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import java.io.IOException;
import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

/**
 * The example illustrates how to use the default method for mixin.
 *
 * @author Taras Ledkov
 * @see MixClass
 * @see Debuggable
 * @see Equalable
 * @see Hashable
 */
public class MixIn {

    /**
     * Implement this interface for a class that must be in debug print
     */
    public interface Debuggable {

        /**
         * Print the class name and all data members to the type string. 
         * Reflection is used for it.
         *
         * @return the string formatted like the following: <pre>
         * State of the: &lt;Class Name&gt;
         * &lt;member name&gt; : &lt;value&gt;
         * ...
         * </pre>
         */
        default String toDebugString() {
            StringBuilder sb = new StringBuilder();
            sb.append("State of the: ").append(
                    this.getClass().getSimpleName()).append("\n");
            for (Class cls = this.getClass();
                    cls != null;
                    cls = cls.getSuperclass()) {
                for (Field f : cls.getDeclaredFields()) {
                    try {
                        f.setAccessible(true);
                        sb.append(f.getName()).append(" : ").
                                append(f.get(this)).append("\n");
                    } catch (IllegalAccessException e) {
                    }
                }
            }
            return sb.toString();
        }
    }

    /**
     * This interface implements the method {@link #equalsReflect} that can be used
     * for simple overriding of the Object.equal method.
     */
    public interface Equalable {

        /**
         * Indicates whether some other object is equal to this one. Uses
         * reflection to access data members. Two objects are equal if all the
         * members (except members from #excludeFields list) are equal.
         *
         * @param obj object to compare
         * @param excludeFields the values of these fields don't affect 
         * the return value
         * @return {@code true} if this object is the same as the obj argument;
         * {@code false} otherwise.
         */
        default boolean equalsReflect(Object obj, String... excludeFields) {
            if (this == obj) {
                return true;
            }
            if (obj == null || getClass() != obj.getClass()) {
                return false;
            }
            Set<String> excludes = new HashSet<>(Arrays.asList(excludeFields));
            for (Class cls = this.getClass();
                    cls != null;
                    cls = cls.getSuperclass()) {
                for (Field f : cls.getDeclaredFields()) {
                    if (excludes.contains(f.getName())) {
                        break;
                    }
                    try {
                        f.setAccessible(true);
                        Object thisFieldVal = f.get(this);
                        Object oFieldVal = f.get(obj);
                        if (thisFieldVal != null) {
                            if (!thisFieldVal.equals(oFieldVal)) {
                                return false;
                            }
                        } else if (oFieldVal != null) {
                            return false;
                        }
                    } catch (IllegalAccessException e) {
                    }
                }
            }
            return true;
        }
    }

    /**
     * This interface implements the method {@link #hashCodeReflect} 
     * that can be used for simple overriding of the Object.hashCode method.
     */
    public interface Hashable {

        /**
         * Returns a hash code value for the object. This method uses reflection
         * to access an object's data members. Hash code is calculated on
         * members hash codes.
         *
         * @param excludeFields the values of these fields don't affect 
         * the return value
         *
         * @return a hash code value for this object.
         */
        default int hashCodeReflect(String... excludeFields) {
            int result = 0;
            Set<String> excludes = new HashSet<>(Arrays.asList(excludeFields));
            for (Class cls = this.getClass();
                    cls != null;
                    cls = cls.getSuperclass()) {
                for (Field f : cls.getDeclaredFields()) {
                    if (excludes.contains(f.getName())) {
                        break;
                    }
                    try {
                        f.setAccessible(true);
                        Object val = f.get(this);
                        result = 31 * result + val.hashCode();
                    } catch (IllegalAccessException e) {
                    }
                }
            }
            return result;
        }
    }

    /**
     * Sample class to demonstrate mixin. This class inherits the behavior 
     * of the {@link Debuggable}, {@link Equalable}, {@link Hashable}
     */
    public static class MixClass implements Debuggable, Equalable, Hashable {

        /**
         * Need to illustrate debug print, compare and hash code calculation
         */
        private final int n = 28;
        /**
         * Need to illustrate debug print, compare and hash code calculation
         */
        private final String strHello = "Hello world";

        /**
         * Use the default method from {@link Equalable} to check equals
         *
         * @param o object to compare
         */
        @Override
        public boolean equals(Object o) {
            return equalsReflect(o);
        }

        /**
         * Use the default method from {@link Hashable} to calculate 
         * the hash code
         */
        @Override
        public int hashCode() {
            return hashCodeReflect();
        }
    }

    /**
     * Illustrate the behavior of the MixClass
     *
     * @param args command-line arguments
     * @throws java.io.IOException internal demo error
     */
    public static void main(final String[] args) throws IOException {
        MixClass a = new MixClass();
        MixClass b = new MixClass();
        System.out.println(a.toDebugString());
        System.out.println(a.equals(b));
    }
}