Testing

What

When testing, we execute a set of test cases. A test case specifies how to perform a test. At a minimum, it specifies the input to the software under test (SUT) and the expected behavior.

Test cases can be determined based on the specification, reviewing similar existing systems, or comparing to the past behavior of the SUT.

For each test case we do the following:

1. Feed the input to the SUT
2. Observe the actual output
3. Compare actual output with the expected output

A test case failure is a mismatch between the expected behavior and the actual behavior. A failure is caused by a defect (or a bug).

Testability

Testability is an indication of how easy it is to test an SUT. As testability depends a lot on the design and implementation. You should try to increase the testability when you design and implement a software. The higher the testability, the easier it is to achieve a better quality software.

What

Unit testing : testing individual units (methods, classes, subsystems, ...) to ensure each piece works correctly.

In OOP code, it is common to write one or more unit tests for each public method of a class.

class Foo{
    String read(){
        //...
    }
    
    void write(String input){
        //...
    }
    
}

class FooTest{
    
    @Test
    void read(){
        //a unit test for Foo#read() method
    }
    
    @Test
    void write_emptyInput_exceptionThrown(){
        //a unit tests for Foo#write(String) method
    }  
    
    @Test
    void write_normalInput_writtenCorrectly(){
        //another unit tests for Foo#write(String) method
    }
}

Stubs

A proper unit test requires the unit to be tested in isolation so that bugs in the dependencies cannot influence the test  i.e. bugs outside of the unit should not affect the unit tests.

Stubs can isolate the SUT from its dependencies.

class Logic {
    Storage s;

    Logic(Storage s) {
        this.s = s;
    }

    String getName(int index) {
        return "Name: " + s.getName(index);
    }
}

interface Storage {
    String getName(int index);
}

class DatabaseStorage implements Storage {

    @Override
    public String getName(int index) {
        return readValueFromDatabase(index);
    }

    private String readValueFromDatabase(int index) {
        // retrieve name from the database
    }
}

Logic logic = new Logic(new DatabaseStorage());
String name = logic.getName(23);

@Test
void getName() {
    Logic logic = new Logic(new DatabaseStorage());
    assertEquals("Name: John", logic.getName(5));
}

class StorageStub implements Storage {

    @Override
    public String getName(int index) {
        if(index == 5) {
            return "Adam";
        } else {
            throw new UnsupportedOperationException();
        }
    }
}

@Test
void getName() {
    Logic logic = new Logic(new StorageStub());
    assertEquals("Name: Adam", logic.getName(5));
}

In addition to Stubs, there are other type of replacements you can use during testing. E.g. Mocks, Fakes, Dummies, Spies.

What

Integration testing : testing whether different parts of the software work together (i.e. integrates) as expected. Integration tests aim to discover bugs in the 'glue code' related to how components interact with each other. These bugs are often the result of misunderstanding of what the parts are supposed to do vs what the parts are actually doing.

What

System testing: take the whole system and test it against the system specification.

System testing is typically done by a testing team (also called a QA team).

System test cases are based on the specified external behavior of the system. Sometimes, system tests go beyond the bounds defined in the specification. This is useful when testing that the system fails 'gracefully' having pushed beyond its limits.

Test case: load a web page that is too big
* Input: load a web page containing more than 5000 characters. 
* Expected behavior: abort the loading of the page and show a meaningful error message. 

System testing includes testing against non-functional requirements too. Here are some examples.

• Performance testing – to ensure the system responds quickly.
• Load testing (also called stress testing or scalability testing) – to ensure the system can work under heavy load.
• Security testing – to test how secure the system is.
• Compatibility testing, interoperability testing – to check whether the system can work with other systems.
• Usability testing – to test how easy it is to use the system.
• Portability testing – to test whether the system works on different platforms.

What

Alpha testing is performed by the users, under controlled conditions set by the software development team.

Beta testing is performed by a selected subset of target users of the system in their natural work setting.

An open beta release is the release of not-yet-production-quality-but-almost-there software to the general population. For example, Google’s Gmail was in 'beta' for many years before the label was finally removed.

What

Eating your own dog food (aka dogfooding), is a creators of a product use their own product to test the product.

What

Developer testing is the testing done by the developers themselves as opposed to professional testers or end-users.

Why

Delaying testing until the full product is complete has a number of disadvantages:

• Locating the cause of such a test case failure is difficult due to a large search space; in a large system, the search space could be millions of lines of code, written by hundreds of developers! The failure may also be due to multiple inter-related bugs.
• Fixing a bug found during such testing could result in major rework, especially if the bug originated during the design or during requirements specification (i.e. a faulty design or faulty requirements).
• One bug might 'hide' other bugs, which could emerge only after the first bug is fixed.
• The delivery may have to be delayed if too many bugs were found during testing.

Therefore, it is better to do early testing, as hinted by the popular rule of thumb given below, also illustrated by the graph below it.

The earlier a bug is found, the easier and cheaper to have it fixed.

Such early testing of partially developed software is usually, and by necessity, done by the developers themselves i.e. developer testing.

What

Here are two alternative approaches to testing a software: Scripted testing and Exploratory testing

1. Scripted testing: First write a set of test cases based on the expected behavior of the SUT, and then perform testing based on that set of test cases.

2. Exploratory testing: Devise test cases on-the-fly, creating new test cases based on the results of the past test cases.

Exploratory testing is ‘the simultaneous learning, test design, and test execution’ [source: bach-et-explained] whereby the nature of the follow-up test case is decided based on the behavior of the previous test cases. In other words, running the system and trying out various operations. It is called exploratory testing because testing is driven by observations during testing. Exploratory testing usually starts with areas identified as error-prone, based on the tester’s past experience with similar systems. One tends to conduct more tests for those operations where more faults are found.

When

Which approach is better – scripted or exploratory? A mix is better.

The success of exploratory testing depends on the tester’s prior experience and intuition. Exploratory testing should be done by experienced testers, using a clear strategy/plan/framework. Ad-hoc exploratory testing by unskilled or inexperienced testers without a clear strategy is not recommended for real-world non-trivial systems. While exploratory testing may allow us to detect some problems in a relatively short time, it is not prudent to use exploratory testing as the sole means of testing a critical system.

Scripted testing is more systematic, and hence, likely to discover more bugs given sufficient time, while exploratory testing would aid in quick error discovery, especially if the tester has a lot of experience in testing similar systems.

In some contexts, you will achieve your testing mission better through a more scripted approach; in other contexts, your mission will benefit more from the ability to create and improve tests as you execute them. I find that most situations benefit from a mix of scripted and exploratory approaches. -- [source: bach-et-explained]

What

Acceptance testing (aka User Acceptance Testing (UAT)): test the delivered system to ensure it meets the user requirements.

Acceptance tests give an assurance to the customer that the system does what it is intended to do. Acceptance test cases are often defined at the beginning of the project, usually based on the use case specification. Successful completion of UAT is often a prerequisite to the project sign-off.

Acceptance vs System Testing

Acceptance testing comes after system testing. Similar to system testing, acceptance testing involves testing the whole system.

Some differences between system testing and acceptance testing:

Note: negative test cases: cases where the SUT is not expected to work normally e.g. incorrect inputs; positive test cases: cases where the SUT is expected to work normally

Passing system tests does not necessarily mean passing acceptance testing. Some examples:

• The system might work on the testbed environments but might not work the same way in the deployment environment, due to subtle differences between the two environments.
• The system might conform to the system specification but could fail to solve the problem it was supposed to solve for the user, due to flaws in the system design.

What

When we modify a system, the modification may result in some unintended and undesirable effects on the system. Such an effect is called a regression.

Regression testing is re-testing the SUT to detect regressions. Noted that to detect regressions, we need to retest all related components, even if they were tested before.

Regression testing is more effective when it is done frequently, after each small change. However, doing so can be prohibitively expensive if testing is done manually. Hence, regression testing is more practical when it is automated.

What

Automated Testing of CLI Apps

A simple way to semi-automate testing of a CLI app is using input/output re-direction.

• First, we feed the app with a sequence of test inputs that is stored in a file while redirecting the output to another file.
  e.g., java AddressBook < input.txt > output.txt
• Next, we compare the actual output file with another file containing the expected output.
  e.g., FC output.txt expected.txt

Let us assume we are testing a CLI app called AddressBook (Example: se-edu/addressbook-level1). Here are the detailed steps:

1. Store the test input in the text file input.txt.

   add Valid Name p/12345 valid@email.butNoPrefix
   add Valid Name 12345 e/valid@email.butPhonePrefixMissing
   

2. Store the output we expect from the SUT in another text file expected.txt.

   Command: || [add Valid Name p/12345 valid@email.butNoPrefix]
   Invalid command format: add 
   
   Command: || [add Valid Name 12345 e/valid@email.butPhonePrefixMissing]
   Invalid command format: add 
   

3. Run the program as given below, which will redirect the text in input.txt as the input to AddressBook and similarly, will redirect the output of AddressBook to a text file output.txt. Note that this does not require any code changes to AddressBook.

   java AddressBook < input.txt > output.txt
   

   A CLI program takes input from the keyboard and outputs to the console. That is because those two are default input and output streams, respectively. But you can change that behavior using < and > operators. For example, if you run AddressBook in the DOS prompt, the output will be shown in the console, but if you run it like this,

   java AddressBook > output.txt 
   

   the Operating System then creates a file output.txt and stores the output in that file instead of displaying it in the console. No file I/O coding is required. Similarly, adding < input.txt (or any other filename) makes the OS redirect the contents of the file as input to the program, as if the user typed the content of the file one line at a time.

   📎 Resources:

   • Using command redirection operators in Windows

4. Next, we compare output.txt with the expected.txt. This can be done using a utility such as Windows FC (i.e. File Compare) command, Unix diff command, or a GUI tool such as Winmerge.

   FC output.txt expected.txt
   

Note that the above technique is only suitable when testing CLI apps, and only if the exact output can be predetermined. If the output varies from one run to the other (e.g. it contains a time stamp), this technique will not work. In those cases we need more sophisticated ways of automating tests.

Test Automation Using Test Drivers

A test driver is the code that ‘drives’ the SUT for the purpose of testing i.e. invoking the SUT with test inputs and verifying the behavior is as expected.

public class PayrollTestDriver {
    public static void main(String[] args) throws Exception {

        //test setup
        Payroll p = new Payroll();

        //test case 1
        p.setEmployees(new String[]{"E001", "E002"});
        // automatically verify the response
        if (p.totalSalary() != 6400) {
            throw new Error("case 1 failed ");
        }

        //test case 2
        p.setEmployees(new String[]{"E001"});
        if (p.totalSalary() != 2300) {
            throw new Error("case 2 failed ");
        }

        //more tests...

        System.out.println("All tests passed");
    }
}

Test Automation Tools

JUnit is a tool for automated testing of Java programs. Similar tools are available for other languages.

public class PayrollTestJUnit {

    @Test
    public void testTotalSalary(){
        Payroll p = new Payroll();

        //test case 1
        p.setEmployees(new String[]{"E001", "E002"});
        assertEquals(p.totalSalary(), 6400);

        //test case 2
        p.setEmployees(new String[]{"E001"});
        assertEquals(p.totalSalary(), 2300);

        //more tests...
    }
}

Most modern IDEs has integrated support for testing tools. The figure below shows the JUnit output when running some JUnit tests using the Eclipse IDE.

Automated Testing of GUIs

If a software product has a GUI component, all product-level testing (i.e. the types of testing mentioned above) need to be done using the GUI. However, testing the GUI is much harder than testing the CLI (command line interface) or API, for the following reasons:

• Most GUIs can support a large number of different operations, many of which can be performed in any arbitrary order.
• GUI operations are more difficult to automate than API testing. Reliably automating GUI operations and automatically verifying whether the GUI behaves as expected is harder than calling an operation and comparing its return value with an expected value. Therefore, automated regression testing of GUIs is rather difficult.
• The appearance of a GUI (and sometimes even behavior) can be different across platforms and even environments. For example, a GUI can behave differently based on whether it is minimized or maximized, in focus or out of focus, and in a high resolution display or a low resolution display.

One approach to overcome the challenges of testing GUIs is to minimize logic aspects in the GUI. Then, bypass the GUI to test the rest of the system using automated API testing. While this still requires the GUI to be tested manually, the number of such manual test cases can be reduced as most of the system has been tested using automated API testing.

There are testing tools that can automate GUI testing.

What

Test coverage is a metric used to measure the extent to which testing exercises the code i.e., how much of the code is 'covered' by the tests.

Here are some examples of different coverage criteria:

• Function/method coverage : based on functions executed  e.g., testing executed 90 out of 100 functions.
• Statement coverage : based on the number of line of code executed  e.g., testing executed 23k out of 25k LOC.
• Decision/branch coverage : based on the decision points exercised  e.g., an if statement evaluated to both true and false with separate test cases during testing is considered 'covered'.
• Condition coverage : based on the boolean sub-expressions, each evaluated to both true and false with different test cases. Condition coverage is not the same as the decision coverage.

• Path coverage measures coverage in terms of possible paths through a given part of the code executed. 100% path coverage means all possible paths have been executed. A commonly used notation for path analysis is called the Control Flow Graph (CFG).
• Entry/exit coverage measures coverage in terms of possible calls to and exits from the operations in the SUT.

How

Measuring coverage is often done using coverage analysis tools. Most IDEs have inbuilt support for measuring test coverage, or at least have plugins that can measure test coverage.

Coverage analysis can be useful in improving the quality of testing  e.g., if a set of test cases does not achieve 100% branch coverage, more test cases can be added to cover missed branches.

What

Dependency injection is the process of 'injecting' objects to replace current dependencies with a different object. This is often used to inject stubs to isolate the SUT from its dependencies so that it can be tested in isolation.

class Logic {
    Storage s;

    Logic(Storage s) {
        this.s = s;
    }

    String getName(int index) {
        return "Name: " + s.getName(index);
    }
}

interface Storage {
    String getName(int index);
}

class DatabaseStorage implements Storage {

    @Override
    public String getName(int index) {
        return readValueFromDatabase(index);
    }

    private String readValueFromDatabase(int index) {
        // retrieve name from the database
    }
}

Logic logic = new Logic(new DatabaseStorage());
String name = logic.getName(23);

@Test
void getName() {
    Logic logic = new Logic(new DatabaseStorage());
    assertEquals("Name: John", logic.getName(5));
}

class StorageStub implements Storage {

    @Override
    public String getName(int index) {
        if(index == 5) {
            return "Adam";
        } else {
            throw new UnsupportedOperationException();
        }
    }
}

@Test
void getName() {
    Logic logic = new Logic(new StorageStub());
    assertEquals("Name: Adam", logic.getName(5));
}

📦 A Foo object normally depends on a Bar object, but we can inject a BarStub object so that the Foo object no longer depends on a Bar object. Now we can test the Foo object in isolation from the Bar object.

How

Polymorphism can be used to implement dependency injection, as can be seen in the example given in [Quality Assurance → Testing → Unit Testing → Stubs] where a stub is injected to replace a dependency.

class Logic {
    Storage s;

    Logic(Storage s) {
        this.s = s;
    }

    String getName(int index) {
        return "Name: " + s.getName(index);
    }
}

interface Storage {
    String getName(int index);
}

class DatabaseStorage implements Storage {

    @Override
    public String getName(int index) {
        return readValueFromDatabase(index);
    }

    private String readValueFromDatabase(int index) {
        // retrieve name from the database
    }
}

Logic logic = new Logic(new DatabaseStorage());
String name = logic.getName(23);

@Test
void getName() {
    Logic logic = new Logic(new DatabaseStorage());
    assertEquals("Name: John", logic.getName(5));
}

class StorageStub implements Storage {

    @Override
    public String getName(int index) {
        if(index == 5) {
            return "Adam";
        } else {
            throw new UnsupportedOperationException();
        }
    }
}

@Test
void getName() {
    Logic logic = new Logic(new StorageStub());
    assertEquals("Name: Adam", logic.getName(5));
}

class Payroll {
    private SalaryManager manager = new SalaryManager();
    private String[] employees;

    void setEmployees(String[] employees) {
        this.employees = employees;
    }

    void setSalaryManager(SalaryManager sm) {
       this. manager = sm;
    }

    double totalSalary() {
        double total = 0;
        for(int i = 0;i < employees.length; i++){
            total += manager.getSalaryForEmployee(employees[i]);
        }
        return total;
    }
}


class SalaryManager {
    double getSalaryForEmployee(String empID){
        //code to access employee’s salary history
        //code to calculate total salary paid and return it
    }
}

class PayrollTest {
    public static void main(String[] args) {
        //test setup
        Payroll p = new Payroll();
        p.setSalaryManager(new SalaryManagerStub()); //dependency injection
        //test case 1
        p.setEmployees(new String[]{"E001", "E002"});
        assertEquals(2500.0, p.totalSalary());
        //test case 2
        p.setEmployees(new String[]{"E001"});
        assertEquals(1000.0, p.totalSalary());
        //more tests ...
    }
}


class SalaryManagerStub extends SalaryManager {
    /** Returns hard coded values used for testing */
    double getSalaryForEmployee(String empID) {
        if(empID.equals("E001")) {
            return 1000.0;
        } else if(empID.equals("E002")) {
            return 1500.0;
        } else {
            throw new Error("unknown id");
        }
    }
}

What

Test-Driven Development(TDD)_ advocates writing the tests before writing the SUT, while evolving functionality and tests in small increments. In TDD you first define the precise behavior of the SUT using test cases, and then write the SUT to match the specified behavior. While TDD has its fair share of detractors, there are many who consider it a good way to reduce defects. One big advantage of TDD is that it guarantees the code is testable.

How

Note that TDD does not imply writing all the test cases first before writing functional code. Rather, proceed in small steps:

1. Decide what behavior to implement.
2. Write test cases to test that behavior.
3. Run those test cases and watch them fail.
4. Implement the behavior.
5. Run the test case.
6. Keep modifying the code and rerunning test cases until they all pass.
7. Refactor code to improve quality.
8. Repeat the cycle for each small unit of behavior that needs to be implemented.