Lecture 6: Specification

1 Specification

1.1 Specification structure

specification of a method consists of several clauses:

• a precondition , indicated by the keyword requires, is an obligation on the client.
• a postcondition , indicated by the keyword effects, is an obligation on the implementer.

1.2 Specifications in Java

preconditions into @param where possible, and postconditions into @return and @throws:

/** * Find a value in an array. * @param arr array to search, requires that val occurs exactly once *            in arr * @param val value to search for * @return index i such that arr[i] = val */static int find(int[] arr, int val)

1.3 Null references

• null values are implicitly disallowed in parameters and return values.
• try to avoid null as much as possible.

1.4 Specifications for mutating methods

• a method should not modify its parameter.
• parameter could be declared with final key word.
• when writing a specification, use the convention that mutation is disallowed unless stated otherwise.

2 Exceptions

2.1 Exceptions for signaling bugs

• ArithmeticException , thrown for arithmetic errors like integer division by zero.
• NumberFormatException , thrown by methods like Integer.parseInt if you pass in a string that cannot be parsed into an integer.

2.2 Exceptions for special results

2.3 Checked and unchecked exceptions

Checked exceptions are called that because they are checked by the compiler:

• If a method might throw a checked exception, the possibility must be declared in its signature.
• If a method calls another method that may throw a checked exception, it must either handle it, or declare the exception itself, since if it isn’t caught locally it will be propagated up to callers.
• As a result, for an unchecked exception the compiler will not check for try - catch or a throws declaration.

2.4 Throwable hierarchy

• RuntimeException , Error , and their subclasses are unchecked exceptions. The compiler doesn’t require them to be declared in the throws clause of a method that throws them, and doesn’t require them to be caught or declared by a caller of such a method.
• All other throwables — Throwable , Exception , and all of their subclasses except for those of the RuntimeException and Error lineage — are checked exceptions. The compiler requires these exceptions to be caught or declared when it’s possible for them to be thrown.

2.5 Exception design considerations

• Exceptions are not just for signaling bugs. They can be used to improve the structure of code that involves procedures with special results.
• You should use an unchecked exception only to signal an unexpected failure (i.e. a bug), or if you expect that clients will usually write code that ensures the exception will not happen, because there is a convenient and inexpensive way to avoid the exception;
• Otherwise you should use a checked exception.

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Reference

[1] 6.005 — Software Construction on MIT OpenCourseWare | OCW 6.005 Homepage at https://ocw.mit.edu/ans7870/6/6.005/s16/