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In Chapter 6, you learned that Java is a call-by-value language: Memory is allocated for each parameter when a method is called, so parameter reassignments inside a method are prevented from propagating outside the method.
When an argument is a reference-type variable, the memory allocated for the parameter holds a copy of the address of the argument. Both addresses point to the same place:
Memory allocated for a Memory that supplies a value reference variable in for a corresponding reference calling method parameter in called method | | v v *----------* Copy operation *----------* | Address | ---------------------> | Address | *----------* *----------* | | | *---------------------------* v v *----------* | Instance | | | *----------*
Because the address is copied, a parameter reassignment inside a method can propagate outside the method.