Is it ok to lose weight by by eating only apple and drinking only apple juice and water for like three or seven days?

Yes: You can lose weight by limiting your food intake, however, you will also be limiting vital nutrients your body needs to work properly. Extreme weight loss diets or plans such as this one are never a good idea as your metabolism will not work properly if your body is nutritionally depleted. You should eat a healthy diet, heavy on a variety of fruits, vegetables, purified water to fuel your body.

Yes...: For a short period of time its fine, and most likely such a diet will be low in calories and you will lose weight. That said, its not a balanced diet and will not sustain a healthy lifestyle including exercise. The best way to lose weight in the long term is to have a balanced diets of protein, fats, and relatively low carbohydrates, combined with regular exercise.