Java泛型类型擦除，编译器输出与OCP书所说的不同

Question

public class Crate<T> {
    private T contents;

    public Crate() {
    }

    public T emptyCrate() {
        return this.contents;
    }

    public void packCrate(T contents) {
        this.contents = contents;
    }

    public static void main(String[] args) {
        Crate<String> stringCrate = new Crate();
        stringCrate.packCrate("content");
        String content = (String)stringCrate.emptyCrate();
    }
}

public class Crate {
   private Object contents;
   public Object emptyCrate() {
      return contents;
   }
   public void packCrate(Object contents) {
      this.contents = contents;
   }

   public static void main(String[] args) {
        Crate stringCrate = new Crate();
        stringCrate.packCrate("content");
        String content = (String)stringCrate.emptyCrate();
    }
}

public class Crate<T> {
    private T contents;

    public Crate() {
    }

    public T emptyCrate() {
        return this.contents;
    }

    public void packCrate(T contents) {
        this.contents = contents;
    }

    public static void main(String[] args) {
        Crate<String> stringCrate = new Crate();
        stringCrate.packCrate("content");
        String content = (String)stringCrate.emptyCrate();
    }
}

Answer 1

这本书试图说明的一点是，运行时并不关心泛型类型参数。它通过在教学上向您“撒谎”类文件不包含任何泛型类型变量或参数来表明情况是这样的，这是可以理解的，但也不是真的。

类文件确实包含字段、方法和类声明中使用的类型变量和泛型参数。你甚至可以用反射得到它们！

// Both of these will print T
System.out.println(Crate.class.getTypeParameters()[0]);
System.out.println(Crate.class.getDeclaredField("contents").getGenericType());

只是运行时并不关心，例如，您是否将不是T的内容分配到一个假定为T类型的字段中。

Crate var1 = new Crate();