A review of Java basics

1. Member variables in Java are divided into instance member variables and class member variables.
2. The purpose of writing a class is to describe the common attributes and functions of a class of things.

3. The type of member variables and local variables can be any data type in Java.

4. Member variables described with the modifier static are class variables.

5. If the name of the variable is the same as the name of the member variable, the member variable is hidden and the member variable is temporarily invalid in this method.

6. Member variables described with the modifier static are class variables.
7. If the name of the local variable is the same as the name of the member variable, the member variable will be hidden.

8. Method definition includes two parts: method declaration and method body.

9. In Java, when a method does not need to return data, the return type must be void.

10. Method overloading means that there can be multiple methods with the same name and type in a class, but the parameters of these methods must be different.

11. In the Java programming language, it allows several methods in a class, all with the same name. This usage is called method overloading.

12. The constructor method is a special method. Its name must be exactly the same as the name of the class in which it is located, and it does not return any data type.

13. Class methods in Java are divided into instance methods and class methods.

14. Because the content of the class body consists of two parts: the definition of member variables and the definition of member methods, operations on member variables can only be placed in member methods.

15. The constructor method in Java has the same name as the class and has no return value. It is automatically called by the new operator when creating an object instance.
16. Only class variables and class methods can be called in class methods, and instance methods cannot be called.

17. Instance methods can operate on both class variables and instance variables, while class methods can only operate on class variables.

18. Classes in Java are templates for creating objects.

19. When an object is created using a class, we also say that an instance of the class is given.

20. Creating an object using a class in Java includes two steps: object declaration and memory allocation for the object.

21. The general format of object declaration is class name object name.

22. A class can create multiple different objects by using the new operator, and these objects will allocate different memory spaces.

23. A class allocates memory for the declared object by using the new operator and the class constructor.

24. After the object is created, by using the operator ".", the object can access variables and call methods.

25. Instance variables of different objects in Java will be allocated different memory spaces.
26. In Java, if the member variables in a class have class variables, then all objects are allocated to the same memory for such variables.

27. The difference between class methods and instance methods is that class methods can be called not only by objects but also directly by class names, but instance methods cannot.

28. Class methods Methods can be called not only by objects but also directly by class names, but instance methods cannot.

29. The methods in the class body are divided into: instance methods and class methods.

30. The package statement, as the first statement of the java source file, indicates the package in which the class defined in the source file is located.

31. In the java program, the system will automatically introduce the java.lang package, so there is no need to use the import statement to introduce the package.

32. In a java program, in order for the program to use a custom package, the location of the custom package must be specified in the classpath environment variable.

33. The java.lang package is the core class library of the Java language. It contains system classes essential for running Java programs.

34. In order to use the classes provided by Java, we can use the statement import to introduce the required classes.

35. Member variables and methods modified with private in Java are called private variables and private methods.
36. Member variables that do not need to be modified in Java are called friendly variables.

37. Access rights are private variables and can only be used within this class and cannot be used outside the class.

38. Member variables and methods modified with public in Java are called shared variables and shared methods.

39. Access permission is a shared variable, which can be used not only in this class, but also in any other class.

40. Access permissions are friendly variables and can only be used in this class and other classes in the same package as this class.

41. Inheritance is a mechanism for creating new classes from existing classes.

42. The class obtained by inheritance in Java is called a subclass, and the inherited class is called a parent class.

43. Multiple inheritance is not supported in Java.

44. In the declaration of the class, create a subclass of the class by using the keyword extends.
45. A class in Java can have 1 parent class.
46. If the extends keyword is not used in the declaration of a class, the class is defaulted by the system to be a subclass of the Object class.

47. Subclasses naturally inherit non-private member variables in their parent class as their own member variables.

48. When the member variables defined in the subclass have the same name as the member variables in the parent class, the member variables of the subclass hide the member variables of the parent class.

49. Subclasses can change the status and behavior of the parent class to their own status and behavior by hiding member variables and rewriting methods.

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