JDK Obejct "equals" method in detail

public boolean equals​(Object obj)

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

Indicates whether one object is equal to another object (objects are equal).

Note: When the class has its own "logical equality" concept, and the parent class does not override the equals method, you need to override this method yourself. But overwriting this method can easily lead to errors, please be careful!

Considering that if you need to override equals, there are far more situations to consider than you think, so try not to overwrite it! Especially in the following situations:

• Each instance of a class is essentially unique.
• It is not necessary for the class to provide "logically equal" testing functions.
• The equals method already covered by the superclass is suitable for you.
• The class is private, or package-level private, you can be sure that its equals method will never be called.

The Object specification to override the equals method is as follows:

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.