Re: unit testing private methods

Daniel Pitts <>
Wed, 13 Aug 2008 12:10:21 -0700
jimgardener wrote:

i am trying out PrivilegedAccessor class (of
along with junit to test some private methods that process double[]
[] .I think this uses reflection to access private methods

public class MyClass {
    private double[] processArray(double[] inarray){
     double[] ret=new double[inarray.length];
     for(int i=0;i<inarray.length;i++){
     return ret;

here is the testcase class
import junit.framework.TestCase;
import junit.extensions.PrivilegedAccessor;

public class MyClassTest extends TestCase{
    public MyClassTest(String name){
        public void testMyClass()throws Exception{
        MyClass mc=new MyClass();

                double[] inputarray=new double[]{1.1,2.2,3.3,4.4};
        double[] ans=new double[]{101.1,102.2,103.3,104.4};
        double[] outputarray=(double[])



when i run the test ,i get an error message like

java.lang.NoSuchMethodException: Method 'processArray(double[])'s
parameter nr1 (double[]) not found

This error originates at the call PrivilegedAccessor.invokeMethod(..)
can someone tell me why this happens?


Unit tests are better off testing public interfaces only. Private
implementation details shouldn't break the test unless they break client
code. They also shouldn't break client code unless they break the test.

Now, if the private method is an implementation of a particular
algorithm that is used in several parts of the same class, you might
consider pulling it out into its own class, making the method public,
and unit testing *that* class. This gives you two benefits: The user of
your class can provide you with a different implementation of that
algorithm if they choose, and any other part of your system that needs
to use that algorithm has access to the new class.

Hope this helps,
Daniel Pitts' Tech Blog: <>

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