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Groovy example source code file (EqualsAndHashCode.java)

This example Groovy source code file (EqualsAndHashCode.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Groovy tags/keywords

annotation, equalsandhashcode, equalsandhashcode, groovyasttransformationclass, groovyasttransformationclass, string, string, target, target

The Groovy EqualsAndHashCode.java source code

/*
 * Copyright 2008-2011 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package groovy.transform;

import org.codehaus.groovy.transform.GroovyASTTransformationClass;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Class annotation used to assist in creating appropriate {@code equals()} and {@code hashCode()} methods.
 * <p/>
 * It allows you to write classes in this shortened form:
 * <pre>
 * import groovy.transform.EqualsAndHashCode
 * {@code @EqualsAndHashCode}
 * class Person {
 *     String first, last
 *     int age
 * }
 * def p1 = new Person(first:'John', last:'Smith', age:21)
 * def p2 = new Person(first:'John', last:'Smith', age:21)
 * assert p1 == p2
 * def map = [:]
 * map[p1] = 45
 * assert map[p2] == 45
 * </pre>
 * The {@code @EqualsAndHashCode} annotation instructs the compiler to execute an
 * AST transformation which adds the necessary equals and hashCode methods to the class.
 * <p/>
 * The {@code hashCode()} method is calculated using Groovy's {@code HashCodeHelper} class
 * which implements an algorithm similar to the one outlined in the book <em>Effective Java.
 * <p/>
 * The {@code equals()} method compares the values of the individual properties (and optionally fields)
 * of the class.  It can also optionally call equals on the super class. Two different equals method
 * implementations are supported both of which support the equals contract outlined in the javadoc
 * for <code>java.lang.Object
 * <p/>
 * To illustrate the 'canEqual' implementation style (see http://www.artima.com/lejava/articles/equality.html
 * for further details), consider this class:
 * <pre>
 * {@code @EqualsAndHashCode}
 * class IntPair {
 *     int x, y
 * }
 * </pre>
 * The generated <code>equals and canEqual methods will be something like below:
 * <pre>
 * public boolean equals(java.lang.Object other)
 *     if (other == null) return false
 *     if (this.is(other)) return true
 *     if (!(other instanceof IntPair)) return false
 *     if (!other.canEqual(this)) return false
 *     if (x != other.x) return false
 *     if (y != other.y) return false
 *     return true
 * }
 *
 * public boolean canEqual(java.lang.Object other) {
 *     return other instanceof IntPair
 * }
 * </pre>
 * If no further options are specified, this is the default style for {@code @Canonical} and
 * {@code @EqualsAndHashCode} annotated classes. The advantage of this style is that it allows inheritance
 * to be used in limited cases where its purpose is for overriding implementation details rather than
 * creating a derived type with different behavior. This is useful when using JPA Proxies for example or
 * as shown in the following examples:
 * <pre>
 * {@code @Canonical} class IntPair { int x, y }
 * def p1 = new IntPair(1, 2)
 *
 * // overriden getter but deemed an IntPair as far as domain is concerned
 * def p2 = new IntPair(1, 1) { int getY() { 2 } }
 *
 * // additional helper method added through inheritance but
 * // deemed an IntPair as far as our domain is concerned
 * {@code @InheritConstructors} class IntPairWithSum extends IntPair {
 *     def sum() { x + y }
 * }
 *
 * def p3 = new IntPairWithSum(1, 2)
 *
 * assert p1 == p2 && p2 == p1
 * assert p1 == p3 && p3 == p1
 * assert p3 == p2 && p2 == p3
 * </pre>
 * Note that if you create any domain classes which don't have exactly the
 * same contract as <code>IntPair then you should provide an appropriate
 * <code>equals and canEqual method. The easiest way to
 * achieve this would be to use the {@code @Canonical} or
 * {@code @EqualsAndHashCode} annotations as shown below:
 * <pre>
 * {@code @EqualsAndHashCode}
 * {@code @TupleConstructor(includeSuperProperties=true)}
 * class IntTriple extends IntPair { int z }
 * def t1 = new IntTriple(1, 2, 3)
 * assert p1 != t1 && p2 != t1 && t1 != p3
 * </pre>
 *
 * The alternative supported style regards any kind of inheritance as creation of
 * a new type and is illustrated in the following example:
 * <pre>
 * {@code @EqualsAndHashCode(useCanEqual=false)}
 * class IntPair {
 *     int x, y
 * }
 * </pre>
 * The generated equals method will be something like below:
 * <pre>
 * public boolean equals(java.lang.Object other)
 *     if (other == null) return false
 *     if (this.is(other)) return true
 *     if (IntPair != other.getClass()) return false
 *     if (x != other.x) return false
 *     if (y != other.y) return false
 *     return true
 * }
 * </pre>
 * This style is appropriate for final classes (where inheritance is not
 * allowed) which have only <code>java.lang.Object as a super class.
 * Most {@code @Immutable} classes fall in to this category. For such classes,
 * there is no need to introduce the <code>canEqual() method.
 * <p/>
 * Note that if you explicitly set <code>useCanEqual=false for child nodes
 * in a class hierarchy but have it <code>true for parent nodes and you
 * also have <code>callSuper=true in the child, then your generated
 * equals methods will not strictly follow the equals contract.
 * <p/>
 * Note that when used in the recommended fashion, the two implementations supported adhere
 * to the equals contract. You can provide your own equivalence relationships if you need,
 * e.g. for comparing instances of the <code>IntPair and IntTriple classes
 * discussed earlier, you could provide the following method in <code>IntPair:
 * <pre>
 * boolean hasEqualXY(other) { other.x == getX() && other.y == getY() }
 * </pre>
 * Then for the objects defined earlier, the following would be true:
 * <pre>
 * assert p1.hasEqualXY(t1) && t1.hasEqualXY(p1)
 * assert p2.hasEqualXY(t1) && t1.hasEqualXY(p2)
 * assert p3.hasEqualXY(t1) && t1.hasEqualXY(p3)
 * </pre>
 *
 * @see org.codehaus.groovy.util.HashCodeHelper
 * @author Paul King
 * @since 1.8.0
 */
@java.lang.annotation.Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@GroovyASTTransformationClass("org.codehaus.groovy.transform.EqualsAndHashCodeASTTransformation")
public @interface EqualsAndHashCode {
    /**
     * Comma separated list of field and/or property names to exclude from the equals and hashCode calculations.
     * Must not be used if 'includes' is used.
     */
    String excludes() default "";

    /**
     * Comma separated list of field and/or property names to include within the equals and hashCode calculations.
     * Must not be used if 'excludes' is used.
     */
    String includes() default "";

    /**
     * Whether to include super in equals and hashCode calculations
     */
    boolean callSuper() default false;

    /**
     * Include fields as well as properties in equals and hashCode calculations
     */
    boolean includeFields() default false;

    /**
     * Generate a canEqual method to be used by equals
     */
    boolean useCanEqual() default true;
}

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