在 Java 8 中使用 .class 进行转换？

Question

我认为 .class 是它被调用的对象的类。然而，这不符合我试图用程序做的事情。我会举一些例子。

//o is of type Object
//this method throws error "cannot find symbol" for the cast method
SampleClass.cast(o);

//sampleClass is an instance of SampleClass
//this method throws error "<identifier> expected"
sampleClass.class.cast(o);

//this method works!
SampleClass.class.cast(o);

//this returns two "<identifier> expected" errors
sampleClass.class.class.cast(o);

//this works too!
((SampleClass) o)

之前我认为sampleClass.class == SampleClass，一个Class类型的对象。我还认为 SampleClass.class 会返回一些很难想象的元数据。现在我知道我真的什么都不知道。感谢任何帮助解决这个难题的方法:)

编辑：谢谢大家！我很高兴能够学习有关 Java 的知识。我很感激帮助

Answer 1

我已经整理了一些可编译的 Java 代码，它们可能会阐明 Class 和强制转换的一些用法。希望它足够自我记录。

您可以Sample.class将“类文字”视为与"something"字符串文字相同的方式。它是一个表达式，它有一个值，它可以赋值给一个变量，它是一个对象。这个对象的类型是Class<Sample>。另一方面Sample是类型，而不是表达式，您不能将其分配给变量。

Class是一个元类，即它是一个描述另一个类的类。你不能比Class<Class> classClass = Class.class;. 有趣的是：还有像int.class.

class Main {
    public static void main(String[] args) {
        // class literal
        Class<Sample> sampleClassLiteral = Sample.class;

        final Sample superObject = new Sample();
        // dynamic class
        // `? extends Sample` because the dynamic type could be a subclass
        Class<? extends Sample> superClassMethod = superObject.getClass();
        // classes are comparable both by == and equals, because `Class` is final and it does not override the default equals implementation
        assert sampleClassLiteral == superClassMethod;
        assert sampleClassLiteral.equals(superClassMethod);

        Sample subObject = new SubSample();
        // dynamic class can be different from the static type of the variable
        Class<? extends Sample> subSampleMethod = subObject.getClass();
        assert subSampleMethod != superClassMethod;

        SubSample staticCast = ((SubSample) subObject);

        List<Object> listOfAny = List.of(superObject, subObject, sampleClassLiteral, "some string", 12);
        // static type check & casting
        final List<Sample> listOfSample = listOfAny.stream()
                .filter(elem -> elem instanceof Sample) // Sample.class::isInstance
                .map(elem -> (Sample) elem)             // Sample.class::cast
                .collect(toList());

        final List<Sample> samples = listOfTargetClass(listOfAny, Sample.class);
        final List<? extends Sample> sameAsSamples = listOfTargetClass(listOfAny, superClassMethod);
        // [superObject, subObject] : does NOT contain `sampleClassLiteral` because it's not of type `Sample`, it is of type `Class<Sample>`
        System.out.println(samples);
        System.out.println(sameAsSamples);
        assert samples.equals(sameAsSamples);

        final List<SubSample> subSamples = listOfTargetClass(listOfAny, SubSample.class);
        System.out.println(subSamples);
    }

    static <T> List<T> listOfTargetClass(Collection<?> anyCollection, Class<T> targetClass) {
        // dynamic type check and casting
        return anyCollection.stream()
                .filter(targetClass::isInstance) // obj -> targetClass.isInstance(obj)
                .map(targetClass::cast) // obj -> targetClass.cast(obj)
                .collect(toList());
    }

    static class Sample { }

    static class SubSample extends Sample { }

}