### nil?
```
nil.nil? # true
false.nil? # false
[].nil? # false
{}.nil? # false
"".nil? # false
```

### empty?
```
nil.empty? # throw error
false.empty? # throw error
[].empty? # true
{}.empty? # true
"".empty? # true
```

### present?
```
nil.present? # false
false.present? # false
[].present? # false
{}.present? # false
"".present? # false
true.present? #true
"abc".present? # true
123.present? # true
```

### blank? (present? 的相反)
```
nil.blank? # true
false.blank? # true
[].blank? # true
{}.blank? # true
"".blank? # true
true.blank? #false
"abc".blank? # false
123.blank? # false
```