Java 8 Custom Collector

In this Article , we’ll Discuss How we can Create our own Custom Collector in java 8 and above.
For this article, We will implement Summary Statistics For BigDecimal number.

Java 8 Summary Statistics

Java 8 Provides Summary Statistics for Long,Int & Double. These Summary classes will help you to get count, min, max, sum, and average values.

• LongSummaryStatistics
• IntSummaryStatistics
• DoubleSummaryStatistics

IntSummaryStatistics Example

In below example we are trying to calculate count, min, max, sum, and average of 1 to 100 Integer.

Calculate Sum,Min,Max,Count & Average

IntSummaryStatistics summaryStatistics = IntStream.range(1, 101)
		.summaryStatistics();
		System.out.println(summaryStatistics);

implementing SummaryStatistics for Big Decimal numbers

Java does not provides inbuilt Summary Statistics class for BigDecimal Number.

Lets try to implement our own BigDecimal SummaryStatistics.

Step 1 : Create Collector

Calculate Sum,Min,Max,Count & Average

import java.math.BigDecimal;
import java.util.function.Consumer;
import java.util.stream.Collector;

public class MyBigDecimalSummaryCollector implements Consumer<BigDecimal> {
	
	public static final int ROUND_HALF_UP = BigDecimal.ROUND_HALF_UP;

	public static final Integer TRANSACTIONS_SCALE = 2;

	private BigDecimal sum = BigDecimal.ZERO.setScale(TRANSACTIONS_SCALE, ROUND_HALF_UP);
	private BigDecimal minimum = BigDecimal.ZERO.setScale(TRANSACTIONS_SCALE, ROUND_HALF_UP);
	private BigDecimal maximum = BigDecimal.ZERO.setScale(TRANSACTIONS_SCALE, ROUND_HALF_UP);
	private int count;

	public static Collector<BigDecimal, ?, MyBigDecimalSummaryCollector> myBigDecimalSummaryStatistics() {

		return Collector.of(MyBigDecimalSummaryCollector::new, MyBigDecimalSummaryCollector::accept,
				MyBigDecimalSummaryCollector::merge);
	}

	@Override
	public void accept(BigDecimal t) {

		if (count == 0) {
			firstElementSetup(t);
		} else {
			sum = sum.add(t);
			minimum = minimum.min(t);
			maximum = maximum.max(t);
			count++;
		}
	}

	public MyBigDecimalSummaryCollector merge(MyBigDecimalSummaryCollector s) {
		if (s.count > 0) {
			if (count == 0) {
				setupFirstElement(s);
			} else {
				sum = sum.add(s.sum);
				minimum = minimum.min(s.minimum);
				maximum = maximum.max(s.maximum);
				count += s.count;
			}
		}
		return this;
	}

	private void setupFirstElement(MyBigDecimalSummaryCollector s) {
		count = s.count;
		sum = s.sum;
		minimum = s.minimum;
		maximum = s.maximum;
	}

	private void firstElementSetup(BigDecimal t) {
		count = 1;
		sum = t;
		minimum = t;
		maximum = t;
	}

	public BigDecimal getAverage() {
		if (count == 0) {
			return BigDecimal.ZERO.setScale(TRANSACTIONS_SCALE, ROUND_HALF_UP);
		}
		return sum.divide(BigDecimal.valueOf(count), TRANSACTIONS_SCALE,
				ROUND_HALF_UP);

	}

	public BigDecimal getSum() {
		return sum;
	}

	public void setSum(BigDecimal sum) {
		this.sum = sum;
	}

	public BigDecimal getMinimum() {
		return minimum;
	}

	public void setMinimum(BigDecimal minimum) {
		this.minimum = minimum;
	}

	public BigDecimal getMaximum() {
		return maximum;
	}

	public void setMaximum(BigDecimal maximum) {
		this.maximum = maximum;
	}

	public int getCount() {
		return count;
	}

	public void setCount(int count) {
		this.count = count;
	}

	@Override
	public String toString() {
		return "MyBigDecimalSummaryCollector [sum=" + sum + ", minimum=" + minimum + ", maximum=" + maximum + ", count="
				+ count + "]";
	}
}

Step 2 :

Use Collector

MyBigDecimalSummaryCollector collect = IntStream.range(1, 101)
	.mapToObj(number -> BigDecimal.valueOf(number))
	.collect(MyBigDecimalSummaryCollector.myBigDecimalSummaryStatistics());
	
	System.out.println(collect);