Defensive Programming

CS 240 – Advanced Programming Concepts

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Defensive Programming: Assertions

Defensive Programming

• Good programming practices that protect you from your own programming mistakes, as well as those of others
• Assertions
• Parameter Checking

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Programming Assumptions

• As we program, we make many assumptions about the state of the program at each point in the code
• A variable's value is in a particular range
• A file exists, is writable, is open, etc.
• Some data is sorted
• A network connection to another machine was successfully opened
• …
• The correctness of our program depends on the validity of our assumptions
• Faulty assumptions result in buggy, unreliable code

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Programming Assumptions Example

int binarySearch(int[] data, int searchValue) {

// What assumptions are we making about the parameter values?

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…

}

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Programming Assumptions Example

int binarySearch(int[] data, int searchValue) {

// What assumptions are we making about the parameter values?

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…

}

• data != null
• data is sorted

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• What happens if these assumptions are wrong?

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Assertions

• Assertions give us a way to make our assumptions explicit in the code

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• assert temperature > 32 && temperature < 212;

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• The parameter to assert is a boolean condition that should be true
• assert condition;

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• If the condition is false, Java throws an AssertionError, which crashes the program

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• Stack trace tells you where the failed assertion is in the code

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Assertion Examples

int binarySearch(int[] data, int searchValue) {

assert data != null;

assert isSorted(data);

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…

}

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String[] someMethod(int y, int z) {

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assert z != 0;

int x = y / z;

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assert x > 0 && x < 1024;

return new String[x];

}

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Assertion Details

• Assertions are little test cases sprinkled throughout your code that alert you when one of your assumptions is wrong
• This is a powerful tool for avoiding and finding bugs

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• Assertions are usually disabled in released software

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• In Java, assertions are DISABLED by default
• To enable them, run the program with the –enableassertions (or -ea) option

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• java –enableassertions MyApp
• java –ea MyApp

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• In Intellij, the –enableassertions option can be specified in the VM options section of the Run/Debug Configurations dialog

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Alternate Assertion Form

• Alternate form of assert
• assert condition : expression;
• If condition is false, expression is passed to the constructor of the thrown AssertionError

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int binarySearch(int[] data, int searchValue) {

assert data != null : ”binary search data is null”;

assert isSorted(data) : ”binary search data is not sorted”;

…

}

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String[] someMethod(int y, int z) {

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assert z != 0 : ”invalid z value”;

int x = y / z;

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assert x > 0 && x < 1024 : x;

return new String[x];

}

Exceptions?

• If one of my assumptions is wrong, shouldn't I throw an exception?

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• No. You should fix the bug, not throw an exception.

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Defensive Programming: Parameter Checking

Parameter Checking Overview

• Another important defensive programming technique is "parameter checking"

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• A method or function should always check its input parameters to ensure that they are valid

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• If they are invalid, it should indicate that an error has occurred rather than proceeding

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• This prevents errors from propagating through the code before they are detected

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• By detecting the error close to the place in the code where it originally occurred, debugging is greatly simplified

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Check Parameter Values

• Two ways to check parameter values
• Assertions
• if statements that throw exception if parameter is invalid

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int binarySearch(int[] data, int searchValue) {

assert data != null;

assert isSorted(data);

…

}

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int binarySearch(int[] data, int searchValue) {

if (data == null || !isSorted(data)) {

throw new InvalidArgumentException();

}

…

}

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Assertions or Parameter Checking?

• Should I use assertions or if/throw to check parameters?

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• If you have control over the calling code, use assertions
• If parameter is invalid, you can fix the calling code

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• If you don't have control over the calling code, throw exceptions
• e.g., your product might be a class library that is called by code you don’t control

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