Why “t instanceof T” is not allowed where T is a type parameter and t is a variable?

Question

Eclipse says that the instanceof operation is not allowed with Type Parameter due to generic type eraser.

I agree that at runtime, no type information stays. But con

Answer 1

If you need T at runtime, you need to provide it at runtime. This is often done by passing the Class which T has to be.

class SomeClass {
    final T t;

    public SomeClass(Class tClass, T t) {
        if(!tClass.isAssignableFrom(t.getClass()) throw new IllegalArgumentException("Must be a " + tClass);
        this.t = t;
    }

    private SomeClass(T t) {
        this.t = t;
    }

    public static  SomeClass of(Class tClass, T t) {
        if(!tClass.isAssignableFrom(t.getClass()) throw new IllegalArgumentException("Must be a " + tClass);
        return new SomeClass(t);
    }
} 

// doesn't compile
SomeClass intSomeClass = SomeClass.of(Integer.class, "one");

Class clazz = Integer.class;
// compiles with a warning and throws an IAE at runtime.
SomeClass intSomeClass = (SomeClass) SomeClass.of(clazz, "one");

// compiles and runs ok.
SomeClass intSomeClass = SomeClass.of(Integer.class, 1);