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);
    }

---

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);

---

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.