java - 如何摆脱这个泛型警告？

Question

我正在尝试模拟一个通用接口(interface)，每当我模拟它时，我都会收到以下警告:

The expression of type GenericInterface needs unchecked conversion to conform to GenericInterface<String>

我的界面是

interface GenericInterface<T>{
    public T get();
}

我的测试是

@Test
public void testGenericMethod(){
    GenericInterface<String> mockedInterface = EasyMock.createMock(GenericInterface.class);
}

我在测试用例的第一行收到警告。

如何删除此通用警告？

Answer 1

摆脱警告的正确步骤是:

首先，证明未经检查的类型转换是安全的，并记录原因

然后才执行未经检查的强制转换，并注释 @SuppressWarnings("unchecked")关于变量声明(不是整个方法)

所以是这样的:

// this cast is correct because...
@SuppressWarnings("unchecked")
GenericInterface<String> mockedInterface =
    (GenericInterface<String>) EasyMock.createMock(GenericInterface.class);

指导方针

以下摘自 Effective Java 2nd Edition:第 24 条:消除未经检查的警告:

• Eliminate every unchecked warning that you can.
• If you can't eliminate a warning, and you can prove that the code that provoked the warning is typesafe, then (and only then) suppress the warning with @SuppressWarning("unchecked") annotation.
• Always use the SuppressWarning annotation on the smallest scope possible.
• Every time you use an @SuppressWarning("unchecked") annotation, add a comment saying why it's safe to do so.

相关问题

What is SuppressWarnings (“unchecked”) in Java?

How do I address unchecked cast warnings?

Type safety: Unchecked cast

重构类型转换

在大多数情况下，也可以在泛型 createMock 中执行未经检查的强制转换。 .它看起来像这样:

static <E> Set<E> newSet(Class<? extends Set> klazz) {
    try {
        // cast is safe because newly instantiated set is empty
        @SuppressWarnings("unchecked")
        Set<E> set = (Set<E>) klazz.newInstance();
        return set;
    } catch (InstantiationException e) {
        throw new IllegalArgumentException(e);
    } catch (IllegalAccessException e) {
        throw new IllegalArgumentException(e);          
    }
}

然后在其他地方你可以简单地做:

// compiles fine with no unchecked cast warnings!
Set<String> names = newSet(HashSet.class);
Set<Integer> nums = newSet(TreeSet.class);

也可以看看

Java Tutorials/Generics

Angelika Langer's Java Generics FAQ