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This article introduces the knowledge of "what is the method of initialization and removal of Java". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1. Object creation and initialization are the same concept and cannot be separated.

2. The constructor name must be exactly the same as the class name. In a sense, the constructor can be regarded as a static method, which is the same for every object of the same class, and the object has not been created when it is executed.

3. The default constructor (default constructor) is parameterless, and the compiler regards the parameterless constructor (which may be self-defined) as the default constructor

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4. The constructor can have parameters and must not return a value (unlike the void method, void is determined by the writer not to return any value.)

5. Constructors can be overloaded (overloading)

6. Overload rules (for all overloads): each overloaded method must have a unique list of parameter types

7. When you new an object, the new expression returns a reference to the newly created object (reference)

8. Creating an object is to name a section of memory space, and the method is the name of an action, which belongs to the category of name management.

9. Argument, the parameter given when the method is called, and the parameter declared when the method is defined

10. The this keyword can only be used within non-static methods. Note that it is within the method, not within the class definition. This is the identifier of the object reference, which refers to an object that is meaningless within the scope of the class definition. It is often used in return statements to return the reference of the current object.

11. You can use this to call another constructor in one constructor, but you can only adjust one. If you think about it, you can see that an object cannot be initialized twice, and obviously the constructor cannot be executed sequentially twice.

12. No constructor can be called in a method that is not a constructor

13. Java's garbage collection mechanism can only reclaim memory allocated by new.

14. The finalize () method. When the garbage collection mechanism releases the memory occupied by the object, it will first call the finalize () method for "hospice care", such as freeing non-new allocated memory in this method.

15. In C++, objects always call destructors (destructor) to clean up themselves, but Java objects are not always collected, and garbage collection is not destruction

16. It is uncertain whether and when the garbage collector will start, so it is impossible to determine when finalize () will be executed. They are unreliable.

17. System.gc () can force garbage collection, that is, to enforce finalize ()

18. Finalize () can also be used to check the termination condition of an object. For example, if an object is an open file, you can use finalize () to check whether it has been closed.

19. Finalize () is only used in very special situations, for example, memory is allocated to objects in a form other than creating objects, that is, memory allocated by native method when Java programs call non-Java programs, such as calling C or C++ 's malloc (). If free () is not called in finalize (), the memory will not be released and leaks will occur.

20. If C++ objects are created in the form of local (local) (without new), they are created on the stack (destructors are called when delete objects are released). Java does not allow the creation of local objects (local objects). You must use new and distribute them in the heap (heap is slower than stack).

Garbage collection not only reclaims space, but also rearranges objects so that the heap pointer points to large chunks of unused space, improving the speed of allocating new memory

22. Different JVM may have different implementation mechanisms for garbage collection, such as stop-copy (stop-and-copy), mark-sweep (mark-and-sweep), and "adaptive" between the two.

23. When the first object is created for a class, the loader loads the class, which is usually the case

24. Lazy evaluation (lazy compilation) is generally used to load, and the compiler compiles the code only when necessary. This is now the method adopted by Java HotSpot technology in JDK. The more times the code is executed, the faster it will be.

25. If the variable defined in the method is not initialized before use, there will be a compile-time error. If the member variable of the class or the reference to the object is not initialized, Java will give the default initial value, where the default initial value of the reference is null. If you want to use this reference, there will be a run-time error, that is, NullPointException

26. The compiler assigns initial values to class member variables before the constructor. No matter where the member variables are in the body of the class definition, the order of variable definitions within the class determines the order of initialization.

27. The complete order in which the first object of a class is created is:

-> allocate space

-> zeroing the storage space (use Java default values to initialize all member variables, set all basic type data in the object to default values, and reference to null)

-> initialize static member variables (using custom default values)

-> initialize non-static member variables (using custom default values)

-> execute constructor

Static member variables are initialized only when they are first accessed or when the first object of the class is created, that is, they are initialized only when necessary (when the class is first loaded) and cannot be changed after initialization

29. Statements initializing static / non-static member variables can be surrounded by {} to form a block (or clause)

30. It is recommended to define an array in the form of int [] A1;, but int A1 []; it is also legal. Java does not allow you to specify the size of the array.

31. An array is a sequence of objects, and it can also be regarded as a special object. Even if the elements in it are of a basic type, an array is also an object. You can use int [] a = new int [20] to allocate storage space, or you can directly use A1 = a2; to assign values. Note that only references are copied when using = assignment.

32. If the elements in the array are not of a basic type, you must new each element once before there is real space for each element, otherwise it is just a reference array

33. If the array is out of bounds, there will be no problem with compilation, but there will be a run-time error, that is, Exception

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