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();
    }   

}