Java Learning Lesson Eighth (constructors and their characteristics)

constructor function:

Constructor, which is the function called when building the creation object, is to initialize the object

features :

1. The function name and the class name are the same
2. You do not need to define a return value type
3. No specific return value

function: initializing an object

Note :

1. Features of the default constructor

2. Multiple constructors exist in the form of overloading

Code Demo:

Class Man{private int age;private String Name;man ()//defines a constructor for a man class {System.out.println ("Sun");} public void Fun () {System.out.println (name+ "" +age);}} public class main{public  static void  Main (string[] args) {mans Jo = new Man ();}} Print Sun,

PS: If a constructor is not defined in a class, then there is a default null argument constructor in the class

If the specified constructor is defined in a class, the default constructor in the class does not have a

Special NOTE: If you define a constructor that has a parameter, the form of the default constructor is also constructed, and the compiler no longer automatically generates

In fact: The default null argument function is: Man () {}

such as a class:
Class Test
{
}
Nothing, but in fact there is a test () {}, the compiler will compile the first to determine whether there is a write constructor, not automatically added, the default constructor is deleted
constructor, which is called when an object is created.


The difference between a general function and a constructor:


1. Constructor: When an object is created, the corresponding constructor is called and the object is initialized
General functions: When an object is created, it is called when a function call is required

2. Constructor, when object is created, call only once
General function: After the object is created, it is called several times to call


When do I use constructors?

When describing a thing, there is something in it that is defined in the constructor.
such as: When the object is created, there is a default age, name

Example code:

Class Man{private int age;private String Name;man ()//defines a man class constructor {age = 20;name = "John";} public void Fun () {System.out.println (name+ "" +age);}} public class main{public  static void  Main (string[] args) {mans Jo = new Man (); Jo.fun ();}}

Overloading of constructors:

class man{private int age;private String Name;man ()//define a man class constructor---------------|{                            |age = 20;                       |name = "John";                                            |}                                             |//objects are age before they are created, names |                             |man (int a,string s) |--------> Reload {|age = A;                             |name =s;                                            |} |man (String s) |                             {|name =s;                             |} ---------------|public void Fun () {System.out.println (name+ "" +age);}} public class Main{public static void Main (string[] args) {mans Jo = new Man (); Jo.fun (); Man JJ = new Man (1, "BLF"); Jj.fun ( ); Man jjj = new Man ("BLF2"); Jj.fun ();//print BLF 0}} 

PS: A class in which the name of the function with the same name must be overloaded.

If a constructor is not called when an object is created, it means that the creation failed.

The memory allocation diagram for the constructor (as): Object to go into the heap, function to go into the stack

Constructor Details:

constructor, to make it easier to modify the information later, add set
Before the name of the constructor, no void is added, and then it is not a constructor, but it becomes a general function (the feature of the constructor 2)


Error code example

Class Man{private int age;private String name;man (int a) {age = A;} The object has an age before it is created, the name man (int a,string s) {ages = A;name =s;fun ();} Public  void Set (String s) {name = s;} public void Fun () {System.out.println (name+ "" +age);}} public class main{public  static void  Main (string[] args) {mans Jo = new Man (); Jo.set ("BLF2"); Jo.fun ();}}

The problem with this piece of code is:

Because no constructors are defined.

The default constructor is that the code is added automatically when the constructor is not created, but the code above has a constructor, but there is no man () {} Looking for a type of constructor, so the error

Java Learning Lesson Eighth (constructors and their characteristics)