Problem in Creating the Instance Intializer Block against JLS

[sakthi moorthy]

Hi All,

I am refering the JLS (Instance Intializers Chapter 8.6) which tells the following thing

An instance initializer declared in a class is executed when an instance of the class
is created (§15.9), as specified in §8.8.7.1.
InstanceInitializer:
Block

It is compile-time error if an instance initializer of a named class can throw a
checked exception unless that exception or one of its supertypes is explicitly
declared in the throws clause of each constructor of its class and the class has at
least one explicitly declared constructor. An instance initializer in an anonymous
class (§15.9.5) can throw any exceptions.

And i tried to create a code as the per the JLS, i not able to suceed.

import java.io.IOException;

public class HHH {

	{  // Instance Intializer block
		throw new IOException(); // line1
	}
	
	
	HHH() throws IOException { // explictly declaring the IOException in throws caluse of constructor && i have atleast one default Constructor too
	}

}

i am getting compile time error "Initializer does not complete normally" in line 1, but as per the above quoted JLS Stanza, it should not give compile time error because
1) I have atleast one default constructor
2) that Constructor is also throwing the same Checked, IOException, what i am throwing in intance intializer block

So can someone tell me where i am going wrong here.

[Stuart A. Burkett]

sakthi moorthy wrote:It is compile-time error if an instance initializer of a named class can throw a
checked exception unless that exception or one of its supertypes is explicitly
declared in the throws clause of each constructor of its class

The important word in that quote is 'can'. There has to be a possibility that an exception can't be thrown, otherwise an instance of the class could never be created in which case there is no point to the class.
Try this

public class HHH {  

    private boolean s = false;
    
    {  // Instance Intializer block  
        if (s)
            throw new IOException(); // line1  
    }  


    HHH() throws IOException { // explictly declaring the IOException in throws caluse of constructor && i have atleast one default Constructor too  
    }  

}  

[sakthi moorthy]

Hi Stuart,


Thanks for the reply. I will tell my understanding of the quoted JLS Sentence.

It is compile-time error if an instance initializer of a named class can throw a
checked exception unless that exception or one of its supertypes is explicitly
declared in the throws clause of each constructor of its class and the class has at
least one explicitly declared constructor

In order to throw a checked exception from Instance Intializer following conditions needs to be statisfied

1) At Least One explicitly declared Constructor Say HHH() in this case has to be present
2) The Same that is HHH() Constructor has to throws A Exception or the Exceptions Super Type, so declared the constructor like following

                HHH() throws IOException { 
                 }  

If above conditions are satisfied, you can have an instance intializer block that can throw a Checked Exception like following

 
{    
        throw new IOException(); // line1  
}  

I am not able to relate my understanding with your code, to be specific i am not able to understand why you have kept if block in instance intializer. If i am not wrong, statements under conditional checking block will be executed only on certain condition or conditions, which may or may not happen,thats why that compilation problem has resolved.

[Stuart A. Burkett]

In your code an IOException will always be thrown, therefore it is impossible to create an instance of your class. Your code is therefore basically useless. The compiler realises this and so doesn't let you do it. In my code there's a path through the initialiser that will not throw an exception (when s is false) and so the compiler does not complain.

See section 8.6 of the Java 7 JLS

It is a compile-time error if an instance initializer cannot complete normally

[sakthi moorthy]

Thanks Stuart i got it now


I wanted to convey one more thing here the code

public class HHH {    
  
    private boolean s = false;  
      
    {  // Instance Intializer block    
        if (s)  
            throw new IOException(); // line1    
    }    
  
  
    HHH() throws IOException { // explictly declaring the IOException in throws caluse of constructor && i have atleast one default Constructor too    
    }    
  
}

will compile fine until the default Constructor HHH() throws IOException or IOException's super type, which means if you want to throw a Exception in Intializer block, the same Exception should be thrown in all Constructors thats what the JLS Says. so the following code wont compile.

public class HHH {

	boolean s = false;
	{ 
		if(s)
		throw new IOException(); // compile time error here.....
	}
	
	
	HHH(){
	}

}

Again thanks Stuart