groovy.transform.EqualsAndHashCode Maven / Gradle / Ivy
/*
* Copyright 2008-2013 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.
*
* It allows you to write classes in this shortened form:
*
* 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
*
* The {@code @EqualsAndHashCode} annotation instructs the compiler to execute an
* AST transformation which adds the necessary equals and hashCode methods to the class.
*
* The {@code hashCode()} method is calculated using Groovy's {@code HashCodeHelper} class
* which implements an algorithm similar to the one outlined in the book Effective Java.
*
* 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 java.lang.Object
*
* To illustrate the 'canEqual' implementation style (see http://www.artima.com/lejava/articles/equality.html
* for further details), consider this class:
*
* {@code @EqualsAndHashCode}
* class IntPair {
* int x, y
* }
*
* The generated equals and canEqual methods will be something like below:
*
* 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
* }
*
* 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:
*
* {@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
*
* Note that if you create any domain classes which don't have exactly the
* same contract as IntPair then you should provide an appropriate
* equals and canEqual method. The easiest way to
* achieve this would be to use the {@code @Canonical} or
* {@code @EqualsAndHashCode} annotations as shown below:
*
* {@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
*
*
* The alternative supported style regards any kind of inheritance as creation of
* a new type and is illustrated in the following example:
*
* {@code @EqualsAndHashCode(useCanEqual=false)}
* class IntPair {
* int x, y
* }
*
* The generated equals method will be something like below:
*
* 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
* }
*
* This style is appropriate for final classes (where inheritance is not
* allowed) which have only 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 canEqual() method.
*
* Note that if you explicitly set useCanEqual=false for child nodes
* in a class hierarchy but have it true for parent nodes and you
* also have callSuper=true in the child, then your generated
* equals methods will not strictly follow the equals contract.
*
* 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 IntPair and IntTriple classes
* discussed earlier, you could provide the following method in IntPair:
*
* boolean hasEqualXY(other) { other.x == getX() && other.y == getY() }
*
* Then for the objects defined earlier, the following would be true:
*
* assert p1.hasEqualXY(t1) && t1.hasEqualXY(p1)
* assert p2.hasEqualXY(t1) && t1.hasEqualXY(p2)
* assert p3.hasEqualXY(t1) && t1.hasEqualXY(p3)
*
* There is also support for including or excluding fields/properties by name when constructing
* the equals and hashCode methods as shown here:
*
* import groovy.transform.*
* {@code @EqualsAndHashCode}(excludes="z")
* {@code @TupleConstructor}
* public class Point2D {
* int x, y, z
* }
*
* assert new Point2D(1, 1, 1).equals(new Point2D(1, 1, 2))
* assert !new Point2D(1, 1, 1).equals(new Point2D(2, 1, 1))
*
* {@code @EqualsAndHashCode}(excludes=["y", "z"])
* {@code @TupleConstructor}
* public class Point2D {
* int x, y, z
* }
*
* assert new Point1D(1, 1, 1).equals(new Point1D(1, 1, 2))
* assert new Point1D(1, 1, 1).equals(new Point1D(1, 2, 1))
* assert !new Point1D(1, 1, 1).equals(new Point1D(2, 1, 1))
*
* Note: {@code @EqualsAndHashCode} is a transform to help reduce boilerplate
* in the common cases. It provides a useful implementation of {@code equals()}
* and {@code hashCode()} but not necessarily the most appropriate or
* efficient one for every use case. You should write custom versions if your
* scenario demands it. In particular, if you have
* mutually-referential classes, the implementations provided may not be suitable
* and may recurse infinitely (leading to a {@code StackOverflowError}). In such cases,
* you need to remove the infinite loop from your data structures or write your own custom methods.
* If your data structures are self-referencing, the code generated by this transform will try to avoid
* infinite recursion but the algorithm used may not suit your scenario, so use with caution if
* you have such structures.
* A future version of this transform may better handle some additional recursive scenarios.
*
* @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 {
/**
* List of field and/or property names to exclude from the equals and hashCode calculations.
* Must not be used if 'includes' is used. For convenience, a String with comma separated names
* can be used in addition to an array (using Groovy's literal list notation) of String values.
*/
String[] excludes() default {};
/**
* List of field and/or property names to include within the equals and hashCode calculations.
* Must not be used if 'excludes' is used. For convenience, a String with comma separated names
* can be used in addition to an array (using Groovy's literal list notation) of String values.
*/
String[] includes() default {};
/**
* Whether to cache hashCode calculations. You should only set this to true if
* you know the object is immutable (or technically mutable but never changed).
*/
boolean cache() default false;
/**
* 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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