Accessing Private Methods and Variables in Test Class

Test methods are defined in a test class, separate from the class they test. This can present a problem when having to access a private class member variable from the test method, or when calling a private method. Because these are private, they aren’t visible to the test class. You can either modify the code in your class to expose public methods that will make use of these private class members, or you can simply annotate these private class members with TestVisible. When you annotate private or protected members with this annotation, they can be accessed by test methods and only code running in test context. 

public class VisibleSampleClass {
    // Private member variables
    @TestVisible private Integer recordNumber = 0;
    @TestVisible private String areaCode = '(415)';
    // Public member variable
    public Integer maxRecords = 1000;
    
    // Private inner class
    @TestVisible class Employee {
        String fullName;
        String phone;
        
        // Constructor
        @TestVisible Employee(String s, String ph) {
            fullName = s;
            phone = ph;
        }
    }
       
    // Private method
    @TestVisible private String privateMethod(Employee e) {
        System.debug('I am private.');
        recordNumber++;
        String phone = areaCode + ' ' + e.phone;
        String s = e.fullName + '\'s phone number is ' + phone;
        System.debug(s);
        return s;
    }
    
    // Public method
    public void publicMethod() {
        maxRecords++;
        System.debug('I am public.');    
    }
    
    // Private custom exception class
    @TestVisible private class MyException extends Exception {}
}

// Test class for VisibleSampleClass
@isTest
private class VisibleSampleClassTest {

    // This test method can access private members of another class 
    // that are annotated with @TestVisible.
    static testmethod void test1() {
        VisibleSampleClass sample = new VisibleSampleClass ();

        // Access private data members and update their values
        sample.recordNumber = 100;
        sample.areaCode = '(510)';
        
        // Access private inner class
        VisibleSampleClass.Employee emp = 
            new VisibleSampleClass.Employee('Joe Smith', '555-1212');
        
        // Call private method
        String s = sample.privateMethod(emp);
        
        // Verify result
        System.assert(
            s.contains('(510)') &&
            s.contains('Joe Smith') &&
            s.contains('555-1212'));
    }
    
    // This test method can throw private exception defined in another class
    static testmethod void test2() {
        // Throw private exception.
        try {
            throw new VisibleSampleClass.MyException('Thrown from a test.');
        } catch(VisibleSampleClass.MyException e) {
            // Handle exception 
        }
    }
    
    static testmethod void test3() {
        // Access public method.
        // No @TestVisible is used.
        VisibleSampleClass sample = new VisibleSampleClass ();
        sample.publicMethod();
    }   

}