Error Handling

What

Well-written applications include error-handling code that allows them to recover gracefully from unexpected errors. When an error occurs, the application may need to request user intervention, or it may be able to recover on its own. In extreme cases, the application may log the user off or shut down the system. --_(source)

What

Exceptions are used to deal with 'unusual' but not entirely unexpected situations that the program might encounter at run time.

How

Most languages allow a method to encapsulate the unusual situation in an Exception object and 'throw'/'raise' that object so that another piece of code can 'catch' it and deal with it. This is especially useful when code segment that encountered the unusual situation does not know how to deal with it.

Exception objects can propagate up the method/function call hierarchy until it is dealt with. Usually, an exception thrown by a method is caught by the caller method. If the called method does not know how to deal with the exception it caught, it can throw/raise the Exception object to its own caller. If none of the callers is prepared to deal with the exception, the exceptions can propagate through the method call stack until it is received by the main method and thrown to the runtime, thus halting the system.

📦 In the code given below, processArray can potentially throw an InvalidInputException. Because of that, processInput method invokes processArray method inside a try{ } block and has a catch{ } block to specify what to do if the exception is actually thrown.

📦 In the code given below, process_array function can potentially raise a ValueError exception. Because of that, process_input function invokes process_array function inside a try clause and has a except clause to specify what to do if the exception is actually raised.

---

Advantages of exception handling in this way:

• The ability to propagate error information through the call stack.
• The separation of code that deals with 'unusual' situations from the code that does the 'usual' work.

When

In general, use exceptions only for 'unusual' conditions. Use normal return statements to pass control to the caller for conditions that are 'normal'.

What

Assertions are used to define assumptions about the program state so that the runtime can verify them. An assertion failure indicates a possible bug in the code because the code has resulted in a program state that violates an assumption about how the code should behave.

If the runtime detects an assertion failure, it typically take some drastic action such as terminating the execution with an error message. This is because an assertion failure indicates a possible bug and the sooner the execution stops, the safer it is.

int timeout = Config.getTimeout(); 

How

Use the assert keyword to define assertions.

x = getX();
assert x == 0 : "x should be 0";
...

Assertions can be disabled without modifying the code.

When

It is recommended that assertions be used liberally in the code. Their impact on performance is considered low and worth the additional safety they provide.

Do not use assertions to do work because assertions can be disabled. If not, your program will stop working when assertions are not enabled.

...
assert writeFile() : "File writing is supposed to return true";

Assertions are suitable for verifying assumptions about Internal Invariants, Control-Flow Invariants, Preconditions, Postconditions, and Class Invariants. Refer to [Programming with Assertions (second half)] to learn more.

Exceptions and assertions are two complementary ways of handling errors in software but they serve different purposes. Therefore, both assertions and exceptions should be used in code.

• The raising of an exception indicates an unusual condition created by the user  (e.g. user inputs an unacceptable input) or the environment  (e.g., a file needed for the program is missing).
• An assertion failure indicates the programmer made a mistake in the code  (e.g., a null value is returned from a method that is not supposed to return null under any circumstances).

What

Logging is the deliberate recording of certain information during a program execution for future reference. Logs are typically written to a log file but it is also possible to log information in other ways  e.g. into a database or a remote server.

Logging can be useful for troubleshooting problems. A good logging system records some system information regularly. When bad things happen to a system  e.g. an unanticipated failure, their associated log files may provide indications of what went wrong and action can then be taken to prevent it from happening again.

How

Most programming environments come with logging systems that allow sophisticated forms of logging. They have features such as the ability to enable and disable logging easily or to change the logging intensity.

import java.util.logging.*;

private static Logger logger = Logger.getLogger("Foo");

// log a message at INFO level
logger.log(Level.INFO, "going to start processing");
//...
processInput();
if(error){
    //log a message at WARNING level
    logger.log(Level.WARNING, "processing error", ex);
}
//...
logger.log(Level.INFO, "end of processing");

What

A defensive programmer codes under the assumption "if we leave room for things to go wrong, they will go wrong". Therefore, a defensive programmer proactively tries to eliminate any room for things to go wrong.

class MainApp{
    Config config;
    
    /** Returns the config object */
    Config getConfig(){
        return config;
    }
}

    /** Returns a copy of the config object */
    Config getConfig(){
        return config.copy(); //return a defensive copy
    }

Enforcing Compulsory Associations

Consider two classes, Account and Guarantor, with an association as shown in the following diagram:

Example:

Here, the association is compulsory i.e. an Account object should always be linked to a Guarantor. One way to implement this is to simply use a reference variable, like this:

class Account {
    Guarantor guarantor;

    void setGuarantor(Guarantor g) {
        guarantor = g;
    }
}

However, what if someone else used the Account class like this?

Account a = new Account();
a.setGuarantor(null);

This results in an Account without a Guarantor! In a real banking system, this could have serious consequences! The code here did not try to prevent such a thing from happening. We can make the code more defensive by proactively enforcing the multiplicity constraint, like this:

class Account {
    private Guarantor guarantor;

    public Account(Guarantor g){
        if (g == null) {
            stopSystemWithMessage("multiplicity violated. Null Guarantor");
        }
        guarantor = g;
    }
    public void setGuarantor (Guarantor g){
        if (g == null) {
            stopSystemWithMessage("multiplicity violated. Null Guarantor");
        }
        guarantor = g;
    }
    …
}

Enforcing 1-to-1 Associations

Consider the association given below. A defensive implementation requires to ensure a MinedCell cannot exist without a Mine and vice versa which requires simultaneous object creation. However, Java can only create one object at a time. Given below are two alternatives implementations, both of which violate the multiplicity for a short period of time.

Option 1:

class MinedCell {
    private Mine mine;

    public MinedCell(Mine m){
        if (m == null) {
            showError();
        }
        mine = m;
    }
    …
}

Option 1 forces us to keep a Mine without a MinedCell (until the MinedCell is created).

Option 2:

class MinedCell {
    private Mine mine;

    public MinedCell(){
        mine = new Mine();
    }
    …
}

Option 2 is more defensive because the Mine is immediately linked to a MinedCell.

Enforcing Referential Integrity

A bidirectional association in the design (shown in (a)) is usually emulated at code level using two variables (as shown in (b)).

class Man {
    Woman girlfriend;

    void setGirlfriend(Woman w) {
        girlfriend = w;
    }
    …
}

class Woman {
    Man boyfriend;

    void setBoyfriend(Man m) {
        boyfriend = m;
    }
}

The two classes are meant to be used as follows:

Woman jean;
Man james;
…
james.setGirlfriend(jean);
jean.setBoyfriend(james);

Suppose the two classes were used this instead:

Woman jean; Man james, yong;
…
james.setGirlfriend(jean);  
jean.setBoyfriend(yong);  

Now James' girlfriend is Jean, while Jean's boyfriend is not James. This situation results as the code was not defensive enough to stop this "love triangle". In such a situation, we say that the referential integrity has been violated. It simply means there is an inconsistency in object references.

One way to prevent this situation is to implement the two classes as shown below. Note how the referential integrity is maintained.

public class Woman {
    private Man boyfriend;

    public void setBoyfriend(Man m) {
        if(boyfriend == m){
            return;
        }
        if (boyfriend != null) {
            boyfriend.breakUp();
        }
        boyfriend = m;
        m.setGirlfriend(this);
    }

    public void breakUp() {
        boyfriend = null;
    }   
    ...
}

public class Man{
    private Woman girlfriend;

    public void setGirlfriend(Woman w) {
        if(girlfriend == w){
            return;
        }
        if (girlfriend != null) {
            girlfriend.breakUp();
        }
        girlfriend = w;
        w.setBoyfriend(this);
    }
    public void breakUp() {
        girlfriend = null;
    }  
   ...
}

When the code james.setGirlfriend(jean) is executed, the code ensures that james break up with any current girlfriend before he accepts jean as the girlfriend. Furthermore, the code ensures that jean breaks up with any existing boyfriends and accepts james as the boyfriend.

When

It is not necessary to be 100% defensive all the time. While defensive code may be less prone to be misused or abused, such code can also be more complicated and slower to run.

The suitable degree of defensiveness depends on many factors such as:

• How critical is the system?
• Will the code be used by programmers other than the author?
• The level of programming language support for defensive programming
• The overhead of being defensive

Design by Contract

Design by contract (DbC) is an approach for designing software that requires defining formal, precise and verifiable interface specifications for software components.

Suppose an operation is implemented with the behavior specified precisely in the API (preconditions, post conditions, exceptions etc.). When following the defensive approach, the code should first check if the preconditions have been met. Typically, exceptions are thrown if preconditions are violated. In contrast, the Design-by-Contract (DbC) approach to coding assumes that it is the responsibility of the caller to ensure all preconditions are met. The operation will honor the contract only if the preconditions have been met. If any of them have not been met, the behavior of the operation is "unspecified".

Languages such as Eiffel have native support for DbC. For example, preconditions of an operation can be specified in Eiffel and the language runtime will check precondition violations without the need to do it explicitly in the code. To follow the DbC approach in languages such as Java and C++ where there is no built-in DbC support, assertions can be used to confirm pre-conditions.