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TestNG extension reducing boilerplate code in tests.

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package de.retest.recheck.testng;

import static java.util.stream.Stream.concat;
import static java.util.stream.Stream.of;

import java.lang.reflect.Field;
import java.util.function.Predicate;
import java.util.stream.Stream;

import lombok.RequiredArgsConstructor;

/**
 * Finds all {@link Field}s matching given {@link Predicate}.
 */
@RequiredArgsConstructor
public class FindFields {

	private final Predicate predicate;

	/**
	 * Create instance of {@link FindFields} to find fields matching the given {@link Predicate}.
	 */
	public static FindFields matching( final Predicate predicate ) {
		return new FindFields( predicate );
	}

	/**
	 * Returns a {@link Stream} of all fields of the given {@link Class}, its super classes and the interfaces
	 * implemented either by the {@link Class} itself or by its super classes, matching the specified {@link Predicate}.
	 *
	 * @param clazz
	 *            to search for fields
	 * @return {@link Field}s of the given {@link Class}, its super classes and the implemented interfaces.
	 */
	public Stream on( final Class clazz ) {
		return concat( getFields( clazz ), getSuperClassFieldsOf( clazz.getSuperclass() ) );
	}

	/**
	 * Return fields directly declared at the given {@link Class} and at the interfaces implemented by the given
	 * {@link Class}.
	 *
	 * @param clazz
	 *            {@link Class} to search for fields
	 * @return {@link Field}s of the given {@link Class} and the interfaces implemented by the given {@link Class}
	 */
	private Stream getFields( final Class clazz ) {
		return concat( getDeclaredFieldsOf( clazz ), getInterfaceFields( clazz ) );
	}

	/**
	 * Return fields declared interfaces implemented by the given {@link Class}.
	 *
	 * @param clazz
	 *            {@link Class} to search for fields
	 * @return {@link Field}s of interfaces implemented by the given {@link Class}
	 */
	private Stream getInterfaceFields( final Class clazz ) {
		return of( clazz.getInterfaces() ).flatMap( this::getFields );
	}

	/**
	 * Return fields declared at super classes of given {@link Class}
	 *
	 * @param clazz
	 *            {@link Class} to search for fields
	 * @return {@link Field}s of the super classes of the given {@link Class}
	 */
	private Stream getSuperClassFieldsOf( final Class clazz ) {
		if ( null == clazz.getSuperclass() || Object.class.equals( clazz.getSuperclass() ) ) {
			return getDeclaredFieldsOf( clazz );
		}
		return concat( getFields( clazz ), getSuperClassFieldsOf( clazz.getSuperclass() ) );
	}

	/**
	 * Return fields directly declared at given {@link Class}.
	 *
	 * @param clazz
	 *            {@link Class} to search for fields
	 * @return {@link Field}s of the given {@link Class}
	 */
	private Stream getDeclaredFieldsOf( final Class clazz ) {
		return Stream.of( clazz.getDeclaredFields() ).filter( predicate );
	}
}