Java: Overriding Equals () and Hashcode ()

The following is summarized from "effective Java".

1. When to rewrite equals ()

When a class has its own unique concept of "logical equivalence" (different from the concept of object identity).

2. Design Equals ()

[1] Use the instanceof operator to check whether the argument is the correct type.

[2] For each "critical field" in a class, check the field in the argument for the corresponding field value in the current object.

[2.1] Use = = Comparison for primitive type fields of non-float and double types;

[2.2] recursively calls the Equals method for the object reference domain;

[2.3] For the float field, use Float.floattointbits (afloat) to convert to int, then use = = comparison;

[2.4] for double fields, convert to int using double.doubletolongbits (adouble), then use = = comparison;

[2.5] For array fields, invoke the Arrays.equals method.

3. When you rewrite equals (), always rewrite hashcode ()

Depending on the Equals method of a class (rewritten), two distinct instances may be logically equal, but, according to the Object.hashcode method, they are just two objects. Therefore, a violation of the "equal object must have an equal hash code".

4. Design Hashcode ()

[1] Store a non-0 constant value, such as 17, in the int variable result;

[2] For each of the key domain F in the object (the Equals method is considered in each field):

[2.1]boolean type, calculation (f. 0:1);

[2.2]byte,char,short type, calculation (int);

[2.3]long type, computed (int) (f ^ (f>>>32));

[2.4]float type, calculated float.floattointbits (afloat);

[2.5]double type, computed double.doubletolongbits (adouble) Gets a long, then executes [2.3];

[2.6] An object reference that recursively calls its Hashcode method;

[2.7] An array field that calls its Hashcode method on each of these elements.

[3] Save the computed hash code to int variable C and execute result=37*result+c;

[4] returns result.