Perhaps, there are other runtime exceptions you wish to prevent or maybe other properties of data that should always hold. In both these cases and others, you you might wish to write your own type-checker. This section of the tutorial is for those who are interested in writing their own type-checkers. If you do not wish to write a new type-checker, feel free to skip this section.
Suppose that you wish to only allow encrypted information to be sent over the internet. To do so, you can write an Encryption Checker.
This project has two source files: Encrypted.java, the type qualifier definition, and EncryptedDemo.java, the example program that use the Encrypted checker. You will need to setup your JRE to use the Checker Framework jars, see Issue 1 below. This will fix the "TYPE_USE cannot be resolved or is not a field" error. The import myqual.Encrypted is never used warning should be ignored.
The Encryption Checker is built on top of the Subtyping Checker. Use the following steps to setup it up with in the Eclipse plugin.
Right Click on the src package, select Checker Framework->Run Custom Checker->Subtyping Checker. You should see the following errors:
incompatible types in assignment. /*@Encrypted*/ int encryptInt = (character + OFFSET) % Character.MAX_VALUE ; found : int required: @Encrypted int EncryptionDemo.java /encryption-checker/src/encrypted incompatible types in argument. sendOverInternet(password); found : String required: @Encrypted String EncryptionDemo.java /encryption-checker/src/encrypted
The first error needs to be suppressed, because the string on the
left is considered "encrypted" in this encryption scheme. All
@SuppressWarnings
should have a comment explaining why suppressing the warning is
the correct action. The comment should also be initialed and
dated. See the correction below.
// The SuppressWarnings is necessary because, the type system is not capable of
//validating that the return value is encrypted. -SOM 01/25/2013
@SuppressWarnings("encrypted")
private /*@Encrypted*/ char encryptCharacter(char character) {
You will see the following error:
incompatible types in argument. sendOverInternet(password); found : String required: @Encrypted String EncryptionDemo.java /encryption-checker/src/encrypted
This is a real error, because the programmer is trying to send a password over the internet without encrypting it first.
The password should be encrypted before it is sent over the internet. The correction is below.
void sendPassword() {
String password = getUserPassword();
sendOverInternet(encrypt(password));
}
Writing a Type annotation requires ElementType.TYPE_USE and/or ElementType.TYPE_PARAMETER which are part of Java 8, but not yet supported by Eclipse. This means that Eclipse will give an error: "TYPE_USE cannot be resolved or is not a field."