Java package
A package is a grouping of related types providing access protection and name space management. Packages in Java is a similar concept to namespaces in C#.
Creaging a package in Java
A package is declared with the package keyword. This statement must be placed at the top of every source file. There can be only one such statement in each source file. The Java source files must be placed in directories that match the package name.
All types defined in the file with the above package are part of the com.zetcode package. A class Being has a fully qualified name com.zetcode.Being .
There are millions of Java programmers worldwide. To avoid potential name conflicts, there is a naming convention in Java. The package names use reversed Internet domain names. The letters are written in lowercase. There can be only one zetcode.com domain name so using a reversed name com.zetcode for the packages will make them unique. A Java source file with a com.zetcode package must be located in the com/zetcode subdirectory. Package names are written in all lower case to avoid conflict with the names of classes or interfaces.
The import keyword is used at the beginning of a source file to specify types (classes, interfaces, enumerations, or annotations) or entire Java packages to be referred to later without including their package names in the reference. Import statements can import static members of a class (methods and variables).
Using the * wildcard character, we can import the whole package at a time. After this import, we can refer to all types of the java.awt package without their fully qualified names.
The java.awt.event subpackage is not imported with the java.awt.* import. Subpackages must be imported independently.
In this case, only Random class is imported. Now the Random class can be referenced using its simple class name.
Core packages in Java
- java.lang — basic language functionality and fundamental types
- java.util — collection data structure classes
- java.io — Java API for file operations
- java.math — multiprecision arithmetics
- java.nio — the Non-blocking I/O framework for Java
- java.net — networking operations, sockets, DNS lookups, .
- java.security — key generation, encryption and decryption
- java.sql — Java Database Connectivity (JDBC) to access databases
- java.awt — basic hierarchy of packages for native GUI components
- javax.swing — hierarchy of packages for platform-independent rich GUI components
- java.beans — Contains classes related to developing beans — components based on the JavaBean architecture.
- java.text — Provides classes and interfaces for handling text, dates, numbers, and messages in a manner independent of natural languages.
- java.rmi — Java API for remote method invocations.
- java.time — The main API for dates, times, instants, and durations.
The java.lang package is available without the use of an import statement.
Practical example
The following example shows how to create a package and import types.
package com.zetcode; import java.util.Random; public class Packages < public static void main(String[] args) < var r = new Random(); int x = r.nextInt(); System.out.println(x); java.time.LocalDateTime dt = java.time.LocalDateTime.now(); System.out.println(dt); >>
The example uses two types: the Random class and the LocalDateTime class. The first class is imported and the second is referred by its fully qualified name.
We declare a package with the package keyword. The Packages.java file must be located in the com/zetcode subdirectory.
This code line enables us to use the Random class without the package name.
Here we use the Random without using its full name.
java.time.LocalDateTime dt = java.time.LocalDateTime.now();
If we did not use the import keyword on a type, we can refer to it only by its full name — java.time.LocalDateTime in our case. The import keyword saves a lot of typing.
$ ls com/zetcode/ Packages.java
The Packages.java source file is placed in the com/zetcode subdirectory. The package name must reflect the directory structure.
$ javac com/zetcode/Packages.java
We compile the source file with the javac tool. The tool is called from the parent directory of the com/zetcode directory.
$ java com.zetcode.Packages 897739291 2022-09-26T12:25:06.699181327
Java package-private visibility
If we do not specify any access modifier (e.g. private , protected , or public ), we have a package-private visibility. In such a case, variables and methods are accessible within the same package. Classes in other packages cannot access classes and members declared with package-private access.
Java default package
If no package is declared, all types defined in that file are part of a default unnamed package. It is recommended to always place your types in a package. Even for small programs.
public class DefaultPackage < public static void main(String[] args) < System.out.println("A class in a default package"); >> The DefaultPackage class is part of the default package.
If we do not specify a package, we do not place the source file in a specific subdirectory.
$ javac DefaultPackage.java $ java DefaultPackage A class in a default package
We compile the code and run the application. The source file and the bytecode is located in the current working directory.
Java automatic imports
Java compiler automatically imports two packages: java.lang and the current package.
package com.zetcode; public class Constants
The Constants class is located in the same package as the AutomaticImports which is referring to its version member.
package com.zetcode; public class AutomaticImports < public static void main(String[] args) < String os = System.getProperty("os.name"); System.out.println(os); System.out.println(Constants.version); >> In this example, we refer to some classes that are automatically imported by Java compiler.
String os = System.getProperty("os.name"); The String and System classes are part of the java.lang package.
System.out.println(Constants.version);
The Constants class is located in the same package as the AutomaticImports class. Therefore, we can access the class and its member without using the fully qualified name or utilizing the import keyword.
$ ls com/zetcode/ AutomaticImports.java Constants.java
Both AutomaticImports.java and Constants.java files are located in the same subdirectory.
$ javac com/zetcode/AutomaticImports.java com/zetcode/Constants.java
$ java com.zetcode.AutomaticImports Linux 1.0
This is a sample output of the com.zetcode.AutomaticImports program.
Java static imports
If we often use some static members, we can use import static statement to refer to them later without a full class name. Static imports should be used with caution.
package com.zetcode; import static java.lang.Math.E; import static java.lang.Math.PI; import static java.lang.Math.abs; public class StaticImport < public static void main(String[] args) < System.out.println(E); System.out.println(PI); System.out.println(abs(-5)); >>
In this example, we refer to two constants and one static method.
import static java.lang.Math.E; import static java.lang.Math.PI; import static java.lang.Math.abs;
We use the import static statement to enable referring to them without their full names.
System.out.println(E); System.out.println(PI); System.out.println(abs(-5));
We refer to these three members without their class name.
$ java com.zetcode.StaticImport 2.718281828459045 3.141592653589793 5
In this article we covered packages in Java. We have shown how organize our code in packages.
Author
My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.
Which Package is Imported by Default in Java?
A package in Java is a collection of similar Java classes, sub-packages, and interfaces. Packages wrap related classes and interfaces together thus, it is easier for programmers.
In Java, two packages java.lang package and a no-name package (also called default package ) are imported by default in all the classes of Java. Default Package doesn’t have a name but it is present by default and thus, it is named the default package . Java Virtual Machine imports these packages by default in Java internally. And that is the reason we are able to access all the classes of these packages.
We are not required to explicitly import java.lang package or any of its classes like we import other packages and their classes such as java.math or java.util .
The default package in Java doesn’t have a name and is a collection of classes whose source files do not contain a package declaration.
Package Declaration
Here package project1 is a package declaration and it tells the compiler that this class is a part of project1.
When we don’t mention package declaration and directly create a class it is placed in the default package or no-name package of Java. This default package is also imported along with java.lang every time a java class file is created.
This class DefaultPackage is part of the default package.
java.lang package
The java.lang package provides fundamental classes of the Java Programming language design. The two important classes Object and Class, instances of which represent classes at runtime are part of this package.
Other important classes of the package are:
- Integer , Character , Double , Float , Long , Number , String , void , etc. which are common data types used in java programs to store data.
- Compiler , Process , ProcessBuilder , Runtime , SecurityManager , System , StackTraceElement , Thread , etc. which are important to run a java program securely and properly.
- StrictMath class which contains common numeric operations.
In the example above, the object of the String class sayHello is created as the String class is part of the java.lang package and it is by default imported. Also, as we don’t declare any package, the class myClass is part of the default package. The class System that we use to take input from the user or to print the output is also part of the java.lang package. Using Object and Class classes we are able to represent MyClass as a class and create its objects such as fun .
Learn more about Packages in Java
Conclusion
- The default package (which is also known as the no-name package as it doesn’t have the name) and java.lang packages are by default imported in the java class by the JVM .
- They contain classes that are fundamental to the design of the Java language. These classes provide basic functions to the java program.