The binary containment operator is represented by the keyword in. It is used for the containment relation which is on the syntactic level of relational expressions as defined by the Java programming language.

The containment operator may be used for two operands of numeric type, two operands of type boolean, or two operands that are each of either reference type or the null type. All other cases result in a compile-time error. The type of a containment expression is boolean.

A containment relation e in s is evaluated as follows: The equality relation e == s is evaluated sequentially. For each yielded result, it is checked whether it is true. If this is the case, the sequential evaluation is terminated, and the result of the containment relation is true. If all comparisons result in false, the result is false.