a live ExchangeRateService can send a request over HTTP to get the actual exchange rate, 
// which is difficult to reproduce in the test environment. 
// Mockito allows us to avoid those difficulties and provides an API for creating empty objects and managing behavior.
// Note: here we use BigDecimal since financial transactions need high accuracy:

public class UsdConverter {

    private ExchangeRateService service;

    public UsdConverter(ExchangeRateService service) {
        this.service = service;
    }

    public BigDecimal convertToUsd(BigDecimal converted) {
        return converted.multiply(service.getUsd());
    }
}

// Using mockito, we create a mock of ExchangeRateService object and define its behavior in case of getUsd method invocation:

import org.junit.Assert;
import org.mockito.Mock;

public class UsdConverterTest {

    @Mock
    private ExchangeRateService service = Mockito.mock(ExchangeRateService.class);

    private UsdConverter converter = new UsdConverter(service);

    @Test
    public void testConvertToUsd() {
        Mockito.when(service.getUsd()).thenReturn(BigDecimal.valueOf(5));

        BigDecimal result = converter.convertToUsd(BigDecimal.valueOf(2));
        BigDecimal expected = BigDecimal.valueOf(10);

        Assert.assertEquals(expected, result);
    }
}

// You can also throw an exception on a method call to test error handling. 
// Another useful feature is checking that method was invoked inside the tested method.