"Rextangle" is a class in which two methods are defined. One is "perimeter ()" (that is, finding the area). When called, it can be written as "Rextangle.perimeter ()".

You can also understand it this way: for example, "car" is a category. If you write "car, bus ()" like this, you will know that this car is big and has many seats. Surely it's not a private car? !

"area = "+ rectangle. Areas (5, 4) are not difficult for you to see this. This area is enclosed in double quotation marks, which means that it will be displayed. This plus sign means that there are two items to be displayed here. If three items are displayed, just add a plus sign and so on. ...

Static int perimeter (int length, int width) {} Here is the definition of this method. It has two variables, one is "length" and the other is "width". The int in front of them is all plastic, with 5 corresponding to length and 4 corresponding to width. ...

that's it ...

Come on, making more mistakes is the best way++