I am trying to write an indicator where outputs[i] = sma[i - shift], such that the output is a SMA lagging by 'shift' number of bars. Here's my code in calculate():

        double[] smaData = new double[endIndex - startIndex + 1 + timePeriod];
        sma.setInputParameter(0, inputs[0]);
        sma.setOptInputParameter(0, timePeriod);
        sma.setOutputParameter(0, smaData);
        IndicatorResult dSmaResult = sma.calculate(startIndex - timePeriod, endIndex);        

        int i, k, j;
        k = dSmaResult.getNumberOfElements();
        j = dAtrResult.getNumberOfElements();
               
        for (i = 0; i < Math.min(k, j); i++) {
           outputs[0][i] = smaData[i + timePeriod];
        }
        
        return new IndicatorResult(startIndex, i);

The problem though, is that instead of receiving lagging bars, the outputs are shifted forward. How would I fix this please?

Try to change to

        for (i = 0; i < Math.min(k - 1, j); i++) {
           outputs[0][i] = smaData[i + timePeriod + 1];
        }
        outputs[0][i] = Double.NaN
        i++;

In this case, to calculate value for the current candle, you need to know sma value for the next candle. For correct calculations on the chart you will need to set lookforward to 1 (getLookforward() method must return 1)

If the shifting of the output on the chart is the only thing you want to get, then consider setting shift parameter to the output. Setting it to +1 or -1 will shift it by one candle