A unique aspect of Java that allows code compiled on one machine to be executed on a machine of a different hardware platform is Java's

Java's bytecode

Explanation:

To execute its operations, java programming languages uses bytecodes.

These bytecodes are literally instructions of a java virtual machine (or JVM). They are generated in form of a class file as soon as the java program is ran and executed. In other words, the java compiler compiles the code and generates the bytecode.

As soon as the bytecode is generated, it can be transferred to a different machine and platform completely and one can run this bytecode on this different machine.