Effective Java: Programming Language Guide in Java

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Effective Java: Programming Language Guide
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public boolean equals(Object o) { if (!(o instanceof MyType)) return false; }
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If this type check were missing and the equals method were passed an argument of the wrong type, the equals method would throw a ClassCastException, which violates the equals contract But the instanceof operator is specified to return false if its first operand is null, regardless of what type appears in the second operand [JLS, 15192] Therefore the type check will return false if null is passed in, so you don't need a separate null check Putting it all together, here's a recipe for a high-quality equals method: 1 Use the == operator to check if the argument is a reference to this object If so, return true This is just a performance optimization, but one that is worth doing if the comparison is potentially expensive 2 Use the instanceof operator to check if the argument is of the correct type If not, return false Typically, the correct type is the class in which the method occurs Occasionally, it is some interface implemented by this class Use an interface if the class implements an interface that refines the equals contract to permit comparisons across classes that implement the interface The collection interfaces Set, List, Map, and MapEntry have this property 3 Cast the argument to the correct type Because this cast was preceded by an instanceof test, it is guaranteed to succeed 4 For each significant field in the class, check to see if that field of the argument matches the corresponding field of this object If all these tests succeed, return true; otherwise, return false If the type in Step 2 is an interface, you must access the argument's significant fields via interface methods; if the type is a class, you may be able to access the fields directly, depending on their accessibility For primitive fields whose type is not float or double, use the == operator for comparisons; for object reference fields, invoke the equals method recursively; for float fields, translate to int values using FloatfloatToIntBits and compare the int values using the == operator; for double fields, translate to long values using DoubledoubleToLongBits and compare the long values using the == operator (The special treatment of float and double fields is made necessary by the existence of FloatNaN, -00f, and the analogous double constants; see the Floatequals documentation for details) For array fields, apply these guidelines to each element Some object reference fields may legitimately contain null To avoid the possibility of a NullPointerException, use the following idiom to compare such fields:
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(field == null ofield == null : fieldequals(ofield))
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This alternative may be faster if field and ofield are often identical object references:
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(field == ofield || (field != null && fieldequals(ofield)))
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For some classes, like CaseInsensitiveString shown earlier, the field comparisons are more complex than simple equality tests It should be apparent from the specification for a class if this is the case If so, you may want to store a canonical form in each object, so that the equals method can do cheap exact comparisons on these canonical forms rather than more costly inexact comparisons This technique is most appropriate for immutable classes (Item 13), as the canonical form would have to be kept up to date if the object could change The performance of the equals method may be affected by the order in which fields are compared For best performance, you should first compare fields that are more likely to differ, less expensive to compare, or, ideally, both You must not compare fields that are not part of an object's logical state, such as Object fields used to synchronize operations You need not compare redundant fields, which can be calculated from significant fields, but doing so may improve the performance of the equals method If a redundant field amounts to a summary description of the entire object, comparing this field will save you the expense of comparing the actual data if the comparison fails 5 When you are finished writing your equals method, ask yourself three questions: Is it symmetric, is it transitive, and is it consistent (The other two properties generally take care of themselves) If not, figure out why these properties fail to hold, and modify the method accordingly For a concrete example of an equals method constructed according to the above recipe, see PhoneNumberequals in Item 8 Here are a few final caveats:
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Always override hashCode when you override equals (Item 8) Don't try to be too clever If you simply test fields for equality, it's not hard to adhere to the equals contract If you are overly aggressive in searching for equivalence, it's easy to get into trouble It is generally a bad idea to take any form of aliasing into account For example, the File class shouldn't attempt to equate symbolic links referring to the same file Thankfully, it doesn't
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Don't write an equals method that relies on unreliable resources It's extremely difficult to satisfy the consistency requirement if you do this For example, javanetURL's equals method relies on the IP addresses of the hosts in URLs being compared Translating a host name to an IP address can require network access, and it isn't guaranteed to yield the same results over time This can cause the URL equals method to violate the equals contract, and it has caused problems in practice (Unfortunately, this behavior cannot be changed due to compatibility requirements) With few exceptions, equals methods should perform deterministic computations on memory-resident objects Don't substitute another type for Object in the equals declaration It is not uncommon for a programmer to write an equals method that looks like the following, and then spend hours puzzling over why it doesn't work properly:
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Effective Java: Programming Language Guide public boolean equals(MyClass o) { }
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The problem is that this method does not override Objectequals, whose argument is of type Object, but overloads it instead (Item 26) It is acceptable to provide such a strongly typed equals method in addition to the normal one as long as the two methods return the same result but there is no compelling reason to do so It may provide minor performance gains under certain circumstances, but it isn't worth the added complexity (Item 37)
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