使用 public static void main(String args[]) 从子 class JAVA 获取 output

Question

I am trying to understand the issue here, it stated the command execution failed, but I cannot seem to find the issue here but I do get it that one of the reasons is public static void main(String args{}) is not functioning as I hope so.

My code is down below:

import java.util.*;

public class Children {
    public static void main(String args[]) {
       public String name;
       public int friends;

       public Children(String name) {
           this.name = name;
           friends = 0;
       }
       public void getFriend(Children k) {
           friends++;
       }

       public int numFriends() {
           return friends;
       }

       public String playsalong() {
           return "joy";
       }

       public String toString() {
           return "a child named "+name;
       }
   }
}

class Girl extends Children {
   public static void main(String args[]) {
       public Girl(String name) {
           super(name);
        friends = 1;
       }

       public void getFriend(Children k) {
           if (k instanceof Girl)
           friends += k.numFriends( );
           else
           friends++;
       }

       public String toString( ) {
           return "a girl named "+ name;
       } 
   }
}

class Boy extends Children {
   public static void main(String args[]) {
       public Boy(String name) {
           super(name);
       }

       public String toString() {
           return "a boy named " +name; 
       } 
   }
}

Answer 1

the access modifiers are applied in instance variable, it can not be applied inside local scope what ever is static field or not, it's for describing the access level for other object, object of classes look at instance membres.

another issue you are trying to define a constructor of class Girl inside the main method and outside the class Girls itself, which is not allowed class consructor should be inside that class and in instance scope not in any other method static or not static.

constructor should be defined in an instance scope of it's class not in other class

so

public Girl(String name) {
       super(name);
    friends = 1;
   }

this code should be defined in Girl class and be sure that is defined inside instance scope not static or local scope.

for other methods you just need an curly brace "}" to end the main method the other methods will be defined speratly.

for the class Children same thing appilied

constructor of class Boy should be replaced by Children constructor and defined outside main method.

method toString is really fine just need curly brace } befor it to encolse the main method, because here you are defining it inside main method.

here is the valid Children class:

public class Children {
   public String name;
   public int friends;
   
   public Children(String name) {
        super()// this optional because parent class define only no- 
                     // argument constructor
       this.name = name;
       friends = 0;
   }

public static void main(String args[]) {

       Children children = new Children();
             children.getFriend();
             children.numFriends();
             children.playsalong();
             children.toString() ;
  
  }

   public void getFriend(Children k) {
       friends++;
   }

   public int numFriends() {
       return friends;
   }

   public String playsalong() {
       return "joy";
   }

   public String toString() {
       return "a child named "+name;
   }
 

}

and here the Girl class definition

class Girl extends Children {
      public Children(String name) {
   // super(name); /*here superclass do not define any constructor 
                  with String argument so this will not compile*/
       super()// this is what required and it optional as said before

       friends = 1;
   }
 public static void main(String args[]) {
            
            Girls girl = new Girl();
   
      }

   public void getFriend(Children k) {
       if (k instanceof Girl)
       friends += k.numFriends( );
       else
       friends++;
   }

   public String toString( ) {
       return "a girl named "+ name;
   } 

}

the boy class definition

class Boy extends Children {
  public Boy(String name) {
     super(name);/* this is fine and it mendatory since parent class 
          Children has a constructor with String parameter, it should 
          be called like this or code will not compile */
    }
   public static void main(String args[]) {

      Boy boy = new Boy("Alex");
   }