Override hashCode, equals, and toString methods from Object class

<Back

All classes in java inherit from java.lang.Object. There are 3 Object methods that are commonly override in subclasses, namely toString, hashCode and equals.

Override toString

Java automatically calls the toString() method if you try to print out an object.
Providing a good toString implementation makes your class much more pleasant to use. The toString method is automatically invoked when an object is passed to println, printf, then string concatenation operator, or assert, or printed by a debugger. When practical, the toString method should return all of the interesting information contained in the object.  To override, you use the following method signature.
@Override public String toString() {...}

Obey the general contract when overriding equals

By default, equals implement each instance of the class is equal only to itself.  When a class has a notion of logical equality that differs from mere object identity, and a superclass has not already overridden equals to implement the desired behavior, it is appropriate to override Object.equals. When you override the equals method, you must adhere to its general contract. According to javadoc:

public boolean equals(Object obj)

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.

Always override hashCode when you override equals

A common source of bugs is the failure to override the hashCode method. You must override hashCode in every class that overrides equals. Failure to do so will result in a violation of the general contract for Object.hashCode, which will prevent your class from functioning properly in conjuction with all hash-based collections, including HashMap, HashSet and Hashtable.

According to javadoc:

public int hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.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 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 hash tables.

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 Java^TM programming language.)

OCPJP test points from the above javadoc are:

1. Within the same program, the result of hashCode() must not change. 
2. If equals() returns true when called with two objects, calling hashCode() on each of these objects must return the same value. 
3. If equals() return false when called with two objects, calling hashCode() on each of those objects does not have to return a different result. This means hashCode() results do not need to be unique when called on unequal objects.