Java BiFunction – Examples

In Java 8, BiFunction interface is a built in functional interface which accepts two arguments and produces a results. This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

Java BiFunction is common use case when we are working with Java HashMap. In HashMap methods compute, computeIfPresent, merge and replaceAll method takes BiFunction as input argument.

1. Java BiFunction methods and Examples

1. R apply(T t, U u) – Applies this function to the given arguments and produces results.
2. default BiFunction andThen(Function after) – Returns a composed function that first applies this function to its input, and then applies the after function to the result.

1.1. BiFunction basic Example

Following example demonstrates simple usage of Java 8 BiFunction interface methods apply(), andThen() methods.

public class Java8BiFunctionExample {

	public static void main(String[] args) {

		// a basic apply() example
		BiFunction<Integer, Integer, Integer> f1 = (a, b) -> a + b;
		System.out.println(f1.apply(10, 20)); // 30

		// a basic andThen() example
		Function<Integer, Integer> f2 = a -> a * a;
		System.out.println(f1.andThen(f2).apply(2, 3));

		// above line f1.andThen(f2).apply(2, 3) is equalant to following code
		Integer j1 = f1.apply(2, 3); // 5
		Integer j2 = f2.apply(j1); // 25

		System.out.println(j2); // 25

	}
}

Output :

30
25
25

1.2. BiFunction with compute and computeIfPresent Example

public class HashMapComputeWithBiFunctionDemo {
	public static void main(String[] args) {

		Map<Integer, String> hashMap = new HashMap<Integer, String>();
		String msg = "Hello ";
		String defaultUser = "Anonymous";

		hashMap.put(7, "Peter");
		hashMap.put(5, "Philip");
		hashMap.put(2, "Martin");
		hashMap.put(4, null);
		System.out.println("Initial HashMap: " + hashMap);

		// Using compute(key, BiFunction)
		// hashMap.compute(4, (key, oldValue) -> oldValue.concat(msg));// throws
		// Excpetion NPE

		BiFunction<Integer, String, String> f1 = (key, oldValue) -> oldValue == null ? defaultUser
				: oldValue.toUpperCase();

		hashMap.compute(4, f1);
		hashMap.compute(2, f1);
		System.out.println("HashMap using compute() => " + hashMap);

		// Using computeIfPresent(key, BiFunction)
		BiFunction<Integer, String, String> f2 = (key, oldValue) -> msg + oldValue + "!";
		hashMap.computeIfPresent(7, f2);
		hashMap.computeIfPresent(3, f2);
		System.out.println("HashMap using computeIfPresent() => " + hashMap);

	}
}

Output :

Initial HashMap: {2=Martin, 4=null, 5=Philip, 7=Peter}
HashMap using compute() => {2=MARTIN, 4=Anonymous, 5=Philip, 7=Peter}
HashMap using computeIfPresent() => {2=MARTIN, 4=Anonymous, 5=Philip, 7=Hello Peter!}

1.3. BiFunction with merge and replaceAll Example

public class HashMapBiFunctionMergeDemo {
	public static void main(String[] args) {

		Map<Integer, String> hashMap = new HashMap<Integer, String>();
		String msg = " King";
		// put()
		hashMap.put(7, "Peter");
		hashMap.put(5, "Philip");
		hashMap.put(2, "Martin");
		hashMap.put(4, null);
		System.out.println("Initial HashMap: " + hashMap);

		// Using merge(key, value, BiFunction)
		BiFunction<String, String, String> f = (old, given) -> given.concat(msg);

		hashMap.merge(7, "Arthur", f);
		hashMap.merge(2, "Martin", f);
		hashMap.merge(4, "Luther", f);
		hashMap.merge(6, "Robert", f);
		System.out.println("HashMap using merge() => " + hashMap);

		// using replaceAll(key, value, BiFunction)
		BiFunction<Integer, String, String> f2 = (key, value) -> value.replace(msg, "");

		hashMap.replaceAll(f2);
		System.out.println("HashMap using replaceAll() => " + hashMap);
	}
}

Initial HashMap: {2=Martin, 4=null, 5=Philip, 7=Peter}
HashMap using merge() => {2=Martin King, 4=Luther, 5=Philip, 6=Robert, 7=Arthur King}
HashMap using replaceAll() => {2=Martin, 4=Luther, 5=Philip, 6=Robert, 7=Arthur}

1.4. BiFunction another example

Following is the basic example to print map key and values, print number of occurrences of value with suffix.

public class BiFunctionExample2 {

	public static void main(String[] args) {

		Map<Integer, String> m = new HashMap<>();
		m.put(1, "Peter");
		m.put(2, "Mike");
		m.put(3, "John");
		m.put(4, "Mike");
		m.put(5, "Peter");
		m.put(6, "Anand");
		m.put(7, "Peter");
		
		// Collections.frequency to get number of occurances
		BiFunction<Integer, String, String> f =
				(key, value) -> "[Key="+key+", "+value+"("+Collections.frequency(m.values(), value)+")]";
		
		m.forEach((k,v)-> System.out.println(f.apply(k, v)));
	}
}

[Key=1, Peter(3)]
[Key=2, Mike(2)]
[Key=3, John(1)]
[Key=4, Mike(2)]
[Key=5, Peter(3)]
[Key=6, Anand(1)]
[Key=7, Peter(3)]

References

1. Java documentation
2. Java Function interface
3. Java HashMap
4. Java 8 Lambda Expression
5. Java 8 BiPredicate Examples
6. Java Predicate Examples