JUnit 4 test classes need to be public and have a public no-arg constructor or no constructor at all (implicit default constructor) and no other public constructors. JUnit 3 test classes need to be public and need either a public no-arg constructor or a public constructor with a single parameter of String type, which calls the matching super constructor. Otherwise the test classes cannot be run by standard JUnit test runners.

Example:

public class MyTest {

  private MyTest() {} // no-arg constructor is private

  @Test
  public void testSomething() {
    assertEquals(1, 1);
  }
}