对 java.util.Collection.contains 的可疑调用
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Suspicious call to java.util.Collection.contains
提问by Cheok Yan Cheng
I am getting the following warning from my NetBeans IDE.
我从 NetBeans IDE 收到以下警告。
Suspicious call to java.util.Collection.contains
Expected type T, actual type Object
May I know what does that means?
我可以知道这是什么意思吗？
This doesn't make sense to me. Both Listand Collectionclass's containsmethod, are using Object as their method parameter.
这对我来说没有意义。无论List和Collection类的contains方法，使用对象作为自己的方法的参数。
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);
}
Code snippet from Collection class
来自 Collection 类的代码片段
/**
* 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 ? e==null : 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
List 类的代码片段
/**
* 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 ? e==null : 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);
回答by dpsthree
In the call to list.contains you are comparing an object to a type T. Casting o to type T should resolve your warning.
在对 list.contains 的调用中，您将对象与类型 T 进行比较。将 o 转换为类型 T 应该可以解决您的警告。
回答by josefx
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.
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。
An example why this warning is necessary:
为什么需要此警告的示例：
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.
代码是有效的，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。
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).
由于存在传递对象的有效用例，您应该能够使用 @SuppressWarnings() 注释来隐藏此警告（仅当您知道自己在做什么时才这样做）。
