Suspicious call to java.util.Collection.contains

Question

I am getting the following warning from my NetBeans IDE.

Suspicious call to java.util.Collection.contains Expected type T, actual type Object 

May I know what does that means?

This doesn't make sense to me. Both List and Collection class's contains method, are using Object as their method parameter.

import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; import java.util.List;  /**  *  * @author yan-cheng.cheok  */ public abstract class AbstractCollection<T> implements Collection<T> {      protected List<T> list = new ArrayList<T>();      public boolean contains(Object o) {         // Suspicious call to java.util.Collection.contains         // Expected type T, actual type Object         return list.contains(o);     } 

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Code snippet from Collection class

/**  * Returns <tt>true</tt> if this collection contains the specified element.  * More formally, returns <tt>true</tt> if and only if this collection  * contains at least one element <tt>e</tt> such that  * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.  *  * @param o element whose presence in this collection is to be tested  * @return <tt>true</tt> if this collection contains the specified  *         element  * @throws ClassCastException if the type of the specified element  *         is incompatible with this collection (optional)  * @throws NullPointerException if the specified element is null and this  *         collection does not permit null elements (optional)  */ boolean contains(Object o); 

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Code snippet from List class

/**  * Returns <tt>true</tt> if this list contains the specified element.  * More formally, returns <tt>true</tt> if and only if this list contains  * at least one element <tt>e</tt> such that  * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.  *  * @param o element whose presence in this list is to be tested  * @return <tt>true</tt> if this list contains the specified element  * @throws ClassCastException if the type of the specified element  *         is incompatible with this list (optional)  * @throws NullPointerException if the specified element is null and this  *         list does not permit null elements (optional)  */ boolean contains(Object o); 

Answer 1

In the call to list.contains you are comparing an object to a type T. Casting o to type T should resolve your warning.

Answer 2

Calling the contains method with an Object instead of the generic type may be a programming error. Since the code is still valid the compiler will only show a warning.

An example why this warning is necessary:

List<Long> l = new ArrayList<Long>(); l.add(1l); l.contains(1); 

The code is valid but would always return false. An error that is normally hidden by contains accepting object instead of a generic type, so the compiler is limited to warnings.

Since there are valid use cases for passing an object, you should be able to use a @SuppressWarnings() annotation to hide this warning (only do this if you know what you are doing).