If you want to use a class in a Java package, you must first import it using an import statement.
The import statement is somewhat similar to the #include in the C language, with the following syntax:
Import Package1[.package2 ...]. ClassName
Packages are package names, ClassName are class names. For example:
1 Import // importing the Date class under the Java.util package 2 Import // Import the Scanner class under the Java.util package 3 Import // Import all classes under the javax.swing package, * denotes all classes
Attention:
- Imports can import only the classes that the package contains, not the packages.
- For convenience, we generally do not import separate classes, but instead import all of the classes under the package, such as import java.util.*;.
The Java compiler defaults to all Java programs importing all the classes in the JDK's Java.lang package (import java.lang.*;), which defines some common classes, such as System, String, Object, Math, and so on. So we can use these classes directly without having to explicitly import them. However, the use of other classes must first be imported.
The "Hello World" program mentioned earlier uses SYSTEM.OUT.PRINTLN (); Statement, the System class is located in the Java.lang package, although we did not explicitly import the class in this package, but the Java compiler has imported it for us by default, or the program will fail to execute. The Java class's search path assumes the following import statement:
1 Import p1. Test;
This statement indicates that you want to import the Test class in the P1 package.
When installing the JDK, we have set the environment variable CLASSPATH to.; %java_home%\lib, and Java_home is D:\Program files\jdk1.7.0_71, so CLASSPATH is equivalent to.;D: \ Program Files\jdk1.7.0_71\lib.
The Java runtime will look in the following path and load the bytecode file Test.class:
- . P1\test.class ("." Represents the current path)
- D:\Program Files\jdk1.7.0_71\lib\p1\test.class
If the desired class file is found under the first path, stop the search, or continue searching for the following path, or compile or run an error if the required class file cannot be found under all paths.
You can add a search path to the CLASSPATH variable, for example.; %java_home%\lib; C:\javalib, then you can put the class file in the C:\javalib directory, the Java Runtime environment will be found.
Series Articles:
Java know how much (1) Language overview
Java know how much (2) virtual machine (JVM) and cross-platform principle
Java know how much (3) employment direction
Java know how much (4) the difference between J2SE, Java EE, J2ME
Java know how much (5) Build Java development environment
Java know how much (6) The first example of a program
Java knows how many (7) classes and objects
Java know how much (8) class library and its organizational structure
Java know how much (9) Import and Java class search path
Java know how much (9) Import and Java class search path
