Specifies some aspects of the method behavior depending on the arguments. Can be used by tools for advanced data flow analysis. Note that this annotation just describes how the code works and doesn't add any functionality by means of code generation.

Method contract has the following syntax:

contract ::= (clause ';')* clause
clause ::= args '->' effect
args ::= ((arg ',')* arg )?
arg ::= value-constraint
value-constraint ::= '_' | 'null' | '!null' | 'false' | 'true'
effect ::= value-constraint | 'fail' | 'this' | 'new' | 'param<N>'

The constraints denote the following:

• _ - any value
• null - null value
• !null - a value statically proved to be not-null
• true - true boolean value
• false - false boolean value

The additional return values denote the following:

• fail - the method throws an exception, if the arguments satisfy argument constraints
• new - (supported in IntelliJ IDEA since version 2018.2) the method returns a non-null new object which is distinct from any other object existing in the heap prior to method execution. If method is also pure, then we can be sure that the new object is not stored to any field/array and will be lost if method return value is not used.
• this - (supported in IntelliJ IDEA since version 2018.2) the method returns its qualifier value (not applicable for static methods)
• param1, param2, ... - (supported in IntelliJ IDEA since version 2018.2) the method returns its first (second, ...) parameter value

Examples:

@Contract("_, null -> null") - the method returns null if its second argument is null
@Contract("_, null -> null; _, !null -> !null") - the method returns null if its second argument is null and not-null otherwise
@Contract("true -> fail") - a typical assertFalse method which throws an exception if true is passed to it
@Contract("_ -> this") - the method always returns its qualifier (e.g. StringBuilder#append(String)). @Contract("null -> fail; _ -> param1") - the method throws an exception if the first argument is null, otherwise it returns the first argument (e.g. Objects.requireNonNull).
@Contract("!null, _ -> param1; null, !null -> param2; null, null -> fail") - the method returns the first non-null argument, or throws an exception if both arguments are null (e.g. Objects.requireNonNullElse in Java 9).

This annotation is the same provided with Jetbrains annotations and used by Nullaway for verifying nullness. We copy the annotation to avoid an external dependency.

This class is internal and is hence not for public use. Its APIs are unstable and can change at any time.