Question

我从NetBeans IDE收到以下警告。

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

我可以知道这意味着什么吗？

这对我没有意义。 List和Collection类的contains方法都使用Object作为方法参数。

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);
    }

Collection class的代码段

/**
 * 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);

列表类的代码段

/**
 * 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

在对list.contains的调用中，您将对象与类型T进行比较。将类型T转换为类型T可解决警告。

Answer 2

使用Object而不是泛型类型调用contains方法可能是编程错误。由于代码仍然有效，编译器只会显示警告。

需要此警告的示例：

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

代码有效但总是返回false。通常由包含接受对象而不是泛型类型隐藏的错误，因此编译器仅限于警告。

由于存在传递对象的有效用例，因此您应该能够使用@SuppressWarnings（）注释来隐藏此警告（仅在您知道自己在做什么时才执行此操作）。

对java.util.Collection.contains的可疑调用

2 个答案: