Hey Abha, JAR files are fundamentally archive files, build on ZIP file format, typically used to collect many Java Class files and have the .jar file extension. So to use the Classes of Java containing predefined functions, all the Classes of Java are grouped into .jar files. And all the .jar files of Java are collected in one library i.e. JRE System Library. When any Java Project in Eclipse IDE uses this JRE System Library, it will have access to all the predefined function of Java
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