How to pass a function as a parameter in Java?

Java 8 and above

Using Java 8+ lambda expressions, if you have a class or interface with only a single abstract method (sometimes called a SAM type), for example:

public interface MyInterface {
    String doSomething(int param1, String param2);
}

then anywhere where MyInterface is used, you can substitute a lambda expression:

class MyClass {
    public MyInterface myInterface = (p1, p2) -> { return p2 + p1; };
}

For example, you can create a new thread very quickly:

new Thread(() -> someMethod()).start();

And use the method reference syntax to make it even cleaner:

new Thread(this::someMethod).start();

Without lambda expressions, these last two examples would look like:

new Thread(new Runnable() { someMethod(); }).start();

Before Java 8

A common pattern would be to 'wrap' it within an interface, like Callable, for example, then you pass in a Callable:

public T myMethod(Callable<T> func) {
    return func.call();
}

This pattern is known as the Command Pattern.

Keep in mind you would be best off creating an interface for your particular usage. If you chose to go with callable, then you'd replace T above with whatever type of return value you expect, such as String.

In response to your comment below you could say:

public int methodToPass() { 
        // do something
}

public void dansMethod(int i, Callable<Integer> myFunc) {
       // do something
}

then call it, perhaps using an anonymous inner class:

dansMethod(100, new Callable<Integer>() {
   public Integer call() {
        return methodToPass();
   }
});

Keep in mind this is not a 'trick'. It's just java's basic conceptual equivalent to function pointers.

Lambda Expressions

To add on to jk.'s excellent answer, you can now pass a method more easily using Lambda Expressions (in Java 8). First, some background. A functional interface is an interface that has one and only one abstract method, although it can contain any number of default methods (new in Java 8) and static methods. A lambda expression can quickly implement the abstract method, without all the unnecessary syntax needed if you don't use a lambda expression.

Without lambda expressions:

obj.aMethod(new AFunctionalInterface() {
    @Override
    public boolean anotherMethod(int i)
    {
        return i == 982
    }
});

With lambda expressions:

obj.aMethod(i -> i == 982);

---

Here is an excerpt from the Java tutorial on Lambda Expressions:

Syntax of Lambda Expressions

A lambda expression consists of the following:

• A comma-separated list of formal parameters enclosed in parentheses. The CheckPerson.test method contains one parameter, p, which represents an instance of the Person class.
  
  Note: You can omit the data type of the parameters in a lambda expression. In addition, you can omit the parentheses if there is only one parameter. For example, the following lambda expression is also valid:

  p -> p.getGender() == Person.Sex.MALE 
      && p.getAge() >= 18
      && p.getAge() <= 25
  

• The arrow token, ->

• A body, which consists of a single expression or a statement block. This example uses the following expression:

  p.getGender() == Person.Sex.MALE 
      && p.getAge() >= 18
      && p.getAge() <= 25
  

  If you specify a single expression, then the Java runtime evaluates the expression and then returns its value. Alternatively, you can use a return statement:

  p -> {
      return p.getGender() == Person.Sex.MALE
          && p.getAge() >= 18
          && p.getAge() <= 25;
  }
  

  A return statement is not an expression; in a lambda expression, you must enclose statements in braces ({}). However, you do not have to enclose a void method invocation in braces. For example, the following is a valid lambda expression:

  email -> System.out.println(email)
  

Note that a lambda expression looks a lot like a method declaration; you can consider lambda expressions as anonymous methods—methods without a name.

---

Here is how you can "pass a method" using a lambda expression:

^{Note: this uses a new standard functional interface, java.util.function.IntConsumer.}

class A {
    public static void methodToPass(int i) { 
        // do stuff
    }
}

import java.util.function.IntConsumer;

class B {
    public void dansMethod(int i, IntConsumer aMethod) {
        /* you can now call the passed method by saying aMethod.accept(i), and it
        will be the equivalent of saying A.methodToPass(i) */
    }
}

class C {
    B b = new B();

    public C() {
        b.dansMethod(100, j -> A.methodToPass(j));   //Lambda Expression here
    }
}

---

The above example can be shortened even more using the :: operator.

public C() {
    b.dansMethod(100, A::methodToPass);
}

You could use Java reflection to do this. The method would be represented as an instance of java.lang.reflect.Method.

import java.lang.reflect.Method;

public class Demo {

    public static void main(String[] args) throws Exception{
        Class[] parameterTypes = new Class[1];
        parameterTypes[0] = String.class;
        Method method1 = Demo.class.getMethod("method1", parameterTypes);

        Demo demo = new Demo();
        demo.method2(demo, method1, "Hello World");
    }

    public void method1(String message) {
        System.out.println(message);
    }

    public void method2(Object object, Method method, String message) throws Exception {
        Object[] parameters = new Object[1];
        parameters[0] = message;
        method.invoke(object, parameters);
    }

}