Testing private methods

Sometimes you need to test the private method of a class. I use reflection to access the private method. Here are the codes:

 * Class with a private method that you want to test.
 * @author Xuan Ngo
public class ClassExample
  private String privateFoo(int a, String b)
    return a+b;
 * Here is the code showing how to access the private method
 *  using reflection.
 * @author Xuan Ngo
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import org.testng.annotations.Test;
import static org.testng.Assert.assertEquals;
public class ClassExampleTest
  public void privateFooTest() throws NoSuchMethodException, IllegalAccessException, InvocationTargetException
    // Get the class of the private method.
    ClassExample oClassExample = new ClassExample();
    Class<?> cNewClassExample = oClassExample.getClass();
    // Change the property of the private method to be accessible.
    Method newPrivateFoo = cNewClassExample.getDeclaredMethod("privateFoo", int.class, String.class);
    // Run the private method.
    Object oActual = newPrivateFoo.invoke(oClassExample, new Integer(169), new String("_ABC"));
    // Test the private method
    String sActual = oActual.toString();
    String sExpected = "169_ABC";
    assertEquals(sActual, sExpected);
Note: When calling, watch out for the exceptions, especially InvocationTargetException:
IllegalAccessException: You're not allowed to call this method
IllegalArgumentException: You can call this method, but not with the supplied arguments
InvocationTargetException(Anything the invoked method throws is wrapped by InvocationTargetException.): You called the method just fine, but it threw an exception
File ClassExample.java196 bytes
File ClassExampleTest.java1.13 KB