What is a java constructor? How to use constructor?

1. How to use the constructor method

Create an object through a class. This process is called instantiation. Instantiation is achieved by calling the constructor (also called the constructor).

1.1. What is a constructor method

The method name is the same as the class name (including upper and lower case)
No return type
Instantiate an object When, the constructor must be called

public class Hero {
 
    String name; 
    float hp; 
    float armor; 
    int moveSpeed; 
    // 方法名和类名一样（包括大小写）
    // 没有返回类型
    public Hero() {
        System.out.println("实例化一个对象的时候，必然调用构造方法");
    }     
    public static void main(String[] args) {        //实例化一个对象的时候，必然调用构造方法
        Hero h = new Hero();
    }
}

1.2. Implicit constructor

The constructor of the Hero class is

public Hero(){ 
}

This parameter-less constructor, If you don’t write it, it will be provided by default

public class Hero {
     
    String name; //姓名
     
    float hp; //血量
     
    float armor; //护甲
     
    int moveSpeed; //移动速度
     
    //这个无参的构造方法，如果不写，
    //就会默认提供一个无参的构造方法
    //  public Hero(){ 
    //      System.out.println("调用Hero的构造方法");
    //  }
 
    public static void main(String[] args) {
        Hero garen =  new Hero();
        garen.name = "盖伦";
        garen.hp = 616.28f;
        garen.armor = 27.536f;
        garen.moveSpeed = 350;
         
        Hero teemo =  new Hero();
        teemo.name = "提莫";
        teemo.hp = 383f;
        teemo.armor = 14f;
        teemo.moveSpeed = 330;
    }  
     
}

1.3 If a constructor with parameters is provided

Once a constructor with parameters is provided
At the same time, there is no Explicitly provide a parameterless constructor
Then the default parameterless constructor is "no"

public class Hero {
      
    String name; //姓名
      
    float hp; //血量
      
    float armor; //护甲
      
    int moveSpeed; //移动速度
      
    //有参的构造方法
    //默认的无参的构造方法就失效了
    public Hero(String heroname){ 
        name = heroname;
    }      
    public static void main(String[] args) {
        Hero garen =  new Hero("盖伦"); 
          
        Hero teemo =  new Hero(); //无参的构造方法“木有了”
    }  
      
}

1.4. Overloading of the constructor

Like ordinary methods, constructors can also be overloaded

 public class Hero {
           
        String name; //姓名
           
        float hp; //血量
           
        float armor; //护甲
           
        int moveSpeed; //移动速度
           
        //带一个参数的构造方法
        public Hero(String heroname){ 
            name = heroname;
        }         
        //带两个参数的构造方法
        public Hero(String heroname,float herohp){ 
            name = heroname;
            hp = herohp;
        }           
        public static void main(String[] args) {
            Hero garen =  new Hero("盖伦"); 
            Hero teemo =  new Hero("提莫",383);
        }
         
    }

Exercise: Constructor method

(Design a constructor method with 4 parameters for Hero
These four parameters are
String heroName
float heroHP
float heroArmor
int heroMoveSpeed)

Answer:

public class Hero {
        
    String name; //姓名
        
    float hp; //血量
        
    float armor; //护甲
        
    int moveSpeed; //移动速度
        
    //带一个参数的构造方法
    public Hero(String heroname){ 
        name = heroname;
    }      
    //带两个参数的构造方法
    public Hero(String heroname,float herohp){ 
        name = heroname;
        hp = herohp;
    }    public Hero(String heroName,float heroHP,float heroArmor,int heroMoveSpeed){ 
        name = heroName;
        hp = heroHP;
        armor = heroArmor;
        moveSpeed=heroMoveSpeed;
    }     
    public static void main(String[] args) {
        Hero garen =  new Hero("盖伦"); 
        Hero teemo =  new Hero("提莫",383);
        Hero db =  new Hero("死哥",400,27,360);
    }
      
}

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