newbie's question about constructor

[SkyGriller tian]

public class Rectangle {
    private int x, y;
    private int width, height;
        
    public Rectangle() {
        this(0, 0, 0, 0);
    }
    public Rectangle(int width, int height) {
        this(0, 0, width, height);
    }
    public Rectangle(int x, int y,
                     int width,
                     int height) {
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
    }
    ...
}

above is what i get from oracle official site. so i tried some exercise myself. below is my code.

public class EnumDemo {
      private Day day;
      public EnumDemo(Day varia){
          this.day=varia;
      }
      public void evaluate(){
      switch(day){
      case MONDAY: 
         System.out.println(day+ " is bad!");
      case FRIDAY:
         System.out.println(day+ " is better!");
      case SUNDAY: case SATURDAY:
         System.out.println(day+ " is the best!");
      default:
         System.out.println("Mid-days are so-so!");
        }
      }
      public static void main(String[]args){
      EnumDemo tryThis=new EnumDemo(Day.THIRSDAY) ;
      tryThis.evaluate();
      }
}

the problem is : when i replace "this.day=varia" with "this(varia)", the system just gave me an error about constructor.

could anyone explain why.
tons of thanks!

[Jeff Verdegan]

1. It helps if you copy/paste the exact, complete error message (though I think I know what it is in this case).

2. It also helps if you carefully read the error message, especially the first couple lines, as it usually tells you exactly what you're doing wrong.

3. Look closely and see if you can spot the key structural difference between what you're doing and what the example is doing.

// example
class Person {
  Person(String name) { ... }

  Person() {
    this("default name");
  }
}

// you
class Person {
  Person(String name) {
    this(name);
  }
}

4. If you're doing it for an academic exercise, that's fine, but note that Java has enums at the language level, so you don't need to simulate one.

[SkyGriller tian]

sorry,i forgot to say that i had a Day.java file in the same package of EnumDemo.java, the content of which is below

public enum Day{
MONDAY, TUESDAY, WEDNESDAY, THIRSDAY, FRIDAY, SATURDAY, SUNDAY
}


the error message is not in english, i don't know how to translate it exactly into english....

[javadoc]
//myversion
public class EnumDemo {
private Day day;
public EnumDemo(Day varia){
this(varia);
}[/javadoc]

[javadoc]
//tutorial's version
public Rectangle(int width, int height) {
this(0, 0, width, height);
}[/javadoc]


as you can see above, the two versions are quite similar... ...

[Chetan Sarnad]

SkyGriller tian wrote:

public class EnumDemo {
      private Day day;
      public EnumDemo(Day varia){
      [color=red]this.day=varia;[/color]
      }
      public void evaluate(){
      switch(day){
      case MONDAY: 
         System.out.println(day+ " is bad!");
      case FRIDAY:
         System.out.println(day+ " is better!");
      case SUNDAY: case SATURDAY:
         System.out.println(day+ " is the best!");
      default:
         System.out.println("Mid-days are so-so!");
        }
      }
      public static void main(String[]args){
      EnumDemo tryThis=new EnumDemo(Day.THIRSDAY) ;
      tryThis.evaluate();
      }
}

the problem is : when i replace "this.day=varia" with "this(varia)", the system just gave me an error about constructor.

could anyone explain why.
tons of thanks!

Can you specify your requirement for using this(varia)?
Wont it be a recursive invocation of the constructor? i.e the constructor calling itself.

And if you want to assign values to your instance variables, the correct syntax would be

this.variableName = value;

[Jeff Verdegan]

SkyGriller tian wrote:
as you can see above, the two versions are quite similar... ...

I already saw how they are similar and how they are different. The difference is very important. It's why you're getting the error. Look closely at my post. It shows the difference.

Even if you cannot translate the error message into English, can you read it and understand it? Between that and my sample code above, you should have an idea of the problem. You can also step through the code yourself, manually, and figure out what each step will do.

Using my example (which is equivalent to your code):

class Person {  
  Person(String name) {  
    this(name);   // WHAT DOES THIS LINE DO?
  }  
}  

...

Person p = new Person();

What do you think will happen if we try to execute that code? In particular, what do you think the commented line will do?

[Campbell Ritchie]

SkyGriller tian wrote: . . . i forgot to say that i had a Day.java file in the same package of EnumDemo.java, . . .

You really ought to tell us all the details. How can we help if we don’t know that sort of information.

the error message is not in english, i don't know how to translate it exactly into english....

Even that information would have been helpful earlier; it would have shown you are making an effort, and somebody else could have tried your code (had it been complete) to get the English error message.

//myversion
public class EnumDemo {
      private Day day;
      public EnumDemo(Day varia){
      this(varia);
      }

//tutorial's version
public Rectangle(int width, int height) {
        this(0, 0, width, height);
    }

as you can see above, the two versions are quite similar... ...

No, they are not similar. They are both incomplete, so we cannot see the differences properly. Please quote the tutorial you found that on, so we can inspect the whole code. The use of this(...); entails constructor overloading. Have you got an overloaded constructor?

[SkyGriller tian]

Chetan Sarnad

your post just exactly presented the error i got.thanks.

and Jeff, thanks, i'm will examine my code again ang again closely! thanks

[Jeff Verdegan]

SkyGriller tian wrote:
and Jeff, thanks, i'm will examine my code again ang again closely! thanks

You're welcome. I was making the same point Chetan was, but I was trying to get you to see for yourself what the error means and why you were getting it.

Do go back and look at my example and the other example and make sure you understand the difference between what they did and what you were doing in the erroneous code.