Whitebox: Invoke the constructor of a class

Whitebox class provides 'invokeConstructor' method, it is used to invoke the constructor of a class and get an instance.

Following are the overloaded forms of invokeConstructor method.

public static <T> T invokeConstructor(Class<T> classThatContainsTheConstructorToTest, Class<?>[] parameterTypes, Object[] arguments) throws Exception

public static <T> T invokeConstructor(Class<T> classThatContainsTheConstructorToTest, Object... arguments) throws Exception

Above methods are useful for testing classes with a private constructor.

Following statements are used to invoke the constructor of a class StringUtil.

StringUtil stringUtil = Whitebox.invokeConstructor(StringUtil.class, "madam");

Find the following working application.

StringUtil.java

package com.sample.util;

public class StringUtil {
 private String str;
 
 private StringUtil(String str){
  this.str = str;
 }

 /**
  * 
  * @param str
  * @return true if the string is palindrome, else false.
  */
 public boolean isStringPalindrome() {
  if (str == null || str.isEmpty()) {
   return true;
  }

  return str.equals(new StringBuilder(str).reverse().toString());
 }

}

StringUtilTest.java

package com.sample.util;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import org.junit.Test;
import org.powermock.reflect.Whitebox;

public class StringUtilTest {

 @Test
 public void isStringPalindrome_palindrome_true() throws Exception{
  StringUtil stringUtil = Whitebox.invokeConstructor(StringUtil.class, "madam");
  assertTrue(stringUtil.isStringPalindrome());
 }
 
 @Test
 public void isStringPalindrome_palindrome_false() throws Exception{
  StringUtil stringUtil = Whitebox.invokeConstructor(StringUtil.class, "madamaa");
  assertFalse(stringUtil.isStringPalindrome());
 }
}

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