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Override equals

Although some people think it might be easy to override the equals method of a class, you can do it wrong in many cases and this can cause problems.

The easiest case is not to override the equals method, then each class is equals only to itself and equals from object is used. Do this only in following cases:

  • When each instance is inherently unique. For example classes as Thread. These represent active entites rather than values. So for value objects this is no applicable.
  • You don’t need a “logical equality” test. For example the java.util.Random where it does not make sense to check if two random numbers are equal.
  • The superclass equals is fully appropriate. E.g. Set -> AbstractSet, List -> AbstractList
  • The class or the package is private and it is assured that it will not been invoced

When to override equals method?

When the logical equality differs from object identity. This is the case for value classes. This class represents a value such as Integer or Date. If these objects will be compared you want to know if the values are the same and not if the objects are the same.

The Java6 specification says about the contract which must be assured when overriding equals:

• Reflexive:Foranynon-nullreferencevaluex,x.equals(x)mustreturntrue.
• Symmetric:Foranynon-nullreferencevaluesxandy,x.equals(y)mustre- turn true if and only if y.equals(x) returns true.
• Transitive:Foranynon-nullreferencevaluesx,y,z,ifx.equals(y)returns true and y.equals(z) returns true, then x.equals(z) must return true.
• Consistent: For any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, pro- vided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) must return false.

How should equals be overridden?

This list was taken from Effective Java 2nd Edition.

  • Use the == operator to check if the argument is a reference to this object.
  • Use the instanceof operator to check if the argument has the correct type.
  • Cast the argument to the correct type.
  • For each “significant” field in the class, check if that field of the argument matches the corresponding field of this object.
  • When you are finished writing your equals method, ask yourself three questions: Is it symmetric? Is it transitive? Is it consistent?
Override hashCode

Always override hashCode when you override equals.

Here is the contract from the Java specification:

Here is the contract, copied from the Object specification [JavaSE6]:
• Whenever it is invoked on the same object more than once during an execu- tion of an application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execu- tion of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

Furthermore two equal objects must have the same hash code.

How to create a good has method?

Here are some tips taken from Effective Java book:

  • Store some constant nonzero value, say 17, in an int variable called result.
  • For each significant field f in your object (each field taken into account by the equals method, that is), do the following:
    • Compute an int has code c for the field:
      • If the field is a boolean, compute (f ? 1 : 0)
      • if the field is a byte, char, short, or int, computer (int) f.
      • If the field is a long, computer (int) (f^>>>32)).
      • If the field is a float, computer Float.floatToIntBits(f).
      • If the field is a double, compute Double.doubleToLongBits(f), and then hash the resulting long as in step for long.
      • If the field is an object reference and this class’s equals method compares the field by recursively invoking equals, recursively invoke hashCode on the field. If a more complex comparison is required, compute a “canonical representation” for this field and invoke hasCode on the canonical representation. If the value of the field is null, return 0 or some other constant.
      • If the field is an array, treat it as if each element were a separate field. That is, compute a hash code for each significant element by applying these rules recursively, and combine these values per step 2 for byte,short etc. If every element in an array field is significant, you can use one of the Arrays.hasCode methods added in release 1.5.
    • Combine the has code c computed in step 2 into result as follows:
      result = 31 * result + c;
  • Return result.
  • When you are finished writing the hashCode method, ask yourself whether equal instances have equal hash codes. Write unit tests to verify your intuition! If equal instances have unequal hash codes, figure out why and fix the problem.

Override toString

If you do not override toString method and print an object you usally see something like this PhoneNumber@434b65. Ok, it is clear that this is a phone number, but wouldn’t it be better to have a representation like “(0221) 77744499″?

So when overriding the toString method it should return all of the interesting information contained in the object. If you have large objects or objects with a not conductive state you can return something like “Yellow pages (2355333 listings)”.

Provide your toString method with good documentation and describe your format. Here is an example taken from Effective Java 2nd Edition:

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/** 
* Returns the string representation of this phone number. 
* The string consists of fourteen characters whose format 
* is "(XXX) YYY-ZZZZ", where XXX is the area code, YYY is 
* the prefix, and ZZZZ is the line number. (Each of the 
* capital letters represents a single decimal digit.) 
* 
* If any of the three parts of this phone number is too small 
* to fill up its field, the field is padded with leading zeros. 
* For example, if the value of the line number is 123, the last 
* four characters of the string representation will be "0123". 
* 
* Note that there is a single space separating the closing 
* parenthesis after the area code from the first digit of the 
* prefix. 
*/
@Override public String toString() { 
    return String.format("(%03d) %03d-%04d", areaCode, prefix, lineNumber);
}

If you want not a specific format make something like this:

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/**
* Returns a brief description of this potion. The exact details 
* of the representation are unspecified and subject to change, 
* but the following may be regarded as typical:
* "[Potion #9: type=love, smell=turpentine, look=india ink]" 
@Override public String toString() { ... }

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