What is a substitute for a function pointer in Java?

There is an Anonymous inner class that can replace function pointers.

If there is a function that receives String as a parameter and returns int, First, define an interface that has only the method.

interface StringFunction {
    int func(String param);
}

public void takingMethod(StringFunction sf) {
   int i = sf.func("my string");
   // // do whatever ...
}

As shown above, the method is accessible from an instance of StringFunction.

ref.takingMethod(new StringFunction() {
    public int func(String param) {
        // // body
    }
});

This is how you call it.

Also, if you use the lambda expression in java 8, you can use ref.taking Method (parameter ->processing); like this.