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Java Tostring Equals Hashcode with annotations

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TEH

TEH stands for __T__oString __E__quals __H__ashCode


TEH uses annotations to implement toString, equals and hashCode, and enforces these 2 rules

  • any attribute used for hashCode will be used for equals and toString
  • any attribute used for equals will be used for toString

The annotations are

  • @ToString
  • @ToStringEquals
  • @ToStringEqualsHashCode

You can activate TEH on a given class by 2 ways

inheritance

class MyTEHObject extends TEHObject{

    @ToStringEqualsHashCode
    long id;

    @ToString
    String description; 
...

annotation

@TEH
class MyTEHObject{

    @ToStringEqualsHashCode
    long id;

    @ToString
    String description

    public String toString() {
        return TEHUtils.toString(this);
    }

    public boolean equals(Object other) {
        return TEHUtils.equals(this, other);
    }

    public int hashCode() {
        return TEHUtils.hashCode(this,super.hashCode());
    }
...

2 unbreakable rules explanations

toString javadoc

Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())* 

equals javadoc

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

hashcode javadoc

Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.

The general contract of hashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java 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 execution 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 {@link java.lang.Object#equals(java.lang.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 hashtables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Complements

  • toString is used for logs, debugger display, junit assertions failures, but rarely for user display
  • equals is used for object equality, based on strong business understanding, or technical unique identification, to compare objects on business rules and junit assertions
  • hashCode is used for internal structure optimisations and is a part of the default toString value

Rules

  • any attribute used for hashCode will be used for equals and toString

    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. (from hashcode javadoc)

  • any attribute used for equals will be used for toString

When a junit assertion like assertEquals is failing, it is preferable that the generated message gives the difference between objects.


Using GWT ?

Checkout [https://github.com/wokier/GWT-TEH]


Release Notes

0.7 2013-04-24 Avoid Class reference. Maven Central Release 0.6 2012-07-10 Performance Optimizations 0.5 2012-06-16 gwt-teh

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