Proposal how to enhance assertj collection matching

AssertJCollectionExample.java

package test;

import static java.util.Objects.isNull;
import static org.assertj.core.api.Assertions.assertThat;
import static org.hamcrest.Matchers.containsInAnyOrder;

import java.util.ArrayList;
import java.util.List;

import org.assertj.core.api.HamcrestCondition;
import org.hamcrest.Matcher;
import org.hamcrest.core.IsNull;
import org.junit.jupiter.api.Test;

public class AssertJCollectionExample {

	/*
	 * + keeps sequence 
	 * - depends on indexes (will not work for sets, sorted sets, queues, iterators)
	 */
	@Test
	void testListIndexMisuse() throws Exception {
		List<ComplexObject> list = createList();

		// this will not work!
		// assertThat(list).element(0).extracting(ComplexObject::getValue).isEqualTo("get0");

		assertThat(list).extracting(ComplexObject::getValue).element(0).isEqualTo("get0");
		assertThat(list).extracting(ComplexObject::getFixedCounter).element(0).isEqualTo(0);
		assertThat(list).extracting(ComplexObject::getValue).element(1).isEqualTo("get1");
		assertThat(list).extracting(ComplexObject::getFixedCounter).element(1).isEqualTo(0);
		assertThat(list).extracting(ComplexObject::getValue).element(2).isEqualTo("get2");
		assertThat(list).extracting(ComplexObject::getFixedCounter).element(2).isEqualTo(2);
	}

	/*
	 * + not depending on indexes
	 * - does not assert that each element is matched at least by one assertion
	 */
	@Test
	void testListAnyMisuse() throws Exception {
		List<ComplexObject> list = createList();

		assertThat(list).anySatisfy(element -> {
			assertThat(element.getValue()).isEqualTo("get0");
			assertThat(element.getFixedCounter()).isEqualTo(0);
		});
		assertThat(list).anySatisfy(element -> {

			assertThat(element.getValue()).isEqualTo("get1");
			assertThat(element.getFixedCounter()).isEqualTo(0);
		});
		assertThat(list).anySatisfy(element -> {
			assertThat(element.getValue()).isEqualTo("get2");
			assertThat(element.getFixedCounter()).isEqualTo(2);
		});
	}

	/*
	 * - does only assert common properties
	 */
	@Test
	void testListAllMisuse() throws Exception {
		List<ComplexObject> list = createList();

		assertThat(list).allSatisfy(element -> {
			assertThat(element.getValue()).startsWith("get");
			assertThat(element.getFixedCounter()).isGreaterThanOrEqualTo(0);
		});
	}

	/*
	 * + not depending on indexes
	 * + keeping sequence property
	 * - not fluent
	 * - type safety warning
	 * - mismatch description is hard for custom matchers
	 */
	@Test
	void testListConditions() throws Exception {
		List<ComplexObject> list = createList();

		assertThat(list).has(new HamcrestCondition(containsInAnyOrder(
			matches("get0", 0),
			matches("get1", 0),
			matches("get2", 2)
		)));
	}

	private Matcher<ComplexObject> matches(String value, int fixedCounter) {
		// TODO replace with a matcher that matches value an fixedCounter
		return IsNull.notNullValue(ComplexObject.class);
	}

	@Test
	void testCouldThisWork() throws Exception {
		List<ComplexObject> list = createList();

		assertThat(list).satisfyInOrder(element -> {
			assertThat(element.getValue()).isEqualTo("get0");
			assertThat(element.getFixedCounter()).isEqualTo(0);
		}, element -> {
			assertThat(element.getValue()).isEqualTo("get1");
			assertThat(element.getFixedCounter()).isEqualTo(0);
		}, element -> {
			assertThat(element.getValue()).isEqualTo("get2");
			assertThat(element.getFixedCounter()).isEqualTo(2);
		});
	}

	private List<ComplexObject> createList() {
		List<ComplexObject> list = new ArrayList<>();
		list.add(createObject(0, false));
		list.add(createObject(1, false));
		list.add(createObject(2, true));
		return list;
	}

	private ComplexObject createObject(int gets, boolean fix) {
		ComplexObject get = new ComplexObject("get" + gets);
		for (int i = 0; i < gets; i++) {
			get.getValue();
		}
		if (fix) {
			get.fix();
		}
		return get;
	}

	private static class ComplexObject {
		private String value;
		private int fixedCounter;
		private int getCounter;

		public ComplexObject(String value) {
			this.value = value;
		}

		public String getValue() {
			getCounter++;
			return value;
		}

		public void fix() {
			this.fixedCounter = getCounter;
		}

		public int getFixedCounter() {
			return fixedCounter;
		}
	}

}