What is the relationship between equals() and hashCode()?

"Objects if two instances are equal (that is, equals() returns true) then they must have the same hashCode() value. This requirement is necessary to ensure that hash table-based data structures operate properly. It is okay if unequal objects have equal or unequal hash values. This rule only applies when the objects are equal.\n\n1 public int hashCode() {\n\n2 final int prime = 31;\n\n3 int result = 1;\n\n4 result = prime * result + ((firstName == null) ? 0 :\n\n5 firstName.hashCode());\n\n6 long temp;\n\n7 temp = Double.doubleToLongBits(gpa);\n\n8 result = prime * result + (int) (temp ^ (temp >>> 32));\n\n9 result = prime * result + ((lastName == null) ? 0 :\n\n10 lastName.hashCode());\n\n11 result = prime * result + nuid;\n\n12 return result;\n\n13 }"