Ensure the Test Class is Public

Your test class must be declared as public:

package com.testingdocs.tutorials;

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

public class TestCaseOne { // Ensure the class is public

 @Test
 public void testExample() {
 assertTrue(true);
 }
}
 

Ensure There is Only One Public Constructor

JUnit test classes should have only one public constructor. If you have multiple constructors, remove the extra ones.

Incorrect:

public class TestCaseOne {
 public TestCaseOne() {} // Default constructor (OK)
 public TestCaseOne(String param) {} // Extra constructor (INVALID)
}
 

Correct:

public class TestCaseOne {
 public TestCaseOne() {} // OK
}
 

Ensure JUnit is Using the Correct Runner

If you are using JUnit 4, add the correct runner annotation:

import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;

@RunWith(JUnit4.class)
public class TestCaseOne {
 @Test
 public void testExample() {
 assertTrue(true);
 }
}
 

For JUnit 5 (Jupiter), no runner annotation is needed:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertTrue;

public class TestCaseOne {
 @Test
 void testExample() {
 assertTrue(true);
 }
}
 

Ensure the Test Class is Not a Nested (Inner) Class

If your test class is an inner class, it must be static:

public class OuterClass {
 public static class TestCaseOne { // Must be static
 @Test
 public void testExample() {
 assertTrue(true);
 }
 }
}