IntelliJ IDEA 13.1.0 Web Help

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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.


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);
            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.


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



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


See Also


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