Top Most Notes on Declarations and Access control

1. Following are invalid declarations,

int a[3];

int a[] = new int[];

int a[][] = new int [][4];

int a[] = new int[2]{1,2};

2. Remember these points,

int [] a1 ,a2 ,a3;

The above declaration symbolizes that a1, a2, a3 are dmensional arrays.

Where as,

int a1 [], a2 , a3 ;

The above declaration symbolizes that a1 is a one dimensional array where as a2 and a3 are simple integer numbers.

→ General rule is that the big brackets i.e  ‘[ ]’ must appear after the type name like int , long etc or after the identifier name. If the  ‘[]’ appears after the type name then all the identifiers after that type name will be arrays of the specified type where as if the  ‘[]’ appears after the identifier name then it only qualifies as the array of the specified dimension.

→array of a primitive type is a class derived from Object class itself and it also implements Cloneable and java.io.serializable.

→ ‘[]’ cannot occur before the identifier name during declaration i.e

int a1,[]a2,a3;//causes error because ‘[]’ appears before a2 and not after a2.

3. While equating array reference variables if the dimensions aren’t appropriate then  compile time error is not caused but runtime error sets in.

4. In a given file only one public class can exist and also the name of the public class and the file must be the same.

5. protected , private access modifiers cannot be applied to top level classes.

6. among the other modifiers that cannot be applied to a top level class are static , synchronized ,native etc but we can apply abstract , final , strictfp.

7. An anonymous class is not local class i.e local class is always a named class. Furthur an anonymous class is always final.

8. A final field can be assigned a value only once (not necessary to do the initialization in the declaration part itself ).

9. Compiler error is caused if the final instance variables which are declared aren’t initialized by the end of each constructor.

10. if the final variable is method local , then even if it isn’t initialized compiler won’t complain ,unless it is used in some expression.

11. A regular inner class may be declared both abstract and private but not abstract and final.

12. Field variables and methods can share the same name as that of the enclosing class.

13. A nested class and the outer class can’t share the same name.

14. But a nested class , method , field variable can share same name provided the names of the nested class and enclosing class are different.

15. A top level class can be declared abstract even if it doesn’t contain any abstract methods. It can even extended one of concrete classes and can implement other interfaces.

16. Return types :-consider

byte m1(){

final int i = 1;

return i ;

}

here the method compiles fine as the integer variable is marked final and it’s value is known at compile time.

byte m2(final int i){

return i ;

}

here though variable ‘i’ is marked final ,it’s value is not determined at compile time and hence it results in compile time error.