what is java constructor

The constructor is a member of the object-oriented system. The constructor can be called a constructor. Its function name is the same as the class name. There is no need to define a return value type and no specific return value. . The constructor is a function called when constructing a created object. Its function is to initialize the object. All objects created must be initialized through the constructor. If there is no constructor defined in a class, the class will have a default constructor with empty parameters. If a specified constructor is defined in a class, there is no default constructor in the class.

When we are born, some people are given names after they are born, but some people are already given names as soon as they are born. So how do we assign a value to an object once it is created in Java?

1. The role of the constructor:

The role of the constructor: perform operations on the object initialization.

As shown in the figure:

2. The difference between constructors and ordinary functions:

(1) General Functions are used to define the functionality that an object should have. The constructor defines some content that the object should have when it is created before calling the function. That is, the initialization content of the object.

(2) The constructor is called by jvm when the object is created to initialize the object. Generally, a function will be executed when the object calls the function after the object is created.

(3) Ordinary functions can be called multiple times using objects, and the constructor is called when the object is created.

(4) The function name of the constructor must be the same as the class name, and ordinary functions only need to comply with the naming rules of identifiers.

(5) The constructor has no return value type.

3. Details to pay attention to in the constructor:

(1) When there is no constructor defined in the class, the system will assign an empty parameter to the class. 's constructor. This is the default constructor in the class. When a constructor is customized in a class, the default constructor is gone.

Note: It can be verified through javap command.

(2) Multiple constructors can be defined in a class to perform different initialization. Multiple constructors exist in a class and are embodied in the form of overloading. Because the names of the constructors are all the same.

class Perosn{
private int id;
private String name;
private int age;
public Perosn(){
cry();
}
public Perosn(int id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public void cry(){
System.out.println("哇哇哇....");
}
}

Question: Every child is required to cry when he is born. This code has two constructors. If every child is required to cry when he is born, cry() needs to be called in different constructors. ) function, but this will cause code duplication problem, so how to solve it? Construct code blocks.

4. Construction code block:

The function of the construction code block: to uniformly initialize all objects.

class Perosn{
private int id;
private String name;
private int age;
{
cry();// 每个Person对象创建出来都会执行这里的代码
}
public Perosn(){
cry();
}
public Perosn(int id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public void cry(){
System.out.println("哇哇哇....");
}
}

Specific functions:

1: Initialize the object. Runs as soon as the object is created and takes precedence over the constructor.

2: Difference from constructor

(1) The difference between constructed code block and constructor function. Constructed code block is to uniformly initialize all objects, and constructor function is to The corresponding object is initialized.

(2) The function of the construction code block: Its function is to extract the common information in all construction methods.

For example, a child cries uniformly upon birth

The code is as follows:

class Boy {
 
String name;
int age;
String gender;
     // 构造代码块,给所有对象进行初始化。
{
System.out.println("哭。。。");
}
     
Boy() {
System.out.println("无参构造");
}
 
Boy(String n, int a, String g) {
name = n;
age = a;
gender = g;
System.out.println("有参构造");
}
 
void run() {
System.out.println("跑...");
}
 
}
 
class Demo9 {
 
public static void main(String[] args) {
 
System.out.println();
Boy b = new Boy();
 
Boy b2 = new Boy("jack", 1, "男");
 
}
}

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