MET01-J. Never use assertions to validate method arguments

Never use assertions to validate arguments of public methods. The Java Language Specification, §14.10, "The assert Statement" [JLS 2015], states that

assertions should not be used for argument checking in public methods. Argument checking is typically part of the contract of a method, and this contract must be upheld whether assertions are enabled or disabled.

A secondary problem with using assertions for argument checking is that erroneous arguments should result in an appropriate run-time exception (such as IllegalArgumentException, IndexOutOfBoundsException, or NullPointerException). An assertion failure will not throw an appropriate exception.

Noncompliant Code Example

The method getAbsAdd() computes and returns the sum of the absolute value of parameters x and y. It lacks argument validation, in violation of MET00-J. Validate method arguments. Consequently, it can produce incorrect results because of integer overflow or when either or both of its arguments are Integer.MIN_VALUE.

public static int getAbsAdd(int x, int y) {
  return Math.abs(x) + Math.abs(y);
}
getAbsAdd(Integer.MIN_VALUE, 1);

Noncompliant Code Example

This noncompliant code example uses assertions to validate arguments of a public method:

public static int getAbsAdd(int x, int y) {
  assert x != Integer.MIN_VALUE;
  assert y != Integer.MIN_VALUE;
  int absX = Math.abs(x);
  int absY = Math.abs(y);
  assert (absX <= Integer.MAX_VALUE - absY);
  return absX + absY;
}

The conditions checked by the assertions are reasonable. However, the validation code is not executed when assertions are disabled.

Compliant Solution

This compliant solution validates the method arguments by ensuring that values passed to Math.abs() exclude Integer.MIN_VALUE and also by checking for integer overflow:

public static int getAbsAdd(int x, int y) {
  if (x == Integer.MIN_VALUE || y == Integer.MIN_VALUE) {
    throw new IllegalArgumentException();
  }
  int absX = Math.abs(x);
  int absY = Math.abs(y);
  if (absX > Integer.MAX_VALUE - absY) {
    throw new IllegalArgumentException();
  }
  return absX + absY;
}

Alternatively, the addition could be performed using type long and the result of the addition stored in a local variable of type long. This alternate implementation would require a check to ensure that the resulting long can be represented in the range of the type int. Failure of this latter check would indicate that an int version of the addition would have overflowed.

Risk Assessment

Using assertions to validate method arguments can result in inconsistent computations, runtime exceptions, and control flow vulnerabilities.

Related Guidelines

Android Implementation Details

The assert statement is supported on the Dalvik VM but is ignored under the default configuration. Assertions may be enabled by setting the system property debug.assert via adb shell setprop debug.assert 1 or by sending the command-line argument --enable-assert to the Dalvik VM.

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