Wednesday, February 6, 2013

SCJP 6 doubts: implicit imports

There are implicit imports that do not need to be declared.
The classes in java.lang package can be used without importing any package, this is an implicit import, so don't be fooled if you see a question that looks like that the import is missing.

What classes can we see in the java.lang package?
JavaSE 6 API just tells us which ones belong to this package: basically its the String, StringBuilder, StringBuffer, Thread, wrappers (Integer, Float,...), Object, System, Runtime, Exception, and a lot of other classes which you can use directly.

The real exam always has line numbers on the left side of the code, so if you see that it does not start with line 1, you don't have to worry about the imports, and assume that the imports (and packages) are right.
If the code starts at line 1, just assume you're looking at the entire file. If you see that the code uses classes that belong to other packages besides java.lang, (e.g class File needs import*) then you can say for sure that the right answer is "compilation fails" due to missing imports.

Default package
If you have a class within the default package (no package declaration is found), and you create another class that has a relashionship with the first one, then no import is needed (because both are in the default package);

1. public class A{
2. int i=0;
3. }

1. public class B extends A{
2. }

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