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This article mainly explains "what are the knowledge points of Java objects". The explanation in this article is simple and clear, easy to learn and understand. Please follow the ideas of Xiaobian and go deep into it slowly to study and learn "what are the knowledge points of Java objects" together!

1. The complexity of the problem directly depends on the type and quality of the abstraction. For the same problem, if the abstract methods are different or the quality is different, the complexity of solving may vary greatly.

The essence of object-oriented thinking is that a program can adapt itself to a particular problem by adding new types of objects.

Five basic characteristics of object-oriented programming (originally from smalltalk)

Everything is an object.

A program is a bunch of objects telling each other what to do by sending messages.

Each object has its own memory made up of other objects.

d. Every object has a type.

All objects of a particular type can receive the same messages

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Class: A collection of objects with the same characteristics (data elements) and behavior (functionality).

An interface defines the requests you can make for a particular object.

The object should be considered a service provider.

7. Reasons for access control:

A, so that client programmers (consumers) can not touch the parts they should not touch

b. Allows library designers (class creators) to change the way classes work inside without worrying about affecting client programmers

Three access specifiers: public, private, protected

9. public: anyone can use it;

private: No one can access it except the class creator and the class's internal methods;

protected: Similar to private, except that an inherited class can access protected members of its parent class, but not private members;

Another default access right is package, where classes can access other class members in the same package, but outside the package, these members are treated like private.

Attempting to access private members will result in compile-time errors

Composition, that is, using existing classes to synthesize new classes; aggregation, that is, dynamically occurring combinations;

When creating a new class, priority should be given to combination, followed by inheritance.

Inheriting a class without redefining inherited methods (i.e., not overloading), then methods in the base class interface will be inherited directly into the derived class.

14. Compared with the parent class, if no new method is added, the relationship between the two is called "is-a"; if a new method is added, it is "is-like-a"

Non-object-oriented compilers generate function calls that use pre-binding, that is, the linker resolves the call to the absolute address of the code to be executed; In OOP, the program does not determine the address of the code until runtime, compiles only to ensure that the called method exists, and performs type checking on the call parameters and return values (strongly typed languages that provide such guarantees, weakly typed languages that do not), but does not know the exact code that will be executed.

In C++, the virtual keyword is used to explicitly declare that you want a method to use dynamic binding, which is not dynamically bound by default; in JAVA, dynamic binding is the default behavior.

Abstract keyword identifies the class as abstract class, abstract class cannot create objects, otherwise it will produce compile-time errors

The abstract keyword can also be used to describe methods that have not yet been implemented, that is, abstract methods, which means: this is an interface method that all types inherited from this class have, but at this point I did not design any concrete implementation for it; abstract methods can only be created inside abstract classes, and when the class is inherited, abstract methods must be implemented, otherwise the inherited class is still abstract (that is, the inherited class must also be modified by the abstract keyword, and the compiler will check this). The point of abstract methods is that you can put a method in an interface without being forced to provide a potentially meaningless method body for the method.

19. No method body definition is allowed in the interface, that is, all methods are abstract, but there is no need to declare the abstract keyword. The order of abstract programs is: ordinary class.

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