If only override hashCode, then after reaching the same hash bucket (when do contains (obj)), java will not know they are equal or not as equals is not redefined. I guess that the default equals ...

The PhoneNumber class’s failure to override hashCode causes the two equal instances to have unequal hash codes, in violation of the hashCode contract.Therefore, the get method is likely to look for ...

When overriding equals(), it’s common practice to also override the hashCode() method. See Comparing Java objects with equals () and hashcode () for more about these methods.

Although I’ll use the new java.util.Objects class instead for JDK 7 projects, Guava’s com.google.common.base.Objects class provides a nice alternative for working in versions of Java prior to ...