用户定义的异常中如何调用toString？

Question

Here how the toString() method executes no explicit call is made to it? can anyone please explain the control flow of it

class MyException extends Exception
{
    String str1;
    MyException(String str2) {
       str1=str2;
    }
    public String toString(){ 
       return ("Output String = "+str1) ;
    }
}

class CustomException{
    public static void main(String args[]){

       try{   throw new MyException("Custom");   }
       catch(MyException e){ System.out.println(e); }
    }
}

Answer 1

When an object is passed to System.out.println it's toString() method is called (so this result can be displayed), whatever this object might be (Exceptions are also objects).

So, when you do:

catch(MyException exp) { 
    System.out.println(e);  
}

System.out.println takes e , call's it's toString() and displays this result onto console.

Answer 2

Since toString() is part of Object , which all objects inherit from, System.out.println(Object o) ( out is a PrintStream ) knows there is a toString() implementation and it uses it ( String.valueOf(Object o) is called first).

Since toString has been overridden - it uses the specified implementation.

Prints an Object and then terminate the line. This method calls at first String.valueOf(x) to get the printed object's string value, then behaves as though it invokes print(String) and then println().

Answer 3

The documentation of the PrintWriter.println(Object) method says:

Prints an Object and then terminates the line. This method calls at first String.valueOf(x) to get the printed object's string value, then behaves as though it invokes print(String) and then println() .

The method String.valueOf(Object) uses the parameter's toString() method unless it is null.

This is why toString() is called.

Answer 4

System.out.println prints an Object and then terminate the line.

This method calls at first String.valueOf(x) to get the printed object's string value, then behaves as though it invokes print(String) and then println() .

String.valueOf(x) return if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned, in your case, it's

Output String = Custom

Answer 5

When you call System.out.println(e) :

     public void println(Object x) {
         String s = String.valueOf(x);
         synchronized (this) {
             print(s);
             newLine();
         }
     }

This method calls String.valueOf(x) :

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

Do you see obj.toString() ? You can reference source-code here PrintStream.println , String.valueOf