【解惑】JVM如何理解Java泛型类

public class TestFan {

	/**
	 * @param args
	 * 
	 * User是一个空class
	 */
	public static void main(String[] args) {
		int i = 0;
		test(null);
	}

	//
	//不添加此方法.编译,运行没问题.输出显示调用的user
	//添加此方法时候.编译期出错The method test(String) is ambiguous for the type TestFan
	public static void test(String str)
	{
		System.out.println("string");
	}
	
	
	public static void test(User users)
	{
		System.out.println("user");
	}
	
	public static void test(Object users)
	{
		System.out.println("object");
	}
}

public class A<T> extends B<T>{
    void method(T param){
        //do somthing
    }
}

final T param

public class TestFan {

	/**
	 * @param args
	 * 
	 * User是一个空class
	 */
	public static void main(String[] args) {
		int i = 0;
		test(null);
	}

	//
	//不添加此方法.编译,运行没问题.输出显示调用的user
	//添加此方法时候.编译期出错The method test(String) is ambiguous for the type TestFan
	public static void test(String str)
	{
		System.out.println("string");
	}
	
	
	public static void test(User users)
	{
		System.out.println("user");
	}
	
	public static void test(Object users)
	{
		System.out.println("object");
	}
}

Object , ? extends Object 

JVM_ENTRY(jobject, JVM_Clone(JNIEnv* env, jobject handle))
  JVMWrapper("JVM_Clone");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  const KlassHandle klass (THREAD, obj->klass());
  JvmtiVMObjectAllocEventCollector oam;

#ifdef ASSERT
  // Just checking that the cloneable flag is set correct
  if (obj->is_javaArray()) {
    guarantee(klass->is_cloneable(), "all arrays are cloneable");
  } else {
    guarantee(obj->is_instance(), "should be instanceOop");
    bool cloneable = klass->is_subtype_of(SystemDictionary::cloneable_klass());
    guarantee(cloneable == klass->is_cloneable(), "incorrect cloneable flag");
  }
#endif

  // Check if class of obj supports the Cloneable interface.
  // All arrays are considered to be cloneable (See JLS 20.1.5)
  if (!klass->is_cloneable()) {
    ResourceMark rm(THREAD);
    THROW_MSG_0(vmSymbols::java_lang_CloneNotSupportedException(), klass->external_name());
  }

  // Make shallow object copy
  const int size = obj->size();
  oop new_obj = NULL;
  if (obj->is_javaArray()) {
    const int length = ((arrayOop)obj())->length();
    new_obj = CollectedHeap::array_allocate(klass, size, length, CHECK_NULL);
  } else {
    new_obj = CollectedHeap::obj_allocate(klass, size, CHECK_NULL);
  }
  // 4839641 (4840070): We must do an oop-atomic copy, because if another thread
  // is modifying a reference field in the clonee, a non-oop-atomic copy might
  // be suspended in the middle of copying the pointer and end up with parts
  // of two different pointers in the field.  Subsequent dereferences will crash.
  // 4846409: an oop-copy of objects with long or double fields or arrays of same
  // won't copy the longs/doubles atomically in 32-bit vm's, so we copy jlongs instead
  // of oops.  We know objects are aligned on a minimum of an jlong boundary.
  // The same is true of StubRoutines::object_copy and the various oop_copy
  // variants, and of the code generated by the inline_native_clone intrinsic.
  assert(MinObjAlignmentInBytes >= BytesPerLong, "objects misaligned");
  Copy::conjoint_jlongs_atomic((jlong*)obj(), (jlong*)new_obj,
                               (size_t)align_object_size(size) / HeapWordsPerLong);
  // Clear the header
  new_obj->init_mark();

  // Store check (mark entire object and let gc sort it out)
  BarrierSet* bs = Universe::heap()->barrier_set();
  assert(bs->has_write_region_opt(), "Barrier set does not have write_region");
  bs->write_region(MemRegion((HeapWord*)new_obj, size));

  // Caution: this involves a java upcall, so the clone should be
  // "gc-robust" by this stage.
  if (klass->has_finalizer()) {
    assert(obj->is_instance(), "should be instanceOop");
    new_obj = instanceKlass::register_finalizer(instanceOop(new_obj), CHECK_NULL);
  }

  return JNIHandles::make_local(env, oop(new_obj));
JVM_END

public class TestFan {

	/**
	 * @param args
	 * 
	 * User是一个空class
	 */
	public static void main(String[] args) {
		int i = 0;
		test(null);
	}

	//
	//不添加此方法.编译,运行没问题.输出显示调用的user
	//添加此方法时候.编译期出错The method test(String) is ambiguous for the type TestFan
	public static void test(String str)
	{
		System.out.println("string");
	}
	
	
	public static void test(User users)
	{
		System.out.println("user");
	}
	
	public static void test(Object users)
	{
		System.out.println("object");
	}
}

Object , ? extends Object 

public class Test {
	class A {
	}

	class B extends A {
	}

	public static void main(String[] args) {
		test(null);
	}

	public static void test(A a) {
		System.out.println("A");
	}

	public static void test(B b) {
		System.out.println("B");
	}
}

public class Test {
	class A {
	}

	class B {
	}

	public static void main(String[] args) {
		test((B) null);
	}

	public static void test(A a) {
		System.out.println("A");
	}

	public static void test(B b) {
		System.out.println("B");
	}
}