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/* package whatever; // don't place package name! */
 
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.function.Function;
 
class Ideone {
    public static void main(String[] args) {
        MyRow row1 = new MyRow(BigDecimal.ONE);
        MyRow row2 = new MyRow(BigDecimal.TEN);
        List<MyRow> rows = Arrays.asList(row1, row2);
 
        rows.stream().map(x -> x.getValue()).reduce(BigDecimal.ZERO, BigDecimal::add);
        //calcSum(rows, (MyRow x) -> x.getValue());
    }
 
    public static BigDecimal calcSum(Collection c , Function<?, BigDecimal> mapFunc) {
        return c.stream().map(mapFunc).reduce(BigDecimal.ZERO, BigDecimal::add);
    }
 
}
 
class MyRow {
    private BigDecimal value;
 
    public MyRow(BigDecimal value) {
        this.value = value;
    }
 
    public BigDecimal getValue() {
        return value;
    }
 
    public void setValue(BigDecimal value) {
        this.value = value;
    }
}

Main.java:20: error: incompatible types: invalid method reference
        return c.stream().map(mapFunc).reduce(BigDecimal.ZERO, BigDecimal::add);
                                                               ^
    no suitable method found for add(Object,Object)
        method BigDecimal.add(BigDecimal,MathContext) is not applicable
          (argument mismatch; Object cannot be converted to BigDecimal)
        method BigDecimal.add(long,long) is not applicable
          (argument mismatch; Object cannot be converted to long)
Note: Main.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

Standard output is empty