What is the parent-delegation model and what concrete problems does it prevent?

Before a class loader tries to define a class itself, it delegates upward to its parent, so the bootstrap loader gets first refusal.

This prevents user code from shadowing core JDK classes (you can't sneak in your own java.lang.String) and avoids loading the same core class twice under different loaders. The gotcha is that two classes with the identical name loaded by different loaders are distinct types, so a cast between them throws ClassCastException even though the bytecode is identical.