CodeQL documentation

Type mismatch on container access

ID: java/type-mismatch-access
Kind: problem
Security severity: 
Severity: error
Precision: very-high
   - reliability
   - correctness
   - logic
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

The contains method of the Collection interface has an argument of type Object. Therefore, you can try to check if an object of any type is a member of a collection, regardless of the collection’s element type. However, although you can call contains with an argument of a different type than that of the collection, it is unlikely that the collection actually contains an object of this type.

Similar considerations apply to other container access methods, such as Map.get, where the argument may also have type Object.


Ensure that you use the correct argument with a call to contains.


In the following example, although the argument to contains is an integer, the code does not result in a type error because the argument does not have to match the type of the elements of list. However, the argument is unlikely to be found (and the body of the if statement is therefore not executed), so it is probably a typographical error: the argument should be enclosed in quotation marks.

void m(List<String> list) {
	if (list.contains(123)) {  // Call 'contains' with non-string argument (without quotation marks)
		// ...

Note that you must take particular care when working with collections over boxed types, as illustrated in the following example. The first call to contains returns false because you cannot compare two boxed numeric primitives of different types, in this case Short(1) (in set) and Integer(1) (the argument). The second call to contains returns true because you can compare Short(1) and Short(1).

HashSet<Short> set = new HashSet<Short>();
short s = 1;
// Following statement prints 'false', because the argument is a literal int, which is auto-boxed
// to an Integer
// Following statement prints 'true', because the argument is a literal int that is cast to a short, 
// which is auto-boxed to a Short


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