What problem does the Spring IoC container solve that plain Java object creation doesn't?

Plain Java means every class manually constructs and wires its own dependencies with `new`, so swapping an implementation (e.g. a mock in tests) means editing every call site. The IoC container inverts that: objects declare what they need via constructor parameters, and the container looks up or creates the right implementation and injects it, so wiring lives in one place (configuration/annotations) instead of scattered across the codebase.