请教OC中self用法

面的获取文本框、标签的文本属性为什么都要用self.num1.text，self.resultLabel.text， 不能直接用num1.text，resultLabel.text？？？

#import "HMViewController.h"

@interface HMViewController ()
 @property (nonatomic, weak) IBOutlet UITextField *num1;
 @property (nonatomic, weak) IBOutlet UITextField *num2;
 @property (nonatomic, weak) IBOutlet UILabel *resultLabel;

@end

@implementation HMViewController

(IBAction)compute
 {
 NSString *num1 = self.num1.text;
 NSString *num2 = self.num2.text;

int result = num1.intValue + num2.intValue;

self.resultLabel.text = [NSString stringWithFormat:@"%d", result];

[self.view endEditing:YES];
 }


@end