Jar packages are classes written by others, and then these classes are packaged. You can introduce these jar packages into your project, and then you can directly use the classes, properties and methods in these jar packages.

JAR(Java ARchive) makes a series of record group into a compressed file, just like Zip. However, like everything else in Java, JAR files are cross-platform, so you don't have to care about specific platform issues.

JAR files are especially useful for Internet applications. Before the JAR file, the Web browser must repeatedly request the Web server to download all the files that make up the "Applet". In addition, every file is uncompressed. But after merging all these files into a JAR file, you only need to send a request to the remote server once. At the same time, due to the adoption of compression technology, all data can be obtained in a shorter time.