How to return JSON from Rest controller?

Question

Spring

I get json with matrix coefficients, I solve this matrix, but I don’t want to send it back, I get a 405 error

. Here is the Rest controller code

package com.solve.gauss.Controllers;

import com.solve.gauss.models.DataMatrix;

import com.solve.gauss.models.SLAU;
import org.springframework.web.bind.annotation.*;

import java.util.HashMap;


@org.springframework.web.bind.annotation.RestController
public class RestController {

    @PostMapping("/solve")
    public HashMap<String, double[]> solve(@RequestBody DataMatrix data) {
        SLAU system = new SLAU(data.size, data.matrix);
        double[] x = system.solveMatrix();
        for(int i=0; i < data.size; i++) System.out.print(x[i] + " ");

        HashMap<String, double[]> json = new HashMap<>();
        json.put("roots", x);
        return json;
    }


}