Ordinary member variables

class variable

constant

mark

Used to instantiate objects, divided into parameterized construction and parameterless construction. If you do not write a constructor, there will be a parameterless constructor by default.

It can only be written in abstract classes or interfaces. It has no method body and must be rewritten.

In the same class, the method names are the same, but the number of parameters and data types are different.

In the inheritance relationship, the subclass redefines the method body of the parent class, and the method name, parameter list, and return value type are exactly the same.

interface interface name {}

The default keyword for member properties is public static final, which is a public static constant and must be initialized.

The method default keyword is public abstract, which is a public abstract method.

Multiple inheritance, multiple implementations

class class name implements interface name {}

You must rewrite the method that implements the interface, or turn the class into an abstract class