[Dart] #1 Variables

What you can learn in this post

1. How to declare variables in Dart

2. What is Null Safety

---

 

1. How to declare variables in Dart

 

[Can be overwritten later]

 

1st. Specify type

when? class property

void main () {
	int i = 12;
    	i = 121212;
}

 

2nd. var

when? local variables in method, small functions  local variables

void main () {
	var name = 'la'
	name = 'lalala'
}

 

3rd. dynamic 

when? available in all types

void main () {
	dynamic name;
	name = 12;
	name = 'nico';
	name = true;
    
	if (name is String){
		name.isEmpty(); //can use functions for String
	}

	if (name is int){
		name.isFinite(); can use functions for Integer
	}
}

 

[Can NOT be overwritten later]

 

1st. final : 앱을 실행하면서 사용자가 입력하는 변수

void main () {
	final String username;
}

 

2nd. const : 앱 스토어에 올라가기 전에 알고 있는 변수 (=has to be known BEFORE complie)

 

[When you DON'T KNOW THE VALUE at first]

1st. late

creating a variable without data

void main () {
    late final String name;
    //do something, go to api
    print(name); //error. part of null safety. 
    
    name = 'nico';
}

 

[what is NULL Safety?]

 

All variables created in Dart are NOT nullable.

which means, it can NOT be NULL.

 

개발자들이 null 변수를 참조하는 코드를 짜더라도, dart 컴파일러에서는 오류가 없이 컴파일 된다.

그러나, run-time 도중에는 null 변수를 참조 오류가 발생한다. 

 

즉, null 참조 오류는 Run-time error이다.  

 

따라서, null safety를 통해 컴파일 도중에 오류를 잡을 필요가 있다 

 

So, if you want to make your variable nullable,

you can add a question mark

 

ERROR

void main () {
	String name = 'nico';
	name = null;
}

 

NO ERROR

void main () {
	String? name = 'nico';
	name = null;
    
	nico?.inNotEmpty; //if Nico is NOT null, give me the isNotEmpty property
}

 

https://nomadcoders.co/dart-for-beginners/lectures/4096

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