如何通过for循环在Java中递增BigDecimal？

Question

How can I increment BigDecimal in java through for loop? This is the codes that I am currently running and I don't understand why it won't increment the BigDecimal instances

package app;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

public class Test {
    private List<BigDecimal> ticketQuantity = new ArrayList<>();
    List<String> someNumber = new ArrayList<>();

    public Test() {

        ticketQuantity.add(new BigDecimal(0));
        ticketQuantity.add(new BigDecimal(0));
        ticketQuantity.add(new BigDecimal(0));
        ticketQuantity.add(new BigDecimal(0));

        someNumber.add("10");
        someNumber.add("10");
        someNumber.add("10");
        someNumber.add("10");
        System.out.println(ticketQuantity);

        int i = 0;
        for (BigDecimal x : ticketQuantity) {
            x.add(new BigDecimal(someNumber.get(i)));
            i++;
        }

        System.out.println(ticketQuantity);
    }

    public static void main(String[] args) {
        new Test();
    }
}

What did I miss? I am hoping that someNumber would equals to [10, 10, 10, 10] but then [0, 0, 0, 0] is printed :(

Answer 1

BigDecimal represents:

Immutable, arbitrary-precision signed decimal numbers

The immutability means you can't modify an instance. The add method will return a new BigDecimal . If you want to change the value in the list, you will have to go through the loop by index and call List.set with the index to replace and the result of BigDecimal.add :

for (int i = 0; i < ticketQuantity.size(); i++) {
    ticketQuantity.set(i, ticketQuantity.get(i).add(someNumber.get(i)));
}

Answer 2

As explained in other answer, add() method will return a new BigDecimal and wouldn't modify the original instance due to BigDecimal being immutable so you need to do as below,

int i = 0;
for (BigDecimal x : ticketQuantity) {
    x = x.add(new BigDecimal(someNumber.get(i)));
    ticketQuantity.set(i, x);
    i++;
}

Answer 3

As others have stated BigDecimal is an immutable class. The method BigDecimal#add actually returns another instance of BigDecimal , doesn't update the operand BigDecimal object. To achieve your object, you can try the following way:

public class Test {
    private List<BigDecimal> ticketQuantity = new ArrayList<>();
    List<String> someNumber = new ArrayList<>();

    public Test() {

        someNumber.add("10");
        someNumber.add("10");
        someNumber.add("10");
        someNumber.add("10");
        System.out.println(ticketQuantity);

        for (String x : someNumber) {
            ticketQuantity.add(new BigDecimal(x));
        }

        System.out.println(ticketQuantity);
    }

    public static void main(String[] args) {
        new Test();
    }
}

Answer 4

add() method adds new elements at the end of list. I think you are trying to overwrite on first four values you 've added previously. so use set(int index,E element);