10.6: Chapter Summary

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If you override = then you should override hash as well.
Override postCopy to correctly implement copying for your objects.
Use self halt. to set a breakpoint.
Return self subclassResponsibility to make a method abstract.
To give an object a String representation you should override printOn:.
Override the hook method initialize to properly initialize instances.
Number methods automatically convert between Floats, Fractions and Integers.
Fractions truly represent rational numbers rather than floats.
All Characters are like unique instances.
Strings are mutable; Symbols are not. Take care not to mutate string literals, however!
Symbols are unique; Strings are not.
Strings and Symbols are Collections and therefore support the usual Collection methods.