Previous | Next | See Also

In this section:

Syntax of the @Contract annotations

The @Contract annotation has two attributes: value and pure.

The @Contract annotation value has the following syntax:

contract ::= (clause  ; )* clause
clause ::= args  ->  effect
args ::= ((arg  , )* arg )?
arg ::= value-constraint
value-constraint ::=  any  |  null  |  !null  |  false  |  true 
effect ::= value-constraint |  fail

The constraints here are:

_ - any value
null - null value
!null - a value statically proved to be not-null
true - true boolean value
false - false boolean value
fail - the method throws exception, if the arguments satisfy argument constraints

The pure attribute is intended for the methods that do not change the state of their objects, but just return a new value. This attribute is either false (by default), or true.

Example

Consider the following code:

private static void printSorted(){
            List <Integer> sorted = Quicksort.sort(null);
            if (sorted != null){
                System.out.println("Sorted array" + sorted);
        }
    }

public static <T extends Comparable<T>> List<T> sort(List<T> list)
    {
        if(list != null){
            List<T> copy = new ArrayList<T>(list);
            sort(copy);
            return copy;
    }
        else {
            return null;
        }
    }

IntelliJ IDEA doesn't complain, because it doesn't know that a null input yields a null output.

Let's decorate the sort() method with @Contract annotation, specifying that null inputs yield null outputs.

contract_annotation

IntelliJ IDEA immediately recognizes that if statement is extraneous, and reports about the condition that is always false:

false_condition_inspection

IntelliJ IDEA suggests a quick fix for the methods of the library classes:

contract_annotation_lib