# Java BigDecimal

In this article we show how to perform high-precision calculations in Java with BigDecimal.

`BigDecimal` represents an immutable, arbitrary-precision signed decimal number. It is used for high-precision arithmetic. `BigDecimal` provides operations for arithmetic, scale manipulation, rounding, comparison, hashing, and format conversion.

`BigDecimal` consists of two parts:

• unscaled value - an arbitrary precision integer
• scale - a 32-bit integer representing the number of digits to the right of the decimal point

For instance, `BigDecimal` 2.18 has the unscaled value of 218 and the scale of 2.

`BigDecimal` is used in areas where high-precision is necessary; for instance, financial or monetary calculations. The arithmetic operations of `BigDecimal` values are done with methods such as `add` or `subtract`; the `+`, `-`, `*`, `\` operators are not overloaded.

## BigDecimal basic arithmetic operations

The following example shows basic arithmetic operations with `BigDecimal`.

Main.java
```import java.math.BigDecimal;
import java.math.RoundingMode;

void main() {

var val1 = new BigDecimal("3.44");
var val2 = new BigDecimal("2.74");

System.out.println(res1);

BigDecimal res2 = val1.subtract(val2);
System.out.println(res2);

BigDecimal res3 = val1.multiply(val2);
System.out.println(res3);

BigDecimal res4 = val1.divide(BigDecimal.TEN, RoundingMode.DOWN);
System.out.println(res4);

BigDecimal res5 = val1.divide(val2, 15, RoundingMode.HALF_UP);
System.out.println(res5);
}
```

In the example, we have addition, subtraction, multiplication, and division operations.

```var val1 = new BigDecimal("3.44");
var val2 = new BigDecimal("2.74");
```

We create two `BigDecimal` values. The numbers are passed as strings.

```BigDecimal res1 = val1.add(val2);
System.out.println(res1);
```

Here we add two `BigDecimal` numbers. The addition operation is performed with the `add` method.

```BigDecimal res4 = val1.divide(BigDecimal.TEN, RoundingMode.DOWN);
System.out.println(res4);
```

When doing the division operation, we also need to specify the rounding mode.

```\$ java Main.java
6.18
0.70
9.4256
0.34
1.255474452554745
```

## BigDecimal precision

`BigDecimal` is used for high-precision arithmetic.

Main.java
```import java.math.BigDecimal;

void main() {

double a = 0.1 + 0.1 + 0.1;
double b = 0.3;

System.out.println(a);
System.out.println(b);
System.out.println(a == b);

var c = new BigDecimal("0.1").add(new BigDecimal("0.1"))
var d = new BigDecimal("0.3");

System.out.println(c);
System.out.println(d);
System.out.println(c.equals(d));
}
```

In the example, we compare the precision of a `double` type with `BigDecimal`. We add three floating point values and compare it with the expected output.

```\$ java Main.java
0.30000000000000004
0.3
false
0.3
0.3
true
```

There is a small margin error for `double`; therefore, the operation is not precise. The `BigDecimal` gives the expected output.

## BigDecimal rounding mode

The `BigDecimal` class gives its user complete control over rounding behavior. If no rounding mode is specified and the exact result cannot be represented, an exception is thrown.

Main.java
```import java.math.BigDecimal;
import java.math.RoundingMode;

void main() {

var x = new BigDecimal("5.54");
BigDecimal x2 = x.setScale(1, RoundingMode.FLOOR);
System.out.println(x2);

var y = new BigDecimal("5.94");
BigDecimal y2 = y.setScale(1, RoundingMode.CEILING);
System.out.println(y2);
}
```

In the example, we round two values in two different rounding modes.

```var x = new BigDecimal("5.54");
BigDecimal x2 = x.setScale(1, RoundingMode.FLOOR);
```

With the `setScale` method, we provide the scale and the rounding mode. In our case, we round the value to one decimal place with a rounding mode `RoundingMode.FLOOR`, which rouds towards negative infinity.

```\$ java Main.java
5.5
6.0
```

## Java BigDecimal comparison

With the `compareTo` method, two `BigDecimal` objects that are equal in value but have a different scale (like 4.0 and 4.00) are considered equal by this method.

Main.java
```import java.math.BigDecimal;

void main() {

var x = new BigDecimal("1.6");
var y = new BigDecimal("1.60");

System.out.println(x.equals(y));
System.out.println(x.compareTo(y));

}
```

The example compares values 1.6 and 1.60 with `equals` and `compareTo`.

```\$ java Main.java
false
0
```

The `compareTo` returns 0 for values that are equal.

## BigDecimal practical example

The following example groups produts by category and calculates the total price of all products in the categories.

Main.java
```import java.math.BigDecimal;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

record Product(String name, String category, BigDecimal price) {}

void main() {

Map<String, Map<BigDecimal, List<Product>>> productsByCategories =
products().stream().collect(
Collectors.groupingBy(Product::category,
Collectors.groupingBy(Product::price)));

productsByCategories.forEach((k, v) -> {

System.out.printf("%s: ", k);

var sum = new BigDecimal("0");

var prices = v.keySet();
for (var price: prices) {

}

System.out.println(sum);
});
}

List<Product> products() {

return List.of(
new Product("apple", "fruit", new BigDecimal("4.50")),
new Product("banana", "fruit", new BigDecimal("3.76")),
new Product("carrot", "vegetables", new BigDecimal("2.98")),
new Product("potato", "vegetables", new BigDecimal("0.92")),
new Product("garlic", "vegetables", new BigDecimal("1.32")),
new Product("ginger", "vegetables", new BigDecimal("2.45")),
new Product("white bread", "bakery", new BigDecimal("1.50")),
new Product("roll", "bakery", new BigDecimal("0.08")),
new Product("bagel", "bakery", new BigDecimal("0.15"))
);
}
```

We have various groceries in three categories: fruit, vegetables, and bakery. Our goal is to group the produts by their categories and calculate the sum of all prices in the respective categories.

```Map<String, Map<BigDecimal, List<Product>>> productsByCategories =
products().stream().collect(
Collectors.groupingBy(Product::category,
Collectors.groupingBy(Product::price)));
```

Using Java streams, we group the products by their categories and then prices.

```productsByCategories.forEach((k, v) -> {

System.out.printf("%s: ", k);

var sum = new BigDecimal("0");

var prices = v.keySet();
for (var price: prices) {

}

System.out.println(sum);
});
```

In the `forEach` loop, we calculate the sum of all prices for each category.

```\$ java Main.java
bakery: 1.73
fruit: 8.26
vegetables: 7.67
```

## Source

In this article we have shown how to do high-precision arithmetic operations in Java with `BigDecimal`.

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