'Why create java extension classes when you can just create classes as part of java program?

I was reading about class loaders in java, came across Extension class loaders. As per my understanding these extension classes are like JRE classes. They will be available to multiple java programs using the that particular JRE, Apart from that does it have any other advantages compared to custom java classes that we create as part of java programs? What if I have only one Java program using my JRE, Is there any use case for creating extension classes?



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