Contains
Contains is a membership-query operation that reports whether a specified value, key, element, or substring occurs in a container or collection.
Clojure: The Essential Reference
.contains is a Java Collection or String method that tests whether a collection contains an element or a string contains a substring.
- How it is used
- Tests vector value membership or string substring presence where Clojure’s `contains?` would answer a different question.
- How deep it goes
- Shows calls and results, including an unsupported map call, without detailing Java method implementations.
- Examples
- Compares value and index checks on a vector, then tests a string substring and an unsupported map call.
- What you can do after
- Apply it via Java interop to check vector values or string substrings without confusing the result with `contains?`.
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100 Java Mistakes and How to Avoid Them
Collection.contains is the Java Collection interface method that tests whether a collection contains an object equal to a specified value.
- How it is used
- Tests collection membership while exposing type-mismatch and null-handling mistakes that often produce silent failures.
- How deep it goes
- Explains the Object parameter, varargs inference, semantic type mismatches, null policies, and static-analysis support.
- Examples
- Uses primitive arrays, File-versus-Path collections, nullable searches, and List.of or Set.of migrations.
- What you can do after
- Diagnose false membership results, guard nullable arguments, expose inferred types, and interpret relevant analyzer warnings.
Read how 100 Java Mistakes and How to Avoid Them explains contains →
Related concepts
- findalternative to · A find operation searches a collection for the first element or entry matching a value or predicate and returns it, or indicates that no match exists.
- binary searchuses · Binary search is an algorithm that locates a target or boundary in an ordered collection by repeatedly comparing with a midpoint and halving the remaining search range.
- extendalternative to · extend is a Ruby method that adds the instance methods of one or more modules to a particular object's singleton class.
- Objectuses · java.lang.Object is the root class of Java’s reference-type hierarchy and defines methods inherited by Java reference types.
- instance_methodproduces · instance_method is a Ruby class method that returns an unbound method object for an instance method.
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