Deep copy is to re-open a new reference address for the variable, which is completely separated from the original, which is equivalent to dividing a new space for itself. Although the content in it is the same, even if one of them is changed, the other is not. will be affected. Shallow copy means that the reference address of the reference type in the stack cannot be changed. The two variables share the same reference address and change the things in the same place. Therefore, the shallow copy causes the change of one variable to cause the same change of the other variable. .