DI has nothing to do with the differences between classes/objects and functions/closures, DI and IoC are about reusable code and being able to switch out implem

DI has nothing to do with the differences between classes/objects and functions/closures, DI and IoC are about reusable code and being able to switch out implementations. You can do IoC in either pattern. In Java functions/closures and objects/classes are the same thing. You have the best of both worlds. You can for instance define a function that takes an argument a and a function b and pass it either a functional lambda or a class that implements said function: static void doSomething(int a, Runnable b) { if (a > 0) { b.run(); } } doSomething(1, () -> System.out.println("hello")); Runnable r = new SomeClassThatImplementsRunnable(); doSomething(0, r);