We discuss this concept with reference to previous example. Given that packages exist and are a good mechanism for compartmentalizing diverse classes from each other, it is easy to see why all of the built in Java classes are stored in packages.
There are no core java classes in the unnamed default package; all of the standard classes are stored in some named package.
Since classes within packages must be fully qualified with their package name or names, it could become tedious to type the long dot-separated package path name for every class you want to use. For this reason, Java includes the import statement to bring certain classes, or entire package into visibility. Once imported, a class can be referred to directly, using only its name.
Note: The import statement is a convenience to the programmer and is not technically needed to write a complete Java program. If you are going to refer to a few dozen classes in your application, however, the import statement will save a lot of typing.
In Java source file, import statements occur immediately following the package statement. (if it exists) and before any class/interface definitions.
The geneal form of import statement:
import pkg1 [.pkg2] [.pkg3].[classname | *];
Note:
1. The star form may increase compilation time. However, the star form has absolutely no effect on the run-time performance or size of your classes.
2. All of the standard Java classes included with Java are stored in a package called java. The basic language functionality is stored in a package inside of the java package called java.lang. Normally, you have to import every package or class that you want to use, but since Java is useless without much of the functionalities in java.lang, it is implicitly imported by the compiler for all programs. This is equivalent to the following line being at the top of all of your programs:
import java.lang.*;
3. If a class with the same name exists in two different packages that you import using the star form, the compiler will remain silent, unless you try to use one of the classes. In that case, you will get a compile-time error and have to explicitly name the class specifying its package.
4. Any place you use a class name, you can use its fully qualified name, which includes its full package hierarchy. When a package is imported, only those items within the package declared as public will be available to non-subclasses in the importing code.
There are no core java classes in the unnamed default package; all of the standard classes are stored in some named package.
Since classes within packages must be fully qualified with their package name or names, it could become tedious to type the long dot-separated package path name for every class you want to use. For this reason, Java includes the import statement to bring certain classes, or entire package into visibility. Once imported, a class can be referred to directly, using only its name.
Note: The import statement is a convenience to the programmer and is not technically needed to write a complete Java program. If you are going to refer to a few dozen classes in your application, however, the import statement will save a lot of typing.
In Java source file, import statements occur immediately following the package statement. (if it exists) and before any class/interface definitions.
The geneal form of import statement:
import pkg1 [.pkg2] [.pkg3].[classname | *];
Note:
1. The star form may increase compilation time. However, the star form has absolutely no effect on the run-time performance or size of your classes.
2. All of the standard Java classes included with Java are stored in a package called java. The basic language functionality is stored in a package inside of the java package called java.lang. Normally, you have to import every package or class that you want to use, but since Java is useless without much of the functionalities in java.lang, it is implicitly imported by the compiler for all programs. This is equivalent to the following line being at the top of all of your programs:
import java.lang.*;
3. If a class with the same name exists in two different packages that you import using the star form, the compiler will remain silent, unless you try to use one of the classes. In that case, you will get a compile-time error and have to explicitly name the class specifying its package.
4. Any place you use a class name, you can use its fully qualified name, which includes its full package hierarchy. When a package is imported, only those items within the package declared as public will be available to non-subclasses in the importing code.