Constructor, this keyword, method overloading, local variables and member variables

1.构造方法的作用：初始化对象

特点：1）,无返回值 2）,方法名和类名相同 3）,可以指定参数

访问修饰符   构造方法名 ( ) {

       //初始化代码

}

2.this 关键字的用法：

1）,调用属性

 this.name = "大黄"

 2）,调用方法

this.print();

3）,调用构造方法：如果使用,Must be placed in the first statement of the constructor

this(); 

thisKeywords can be used to differentiate 

3.方法的重载：在同一个类中,Multiple methods appear with the same method name,参数列表不同（参数的个数、参数的类型、参数顺序）的现象.

1. 同一个类中
2. 方法名相同
3. 参数个数和类型不同
4. 与返回值和访问修饰符无关 

public class Sum {

        //Define several common methods
        public int sum(int num1,int num2){
            return num1+num2;
        }
        
        public double sum(double num1,double num2){
            return num1+num2;
        }
        
        public double sum(int num1,double num2){
            return num1+num2;
        }

 }

4.成员变量和局部变量

变量作用域：Variables follow their location,Can be divided into member variables（全局变量）、There are two types of local variables

成员变量（类内方法外）：
              Variables outside other structures in the acting class,
              The scope of member variables is available throughout the class（Non-static member variables cannot be used in static methods,Static member variables can be used）
             The member variable system assigns it a default value
             在同一个类中,There cannot be a global variable with the same name,全局变量和局部变量可以同名,在使用的时候,局部变量具有更高的优先级 

public class Demo01 {

    String name;
    int num1 = 1000;//成员变量
    public void test(){
        int num1;
        System.out.println(name);
        num1 = 100;
        //Local variables must be assigned a value before being used
        System.out.println(num1);
    }
    

局部变量（方法内）：
              Variables in action methods or other structures,
              The scope of a local variable is limited to the structure in which the local variable is defined
              局部变量没有默认值,Assign values ​​before use,否则会报错
              in different methods（Get inside other structures）可以有相同名称的局部变量,There cannot be local variables with the same name within the same method or structure

public class Demo01 {

    String name;
    int num1 = 1000;//成员变量
    public void test(){
        int num1;//方法内的局部变量
        System.out.println(name);
        num1 = 100;
        //Local variables must be assigned a value before being used
        System.out.println(num1);
    }