![]() Therefore b <= 255 still holds and the loop restarts from 0.įor unsigned integer values, the operations +, -, *, and << are computed modulo 2 n, This overflows (since the maximum value for a byte is 255) and results in b = 0. Since ArithmeticException inherits from RuntimeException, we do not need to declare it in a throws clause.Why does this loop run forever? var b byteĪfter the b = 255 iteration, b++ is executed. ![]() These methods throw an exception when an integer overflow would otherwise occur any other conforming error handling is also acceptable. The checks for the other integral types are analogous. The following code example shows the necessary precondition checks required for each arithmetic operation on arguments of type int. ![]() Operations on objects of type BigInteger can also be significantly less efficient than operations on the original primitive integer type. However, it requires the use of method calls for each operation in place of primitive arithmetic operators, which can obscure the intended meaning of the code. The BigInteger technique is conceptually the simplest of the three techniques because arithmetic operations on BigInteger cannot overflow. Unfortunately, it cannot be applied to operations involving type long, as there is no bigger type to upcast to. The checks it requires are simpler than those of the previous technique it is substantially more efficient than using BigInteger. The upcast technique is the preferred approach when applicable. This approach can be somewhat more difficult to implement and to audit than either of the other two approaches. The precondition testing technique requires different precondition tests for each arithmetic operation. Consequently, compliant code performs only a single range check just before converting the final result to the original smaller type and throws an ArithmeticException if the final result is outside the range of the original smaller type. The arithmetic operations implemented as methods of this type cannot overflow instead, they produce the numerically correct result. Type BigInteger is the standard arbitrary-precision integer type provided by the Java standard libraries. Convert the inputs into objects of type BigInteger and perform all arithmetic using BigInteger methods. This approach cannot be used for type long because long is already the largest primitive integer type. ![]() Downcast the final result to the original smaller type before assigning to a variable of the original smaller type. Note that the range check must be performed after each arithmetic operation larger expressions without per-operation bounds checking can overflow the larger type. Check each intermediate result for overflow of the original smaller type and throw an ArithmeticException if the range check fails. Cast the inputs to the next larger primitive integer type and perform the arithmetic in the larger size. Throw an ArithmeticException when the operation would overflow if it were performed otherwise, perform the operation. Check the inputs to each arithmetic operator to ensure that overflow cannot occur. Comparison of Compliant Techniquesįollowing are the three main techniques for detecting unintended integer overflow: For operands of opposite signs, this approach can result in integer overflow, consequently violating the compareTo() contract. Consequently, an apparent but incorrect optimization would be to subtract the operands and return the result. The meaning of the return value of the compareTo() method is defined only in terms of its sign and whether it is zero the magnitude of the return value is irrelevant. Failure to account for integer overflow has resulted in failures of real systems, for example, when implementing the compareTo() method. The silent wrap can result in incorrect computations and unanticipated outcomes. When a mathematical operation cannot be represented using the supplied integer types, Java's built-in integer operators silently wrap the result without indicating overflow. Because the () method returns the absolute value of any number, it can also overflow if given the minimum int or long as an argument. Because the ranges of Java types are not symmetric (the negation of each minimum value is one more than each maximum value), even operations such as unary negation can overflow if applied to a minimum value.
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