Java equals() and hashCode() Contract

importjava.util.HashMap;

publicclassApple{
privateStringcolor;

publicApple(Stringcolor){
this.color=color;
}

publicbooleanequals(Objectobj){
if(!(objinstanceofApple))
returnfalse;
if(obj==this)
returntrue;
returnthis.color==((Apple)obj).color;
}

publicstaticvoidmain(String[]args){
Applea1=newApple("green");
Applea2=newApple("red");

//hashMapstoresappletypeanditsquantity
HashMap<Apple,Integer>m=newHashMap<Apple,Integer>();
m.put(a1,10);
m.put(a2,20);
System.out.println(m.get(newApple("green")));
}
}

Inthisexample,agreenappleobjectisstoredsuccessfullyinahashMap,butwhenthemapisaskedtoretrievethisobject,theappleobjectisnotfound.Theprogramaboveprintsnull.However,wecanbesurethattheobjectisstoredinthehashMapbyinspectioninthedebugger(snapshotbelow).

2.ProblemcausedbyhashCode()Theproblemiscausedbytheun-overriddenmethod“hashCode()”.Thecontractbetweenequals()andhasCode()isthat:
1.Iftwoobjectsareequal,thentheymusthavethesamehashcode.
2.Iftwoobjectshavethesamehashcode,theymayormaynotbeequal.TheideabehindaMapistobeabletofindanobjectfasterthanalinearsearch.Usinghashedkeystolocateobjectsisatwo-stepprocess.InternallytheMapstoresobjectsasanarrayofarrays.Theindexforthefirstarrayisthehashcode()valueofthekey.Thislocatesthesecondarraywhichissearchedlinearlybyusingequals()todetermineiftheobjectisfound.hashCode()indefaultimplementationinObjectclassreturnsdistinctintegersfordistinctobjects.Therefore,intheexampleabove,sameobjectshavedifferenthashCode.HashCodeislikeasequenceofgaragesforstorage,differentstuffcanbestoredindifferentgarages.Itismoreefficientifyouorganizestufftodifferentplaceinsteadofthesamegarage.Soit’sgoodtoequallydistributethehashCodevalue.SothesolutionistoaddhashCodemethodtoclass.HereIjustusethecolorstring’slengthfordemonstration.
publicint
hashCode(){
returnthis.color.length();
}