Java Reference
In-Depth Information
Suppose you want to use class
A
from package
p2
in the previous example. That is, there is a class
p2.A
and you
want to declare the instance variable
var
of type
p2.A
in class
B
instead of
p1.A
. Let's try to solve it by importing class
p2.A
using a single-type-import declaration, like so:
// B.java - Includes a new import declaration
package p1;
import p2.A;
class B {
A var; // We want to use p2.A when you use A
}
class A {
// Code goes here
}
When you compile the modified
B.java
file, you will get the following compilation error:
"B.java": p1.A is already defined in this compilation unit at line 2, column 1
What is wrong with the modified source code? When you remove the single-type-import declaration from it,
it compiles fine. It means it is the single-type import declaration that is causing the error. Before you resolve this
compiler error, you need to learn about a new rule about single-type import declarations. The rule is
It is a compile time error to import more than one type with the same simple name using multiple
single-type-import declarations.
Suppose you have two classes,
p1.A
and
p2.A
. Note that both classes have the same simple name
A placed in
two different packages. According to the above rules, if you want to use the two classes,
p1.A
and
p2.A
, in the same
compilation unit, you cannot use two single-type-import declarations.
// Test.java
package pkg;
import p1.A;
import p2.A; // A compile-time error
class Test {
A var1; // Which A to use p1.A or p2.A?
A var2; // Which A to use p1.A or p2.A?
}
The reason behind this rule is that the compiler has no way to know which class (
p1.A
or
p2.A
) to use when you
use simple name
A
in the code. Java might have solved this issue by using the first imported class or last imported
class, which might have been error prone. Java decided to nip the problem in the bud by giving you a compiler error
when you import two classes with the same simple names, so you cannot make silly mistakes like this and end up
spending hours resolving them.
