Initialzer's

[saravana.T kumar]

HI, Sorry for the error in the questions IT is editited correctly now
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1.Static and Instance variable initializer expressions.
Cannot pass on the checked exceptions. Must catch and handle them.

2.Static Initialzer
Cannot pass on the checked exceptions. Must catch and handle them.

Q1.why cannot we pass checked exception's to static and instance VARIBLE initializer.
(The word PASS is what i cannot understand. How can someone pass exception in this context(intiliazer)).

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Q2.can we pass exceptions from constructor to constructor.?

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3.Instance initializer blocks.

Instance Initializers in anonymous classes can throw any exception

Q3.What does it mean by the above statement?

Q. IF the above code is correct then why does this not compile?
new Runnable()
{
{
synchronized(this)
{wait(); //throws checked exception
}
}//instance block
public void run()
}

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Please explain,Thank you

[ July 30, 2007: Message edited by: saravana.T kumar ]
[ July 30, 2007: Message edited by: saravana.T kumar ]

[Deepak Jain]

Static initializers: These are static blocks that are declared within a class with keyword static { .... } and are used to initialize the static memebers of the class, like static methods, static blocks cannot access the instance members of the class.

Instance variable initialiazers: These are instance blocks {....} that are used to initialize the instance variables of the class and like instance methods can access the static and instance members of the class.

class Test {
 
static int i;
int j;
//Static block
static {
  i = 10;
  // j = 20; Compiler error as explained above.
}
 
//Instance block
{
  j = 20;
  i = 10; // as explained above instance blocks can access both static and
// non-static members of the class.
}
 
}// Class declaration ends
 

Now checked exceptions are compile time exceptions and they cannot be thrown out of the above two blocks via the throw keyword, they have to be caught and handled within the blocks. Just imagine, static block is executed whhen the class is loaded if JVM would have allowed us to throw exceptions out of these blocks then who would handle them, JVM cannot handle user exceptions thrown from these blocks.

Example:

public class Test {
 
static {
 
FileInputStream fin = new FileInputStream("a.txt");
}

{
FileInputStream fin = new FileInputStream("a.txt");
}
}

The above code will cause compiler error:

public class Test {
 
static {
try {
FileInputStream fin = new FileInputStream("a.txt");
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

{
try {
FileInputStream fin = new FileInputStream("a.txt");
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

Now it will work.
I could not understand your last question on , Could you please explain in detail and re-phrase your question.
Thanks
Deepak

[Manfred Klug]

Originally posted by Deepak Jain:
Now checked exceptions are compile time exceptions and they cannot be thrown out of the above two blocks via the throw keyword, they have to be caught and handled within the blocks.

You can throw checked exceptions in an instance initializer block if you declare the exception on the constructor(s).

import java.io.*;
 
public class Test {
    {
        FileInputStream fin = new FileInputStream("a.txt");
    }
 
    public Test() throws IOException {}
}

[Deepak Jain]

Hi Manfred,
Your explanation made me think the code inside the instance block is simply moved to the constructor and hence when we change the signature of the constructor to throws IOException there will be no compilation error in instance block even if the IOException is not caught in it. But that was not the case.I modified the code to include a overloaded constructor which led to compilation error in instance block and again modifying the construtor to throw IOException resolved it.

Does this mean each and every constructor must throw all the checked exceptions that are thrown in instance blocks if it doesnt handle it?

Also when excatly the code placed in the instance block is executed, before or after the construtor invocation?

Thanks
Deepak

[Deepak Jain]

Execeptions thrown in instance initializers in annonymouse inner classes muse be
a) Either caught in the initializer.
OR
b) The entire class definition must be within try {...} catch(){}
OR
c) The method that defines the annonymouse inner class must throw the exception.

Also the code that you had mentioned will wait for ever after invoking wait()
Thanks
Deepak

[Manfred Klug]

Originally posted by Deepak Jain:
Does this mean each and every constructor must throw all the checked exceptions that are thrown in instance blocks if it doesnt handle it?

Yes, since the initializer block is always executed, every constructor may throw the exception(s).

Also when excatly the code placed in the instance block is executed, before or after the construtor invocation?

Between the invocation of the super constructor and the body of the constructor.

[Sahid Khan]

Does this mean each and every constructor must throw all the checked exceptions that are thrown in instance blocks if it doesnt handle it?

Another insight to this, instance initialization block becomes part of your constructor. compiler inserts all instance initialization block at the first of all constructors unless that constructor calls another constructor using this keyword. So eventually instance initialization block gets called from all constructors.

Thanks.

[Anonymous]

Hi Sahid, from your statement

Another insight to this, instance initialization block becomes part of your constructor. compiler inserts all instance initialization block at the first of all constructors unless that constructor calls another constructor using this keyword. So eventually instance initialization block gets called from all constructors.

Initialzation block would get called from all the constructors, if we call the constructors explicitly. Explicitly I mean here is not calling a constructor from another, but using keyword "new".

[Manfred Klug]

Originally posted by Chandra Kota:
Explicitly I mean here is not calling a constructor from another, but using keyword "new".

And during the processing of new, the constructor is called.