Some classes require another class, a dependency. While the dependency can be defined in the dependant class’s own constructor, this introduces several problems such as tightly coupling the two classes and introducing difficulties during unit testing. To solve this, we can inject the dependency as an argument into the dependant class’s constructor or setter.
public class Order {
private Customer customer;
// Constructor Injection
public Order(Customer customer) {
this.customer = customer;
}
// Setter Injection
public setCustomer(Customer customer) {
this.customer = customer;
}
}
By injecting dependencies, we can more easily test this class without having to also test the dependency which can be independantly tested, if necessary. The injection pattern means that a mocked dependency can be injected in the original dependency’s place.
public class MockCustomer extends Customer {
// Stub methods overriding the super class methods
}
public class OrderTest {
@Test
public void testMock() {
Order order = new Order(new MockCustomer);
}
}