Annotation Type EqualsAndHashCode


  • @Documented
    @Retention(RUNTIME)
    @Target(TYPE)
    public @interface EqualsAndHashCode
    Class annotation used to assist in creating appropriate equals() and hashCode() methods.

    It allows you to write classes in this shortened form:

     import groovy.transform.EqualsAndHashCode
     @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 @EqualsAndHashCode annotation instructs the compiler to execute an AST transformation which adds the necessary equals and hashCode methods to the class.

    The hashCode() method is calculated using Groovy's HashCodeHelper class which implements an algorithm similar to the one outlined in the book Effective Java.

    The 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:

     @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 @Canonical and @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:
     @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
     @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 @Canonical or @EqualsAndHashCode annotations as shown below:
     @EqualsAndHashCode
     @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:
     @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 @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.*
     @EqualsAndHashCode(excludes="z")
     @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))
    
     @EqualsAndHashCode(excludes="y,z")
     @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))
     
    Since:
    1.8.0
    Author:
    Paul King
    See Also:
    HashCodeHelper
    • Optional Element Summary

      Optional Elements 
      Modifier and Type Optional Element Description
      boolean callSuper
      Whether to include super in equals and hashCode calculations
      java.lang.String excludes
      Comma separated list of field and/or property names to exclude from the equals and hashCode calculations.
      boolean includeFields
      Include fields as well as properties in equals and hashCode calculations
      java.lang.String includes
      Comma separated list of field and/or property names to include within the equals and hashCode calculations.
      boolean useCanEqual
      Generate a canEqual method to be used by equals
    • Element Detail

      • excludes

        java.lang.String excludes
        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.
        Default:
        ""
      • includes

        java.lang.String includes
        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.
        Default:
        ""
      • callSuper

        boolean callSuper
        Whether to include super in equals and hashCode calculations
        Default:
        false
      • includeFields

        boolean includeFields
        Include fields as well as properties in equals and hashCode calculations
        Default:
        false
      • useCanEqual

        boolean useCanEqual
        Generate a canEqual method to be used by equals
        Default:
        true