A Foo object normally depends on a Bar object, but you can inject a BarStub object so that the Foo object no longer depends on a Bar object. Now you can test the Foo object in isolation from the Bar object.

Here is another example of using polymorphism to implement dependency injection:

Suppose you want to unit test Payroll#totalSalary() given below. The method depends on the SalaryManager object to calculate the return value. Note how the setSalaryManager(SalaryManager) can be used to inject a SalaryManager object to replace the current SalaryManager object.

class Payroll {
    private SalaryManager manager = new SalaryManager();
    private String[] employees;

    void setEmployees(String[] employees) {
        this.employees = employees;
    }

    void setSalaryManager(SalaryManager sm) {
        this.manager = sm;
    }

    double totalSalary() {
        double total = 0;
        for (int i = 0; i < employees.length; i++) {
            total += manager.getSalaryForEmployee(employees[i]);
        }
        return total;
    }
}


class SalaryManager {
    double getSalaryForEmployee(String empID) {
        // code to access employee’s salary history
        // code to calculate total salary paid and return it
    }
}

During testing, you can inject a SalaryManagerStub object to replace the SalaryManager object.

class PayrollTest {
    public static void main(String[] args) {
        // test setup
        Payroll p = new Payroll();
        // dependency injection
        p.setSalaryManager(new SalaryManagerStub());
        // test case 1
        p.setEmployees(new String[]{"E001", "E002"});
        assertEquals(2500.0, p.totalSalary());
        // test case 2
        p.setEmployees(new String[]{"E001"});
        assertEquals(1000.0, p.totalSalary());
        // more tests ...
    }
}


class SalaryManagerStub extends SalaryManager {
    /** Returns hard coded values used for testing */
    double getSalaryForEmployee(String empID) {
        if (empID.equals("E001")) {
            return 1000.0;
        } else if (empID.equals("E002")) {
            return 1500.0;
        } else {
            throw new Error("unknown id");
        }
    }
}