Explaining Monads with a Java example

It’s important to note that the **flatMap**** method in the **Stream** class is not a traditional monadic “bind” operator, it is a method that allows you to transform a **Stream<T>** into a **Stream<U>** by applying a function **T -> Stream<U>** **to each element in the original stream, it does not return a Monad instance.

Java Method	Monad Law
**flatMap(Function<T, Monad<U>> f)**	**bind** or **>>=**
**join()**	**join**
**returnValue(T value)**	**return** or **pure**

• flatMap method is used for chaining computations and handling errors and absent values, it corresponds to bind or >>= operator in other languages.

**returnValue**** and **join** **are related to the monad laws, but not equivalent.

• join is related to the left identity law, this method is used to flatten nested monadic values.

• returnValue is related to the right identity law, this method is used to wrap a plain value in a monadic context.

In Java, the **flatMap**** method allows you to chain computations together and handle errors and absent values, so there is no need for a separate **bind** operator with specific associativity properties. In another words, in Java, the order of the operations is defined by the order of the method calls, so the ==**flatMap**==== **==method is executed in the order it appears in the code, and it does not have any specific associativity.

For example, if you have the following code:

public class Monad<T> {
    private T value;
 
    public Monad(T value) {
        this.value = value;
    }
 
    public T getValue() {
        return value;
    }
 
    public <R> Monad<R> flatMap(Function<T, Monad<R>> f) {
        return f.apply(value);
    }
 
		public Monad<T> join() {
        return value;
    }
 
		public static <T> Monad<T> returnValue(T value) {
        return new Monad<>(value);
    }
 
}
 
// Example 1: 
 
Monad<Integer> m1 = new Monad<>(1);
Monad<Integer> m2 = m1.flatMap(x -> new Monad<>(x + 1));
Monad<Integer> m3 = m2.flatMap(x -> new Monad<>(x * 2));
 
// Example 2:
 
Monad<Integer> monad = Monad.returnValue(5);
Monad<String> monadString = monad.flatMap(x -> Monad.returnValue(x.toString()));
String result = monadString.getValue(); // result is "5"
 
// Chaining:
 
Monad<Integer> monad = Monad.returnValue(5);
String result = monad
                .flatMap(x -> Monad.returnValue(x * 2))
                .flatMap(x -> Monad.returnValue(x.toString()))
                .getValue();
								// result is "10"

Java 8’s **Stream**** class has a **flatMap** method that is similar in functionality to the **flatMap** method of monads. It takes a function as an argument, applies it to each element of the stream, and flattens the resulting streams into a single stream. In summary, Java 8’s **Stream** class has a **flatMap** method,== but it is not a monad because it does not obey the three monad laws**==, it is a functional tool that can be used to perform functional-style operations on collections of data.

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