EqualsAndHashCode (Groovy 2.4.5)
Date: 2015-09-17

groovy.transform

## Annotation Type EqualsAndHashCode

-     @Documented
      @Retention(value=RUNTIME)
      @Target(value=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)

       // overridden 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))

  Note: `@EqualsAndHashCode` is a transform to help reduce boilerplate in the common cases. It provides a useful implementation of `equals()` and `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 `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.
  Since:
    1.8.0
  Author:
    Paul King
  See Also:
    `HashCodeHelper`

- - ### Optional Element Summary

    Optional Elements
    Modifier and Type | Optional Element and Description
    --- | ---
    `boolean` | `cache` Whether to cache hashCode calculations.
    `boolean` | `callSuper` Whether to include super in equals and hashCode calculations.
    `String[]` | `excludes` 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.
    `String[]` | `includes` 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

          public abstract String[] excludes

      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.
      Default:
        {}

    - #### includes

          public abstract String[] includes

      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.
      Default:
        {}

    - #### cache

          public abstract boolean cache

      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).
      Default:
        false

    - #### callSuper

          public abstract boolean callSuper

      Whether to include super in equals and hashCode calculations.
      Default:
        false

    - #### includeFields

          public abstract boolean includeFields

      Include fields as well as properties in equals and hashCode calculations.
      Default:
        false

    - #### useCanEqual

          public abstract boolean useCanEqual

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