How to iterate over lambda functions in Java

Shivansh Kuchhal :

I was able to do it in Python and my Python code is:

signs = {"+" : lambda a, b : a + b, "-" : lambda a, b : a - b}

a = 5
b = 3
for i in signs.keys():
    print(signs[i](a,b))

And the output is:

8
2

How do I do this same thing in Java through HashMap?

YCF_L :

You can use BinaryOperator<Integer> in this case like so :

BinaryOperator<Integer> add = (a, b) -> a + b;//lambda a, b : a + b
BinaryOperator<Integer> sub = (a, b) -> a - b;//lambda a, b : a - b

// Then create a new Map which take the sign and the corresponding BinaryOperator
// equivalent to signs = {"+" : lambda a, b : a + b, "-" : lambda a, b : a - b}
Map<String, BinaryOperator<Integer>> signs = Map.of("+", add, "-", sub);

int a = 5; // a = 5
int b = 3; // b = 3

// Loop over the sings map and apply the operation
signs.values().forEach(v -> System.out.println(v.apply(a, b)));

Outputs

8
2

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Note for Map.of("+", add, "-", sub); I'm using Java 10, If you are not using Java 9+ you can add to your map like so:

Map<String, BinaryOperator<Integer>> signs = new HashMap<>();
signs.put("+", add);
signs.put("-", sub);

Ideone demo

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Good practice

As already stated by @Boris the Spider and @Holger in the comments, Its better to use IntBinaryOperator to avoid boxing, in the end your code can look like this :

// signs = {"+" : lambda a, b : a + b, "-" : lambda a, b : a - b}
Map<String, IntBinaryOperator> signs = Map.of("+", (a, b) -> a + b, "-", (a, b) -> a - b);
int a = 5; // a = 5
int b = 3; // b = 3
// for i in signs.keys(): print(signs[i](a,b))
signs.values().forEach(v -> System.out.println(v.applyAsInt(a, b)));