EXP06-J. Do not use side-effecting expressions in assertions

The assert statement is a convenient mechanism for incorporating diagnostic tests in code. Expressions used with the standard assert statement must avoid side effects. Typically, the behavior of the assert statement depends on the status of a runtime property. When enabled, the assert statement is designed to evaluate its expression argument and throw an AssertionError if the result of the expression is false. When disabled, assert is defined to be a no-op; any side effects resulting from evaluation of the expression in the assertion are lost when assertions are disabled. Consequently, programs must not use side-effecting expressions in assertions.

Noncompliant Code Example

This noncompliant code is attempting to delete all the null names from the list in an assertion. However, the boolean expression is not evaluated when assertions are disabled.

private ArrayList<String> names;

void process(int index) {
  assert names.remove(null); // side-effect 
  // ...
}

Compliant Solution

Avoid the possibility of side effects in assertions. This can be achieved by decoupling the boolean expression from the assertion.

private ArrayList<String> names;

void process(int index) {
  boolean nullsRemoved = names.remove(null);
  assert nullsRemoved; // no side-effect 
  // ... 
}

Risk Assessment

Side effects in assertions result in program behavior that depends on whether assertions are enabled or disabled.

Automated Detection

Automated detection of assertion operands that contain locally visible side effects is straightforward. Some analyses could require programmer assistance to determine which method invocations lack side effects.

Related Guidelines

Bibliography

EXP05-J. Do not write more than once to the same variable within an expression      02. Expressions (EXP)      03. Numeric Types and Operations (NUM)