What's the nearest substitute for a function pointer in Java?

Anonymous inner class

Say you want to have a function passed in with a String param that returns an int.
First you have to define an interface with the function as its only member, if you can't reuse an existing one.

interface StringFunction {
    int func(String param);
}

A method that takes the pointer would just accept StringFunction instance like so:

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

And would be called like so:

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

EDIT: In Java 8, you could call it with a lambda expression:

ref.takingMethod(param -> bodyExpression);

For each "function pointer", I'd create a small functor class that implements your calculation. Define an interface that all the classes will implement, and pass instances of those objects into your larger function. This is a combination of the "command pattern", and "strategy pattern".

@sblundy's example is good.

When there is a predefined number of different calculations you can do in that one line, using an enum is a quick, yet clear way to implement a strategy pattern.

public enum Operation {
    PLUS {
        public double calc(double a, double b) {
            return a + b;
        }
    },
    TIMES {
        public double calc(double a, double b) {
            return a * b;
        }
    }
     ...

     public abstract double calc(double a, double b);
}

Obviously, the strategy method declaration, as well as exactly one instance of each implementation are all defined in a single class/file.