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Android is a phone os and use a Google modified Java version. Java has specific implementation for different system. Limited system can't contain the full framwork or It has specific framework. Java SE for computer Java Card for SIM card Java ME for embedded hardware (phone, TV, whasher and device with limitated hardware) If you use Java SE, your .class works where you have the relative jre for the specific system. So the same .class work on Linux, Mac OS or Windows without recompile the .java source. In 1995 Java was revolutionary with compile one time and execute evreywhere, in a world of compiler specific for a machine in which the source was for that compiler that was for that machine.
why people say Java is multiplatform, While apps written with Java usually work on the one platform?