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5.1.2 Solving the Fragile Superclass Problem
The Java language solves the fragile superclass problem in several stages. The Java compiler doesn't compile references down to numeric values--instead, it passes symbolic reference information through to the byte code verifier and the interpreter. The Java interpreter performs final name resolution once, when classes are being linked. Once the name is resolved, the reference is rewritten as a numeric offset, enabling the Java interpreter to run at full speed.
Finally, the storage layout of objects is not determined by the compiler. The layout of objects in memory is deferred to run time and determined by the interpreter. Updated classes with new instance variables or methods can be linked in without affecting existing code.

At the small expense of a name lookup the first time any name is encountered, the Java language eliminates the fragile superclass problem. Java programmers can use object-oriented programming techniques in a much more straightforward fashion without the constant recompilation burden engendered by C++. Libraries can freely add new methods and instance variables without any effect on their clients. Your life as a programmer is simpler.

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