How to use java package

A package in Java is a class library unit. The package contains a group of classes, which are organized together under a single namespace. This namespace is the package name.

#So how to use Java package?

You can use the import keyword to import a package. For example, use import java.util.* to import all classes in the namespace java.util package. The so-called importing all the classes in this package means that after the import declares the package name, the classes in the package can be used directly in the subsequent program.

For example:

import java.util.*  
  
public class SingleImport  
{  
    public static void main(Strin[] args)  
    {  
        ArrayList list=nwe ArrayList();  
    }  
}

Here ArrayList is a class in the java.util package, but because the import keyword is used for the program to load the java.util package, so There is no definition and declaration of the ArrayList class here, nor is there any qualified name in front of the class, so you can use this class directly.

The reason why we want to import package names is to provide a mechanism for managing name spaces.

We know that if there are two classes, class A and class B, both contain a method f() with the same feature tag (parameter list), even if these two methods f() are used at the same time in the same piece of code ), there will be no conflict. The reason is that there are two different class names in front as qualified names, so there will be no conflict even if the two methods have the same name. But what if the class names conflict with each other? Suppose you write an Apple class and install it on a machine that already has an Apple class written by someone else. How do we solve this problem?

Because it is not easy to figure out which classes are installed on a machine, there is always the possibility of potential conflicts between names. It is very important in Java to have full control over the namespace and create a unique combination of identifiers for each class. If you want to write a class library or program that is friendly to other Java programs that coexist on the same machine, you need to consider how to prevent conflicts between class names.

When writing a Java source code file, this file is usually called a compilation unit.

Each compilation unit must have a suffix name of .java, and there is only one public class in the compilation unit, otherwise the compiler will not accept it.

The name of the public class must be the same as the name of the file (including upper and lower case, but not including the suffix .java). If there are additional classes in the compilation unit, these classes are not visible to the world outside the package because they are not public classes and they are mainly used to provide support for the main public class.

When compiling a .java file (i.e. a compilation unit), each class in the .java file will have an output file, and the name of the output file is the same as the name of each class in the .java file. The names are the same, but with an additional suffix of .class. Therefore, after compiling a small number of .java files, a large number of .class files will be obtained.

Each .java file will have a public class after compilation, and any number of non-public classes. Therefore, each .java file is a component. If you want many such components to belong to the same group, you can use the keyword package in each .java file. And this group is a class library.

The above is the detailed content of How to use java package. For more information, please follow other related articles on the PHP Chinese website!