"unchecked or unsafe operations." - when this type of exception thrown?

[pjerald]

package test;
 
import java.util.ArrayList;
 
public class MyArrayList{
    public static void main(String args[])
    {
        ArrayList al = new ArrayList();
        al.add("Jerald");
        al.add("PJerald");
        al.add(1);
    }
}
 

I compiled my code as follows. It scold some thing that i dont know..

[$ ~]$ javac MyArrayList.java -d .; java test.MyArrayList
Note: MyArrayList.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

[r035198x]

package test;
 
import java.util.ArrayList;
 
public class MyArrayList{
    public static void main(String args[])
    {
        ArrayList al = new ArrayList();
        al.add("Jerald");
        al.add("PJerald");
        al.add(1);
    }
}
 

I compiled my code as follows. It scold some thing that i dont know..

[$ ~]$ javac MyArrayList.java -d .; java test.MyArrayList
Note: MyArrayList.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Did you recompile with -Xlint unchecked for the details?

[JosAH]

@OP: when Sun decided to introduce 'generics' they also decided that some
objects should be part of it: e.g. their Collection framework. This simply implies
that you don't just have an Arraylist anymore, you have an ArrayList that contains
a certain type, even if the type is just an Object. If you don't specify a type for
the objects to be stored in the ArrayList, all the compiler can do is whine at you.

Just because of backwards compatibility it has to accept that you want to store
just an object (or a derivative thereof) in your ArrayList, but it whines at you.

That's what you saw and you have two options:

1) ignore it and go on programming 'old style' which is semantically valid;
2) agnowledge the fact that you're just storing T type objects in an ArrayList.

For 2) you have to specify the type T using that funny <...> notation and you
have to fill in the dots as in:

ArrayList<String> myList= new ArrayList<String>();
 

It's all about static type checking where the compiler is boss ...

kind regards,

Jos

[pjerald]

@OP: when Sun decided to introduce 'generics' they also decided that some
objects should be part of it: e.g. their Collection framework. This simply implies
that you don't just have an Arraylist anymore, you have an ArrayList that contains
a certain type, even if the type is just an Object. If you don't specify a type for
the objects to be stored in the ArrayList, all the compiler can do is whine at you.

Just because of backwards compatibility it has to accept that you want to store
just an object (or a derivative thereof) in your ArrayList, but it whines at you.

That's what you saw and you have two options:

1) ignore it and go on programming 'old style' which is semantically valid;
2) agnowledge the fact that you're just storing T type objects in an ArrayList.

For 2) you have to specify the type T using that funny <...> notation and you
have to fill in the dots as in:

ArrayList<String> myList= new ArrayList<String>();
 

It's all about static type checking where the compiler is boss ...

kind regards,

Jos

Oh I feel sorry jos and r035198x, since i signed out and left yesterday, but you people took pain and answered. Dear r035198x, then i compiled my code with -Xlint. It showed me three warnings something like this,

[$ ~]$ javac -Xlint MyArrayList.java
MyArrayList.java:9: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
al.add("Jerald");
^
MyArrayList.java:10: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
al.add("PJerald");
^
MyArrayList.java:11: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
al.add(1);
^
3 warnings

Here with this
1) what is actually unchecked ?
2) should i specify explicitly the type of object which i going to add in the ArrayList( Am i wright jos?) ?

Thanks,
P.Jerald

[r035198x]

Oh I feel sorry jos and r035198x, since i signed out and left yesterday, but you people took pain and answered. Dear r035198x, then i compiled my code with -Xlint. It showed me three warnings something like this,

[$ ~]$ javac -Xlint MyArrayList.java
MyArrayList.java:9: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
al.add("Jerald");
^
MyArrayList.java:10: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
al.add("PJerald");
^
MyArrayList.java:11: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList
al.add(1);
^
3 warnings

Here with this
1) what is actually unchecked ?
2) should i specify explicitly the type of object which i going to add in the ArrayList( Am i wright jos?) ?

Thanks,
P.Jerald

Now read Jos' response above.

[pjerald]

Now read Jos' response above.

i think my question number two is the answer!. ie., before we use ArrayList, We must specify what type of object we are going to store. am i right ?

[r035198x]

i think my question number two is the answer!. ie., before we use ArrayList, We must specify what type of object we are going to store. am i right ?

Yep .

[pjerald]

Yep .

k
thanks, r035198x.
regards,
P.Jerald

[JosAH]

Note that you have to rethink the statement at line 11 of your op: a1.add(1).
An int is definitely not a String and it won't be 'autoboxed' to a String either.

kind regards,

Jos