Example of PowerMock usage for mocking private methods and attributes.

PowerMockPrivateMethods.java

import java.util.ArrayList;
import java.util.List;

import org.junit.Assert;
import org.junit.Test;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.reflect.Whitebox;

/**
 * The class that holds all unit tests for
 * the Employee class.
 * @author Deep Shah
 */
public class EmployeeTest {

	@Test()
	public void shouldNotDoAnythingIfEmployeeWasSaved() {
		Employee employee = PowerMockito.mock(Employee.class);
		PowerMockito.doNothing().when(employee).save();
		try {
			employee.save();
		} catch(Exception e) {
			Assert.fail("Should not have thrown an exception");
		}
	}

	@Test(expected = IllegalStateException.class)
	public void shouldThrowAnExceptionIfEmployeeWasNotSaved() {
		Employee employee = PowerMockito.mock(Employee.class);
		PowerMockito.doThrow(new IllegalStateException()).when(employee).save();

		employee.save();
	}

	@Test
	public void shouldVerifyThatNewEmployeeIsAddedToTheDepartment() {

		final Department department = new Department();
		final Employee employee = new Employee();

		//Adding the employee to the department.
		department.addEmployee(employee);

		//Getting the privately held employees list
		//from the Department instance.
		final List<Employee> employees = Whitebox.getInternalState(department, "employees");
		//Asserting that the employee was added to the
		//list.
		Assert.assertTrue(employees.contains(employee));
	}

	@Test
	public void shouldAddNewEmployeeToTheDepartment() {
		final Department department = new Department();
		final Employee employee = new Employee();
		final ArrayList<Employee> employees = new ArrayList<Employee>();
		//Setting the privately held employees list with
		// our test employees list.
		Whitebox.setInternalState(department, "employees", employees);
		//Adding the employee.
		department.addEmployee(employee);
		//Since we substituted the privately held employees
		//within the department instance
		//we can simply assert whether our list has the
		// newly added employee or not.
		Assert.assertTrue(employees.contains(employee));
	}

	@Test
	public void shouldVerifyThatMaxSalaryOfferedForADepartmentIsCalculatedCorrectly() throws Exception {

		final Department department = new Department();
		final Employee employee1 = new Employee();
		final Employee employee2 = new Employee();

		employee1.setSalary(60000);
		employee2.setSalary(65000);

		//Adding two employees to the test employees list.
		final ArrayList<Employee> employees = new ArrayList<Employee>();

		employees.add(employee1);
		employees.add(employee2);

		//Substituting the privately held employees list
		// with our test list.
		Whitebox.setInternalState(department, "employees", employees);
		//Getting the value of maxSalary from the private
		// field.
		
		ArrayList<Employee> emps = Whitebox.getInternalState(department, "employees");
		
		for (Employee emp : emps) {
			System.out.println(emp.getSalary());
		}

		Whitebox.invokeMethod(department, "updateMaxSalaryOffered");


		final long maxSalary = Whitebox.getInternalState(department, "maxSalaryOffered");
		Assert.assertEquals(65000, maxSalary);
	}
}