MNI parameters and results

This page follows MNI introduction. Continue with the same project. The Java entry will receive integers and return their sum.

Receive integers with MCInt

src/main/java/example/mni/Arithmetic.java:

package example.mni;

import top.mcfpp.annotations.MNIFunction;
import top.mcfpp.core.lang.MCInt;
import top.mcfpp.util.ValueWrapper;

public final class Arithmetic {
    @MNIFunction(normalParams = {"int", "int"}, returnType = "int")
    public static void add(
            MCInt left, MCInt right, ValueWrapper<MCInt> result) {
        result.set((MCInt) left.plus(right));
    }

    @MNIFunction(
        readOnlyParams = {"int"},
        normalParams = {"int"},
        returnType = "int"
    )
    public static void addBonus(
            MCInt bonus, MCInt score, ValueWrapper<MCInt> result) {
        result.set((MCInt) score.plus(bonus));
    }
}

add receives two ordinary integers. addBonus receives a fixed reward and an ordinary score. Both return a result through ValueWrapper<MCInt>.

The central operation in add is:

result.set((MCInt) left.plus(right));

left.plus(right) asks the compiler to perform MCFPP integer addition. The compiler produces the appropriate calculation and supplies its result to result.

Declare the two interfaces

src/main/mcfpp/main.mcfpp:

func add(left: int, right: int): int = example.mni.Arithmetic.add
func addBonus<bonus: int>(score: int): int = example.mni.Arithmetic.addBonus

func main() {
    external count: int = 2
    print(add(count, 3))
    print(addBonus<5>(count))
}

Run mcfpp build. Then call main in the game. The outputs are 5 and 7.

external count uses game runtime storage. During compilation, Java receives a compiler object that describes access to this integer. Generated addition commands read the actual integer when the datapack executes.

<5> supplies the reward during compilation. Read Compilation parameters for the difference between angle brackets and parentheses.

Match the three parameter descriptions

An ordinary MNI wrapper describes its parameters in three corresponding places:

MCFPP declaration	Java annotation	Java method parameters
left: int, right: int	normalParams = {"int", "int"}	MCInt left, MCInt right
<bonus: int>	readOnlyParams = {"int"}	MCInt bonus before ordinary parameters
: int	returnType = "int"	Final ValueWrapper<MCInt> result

The source declaration defines the caller's interface. The annotation describes the Java entry signature. Java parameters use corresponding compiler types in declaration order.

For these free functions, the order is: readonly parameters, ordinary parameters, then the result wrapper. Member entries also receive a caller object before the result wrapper. Read MNIFunction API for those signatures.

Where Java int and MCInt apply

Form	Meaning	Typical use
Java int	An integer that Java currently holds	Configuration, ordinary calculations, and tests
Java MCInt	An MCFPP integer value represented by the compiler	Source inputs and generated integer operations
MCFPP int	An integer type in the datapack program	Variable, parameter, and result declarations

MCInt can represent either a known integer or runtime data. Compiler operations such as plus support both inputs through the same entry. Reading an ordinary Java integer requires a value known during compilation.

Common value type mappings

Use the corresponding compiler value class for other inputs and results. Import the classes listed below:

MCFPP type	Java class
int	top.mcfpp.core.lang.MCInt
float	top.mcfpp.core.lang.MCFloat
bool	top.mcfpp.core.lang.bool.ScoreBool
string	top.mcfpp.core.lang.nbt.MCString

For example, use MCString input for a string input and ValueWrapper<MCString> result for a string result. The annotation and source declaration still use string.

Return a value or generate an operation

An entry with a result calls result.set(...) on every successful completion path. This delivers an MCFPP value to the caller. The Java method itself still returns void.

Read Generate commands for messages and other operations without results. Read MNIFunction API for annotation field details.