Undefined behavior：Objective-C category

Test_OC_Category.m

#import <Foundation/Foundation.h>

@interface TestCls: NSObject

@property(nonatomic, copy) NSString *name;

@end

@implementation TestCls

- (instancetype)init {
	if (self = [super init]) {
		[self testFunction];
	}
	return self;
}

- (void)testFunction {
	NSLog(@"testFunction %@", self.name);
}

@end

@interface TestCls (TestCategory)

- (void)testFunction;

@end

@implementation TestCls (TestCategory)

- (void)testFunction {
	NSLog(@"another testFunction");
}

- (Class)class {
	return [NSString class];
}

@end

int main() {
	TestCls *testCls = [[TestCls alloc] init];
	[testCls testFunction];
	NSLog(@"which class is testCls %@", [testCls class]);
	NSString *testStr = @"testStr";
	NSLog(@"which class is testStr %@", [testStr class]);
}

@implementation NSString (Test_Category)

- (Class)class {
	return [NSNumber class];
}

@end

Test_Swift_Extension.swift

import Foundation

class TestCls: NSObject {
	var name: String
	var anothername: String

	override init() {
		self.name = ""
		self.anothername = ""
		super.init()
	}

	func testFunction() {
		print("testFunction")
	}
}

protocol TestPrtl {
	func testFunction()
}

extension TestPrtl {
	func testFunction() {
		print("another testFunction")
	}
}

extension TestCls: TestPrtl {
/* compiler will report error:
 * Invalid redeclaration of 'testFunction()' **/
	
/* If we comment testFunction below to resolve compiler error
 * program run well but the console whill log "testFunction" instead of "another testFunction" **/
	func testFunction() {

	}
}

let testCls = TestCls()
testCls.testFunction()

Objective-C Category make program behavior undefined: it's so ridiculous as support change TestCls type to NSString!!!!

On the opposite way, Swift is more Safe, even though we implementation ```TestPrtl``, and default implementation of testFunction not be changed.