java - Comparing two different objects using contains(Object o) returns false, when equals(Object o) returns true -

[fixed problem below]

i have class:

class doc() {   private string d;    doc(string d) {     d = d;   }    @override   public boolean equals(object o) {     if (o instanceof string) {       if (o.equals(d)) return true;       else return false;     } else return false;   }    @override   public int hashcode() {     ...   } } 

if run test, initiating doc() using e.g. "abc" parameter , use doc().equals("abc") returns true - expect. if use:

arraylist docs = new arraylist<doc>(); doc d = new doc("123"); docs.add(d); system.out.println(docs.contains("123")); 

it returns false, , quite unsure of why. have tried adding output .equals() method check if used, , is. code hashcode() bogus (i know should proper, i'm not sure how) - ever going use equals().

i read somewhere equals() checks hashcode(), however; if add system.out.println(".") first line in hashcode() doesn't output anything, doesn't seem called. best guess can't compare 2 objects of different class - correct? or overlooking else?

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for future reference, problem solved using suggestion of @jonskeet - code now:

class doc() {   private string d;   doc(string d) {     d = d;   }    public string getd() {     return d;   }    @override   public boolean equals(object o) {     if (o instanceof doc) {       doc tmp = (doc) o;       if (tmp.getd().equals(d)) return true;       else return false;     } else return false;   }    @override   public int hashcode() {     ...   } } 

you're assuming contains() call memberincollection.equals(candidate) rather candidate.equals(memberincollection). documentation goes against that:

more formally, returns true if , if list contains @ least 1 element e such (o==null ? e==null : o.equals(e)).

fundamentally, equals method breaks normal contract, because isn't symmetric - non-null values of x , y, x.equals(y) == y.equals(x) should true. that's not case code:

new doc("123").equals("123") // true "123".equals(new doc("123")) // false 

(additionally, equals method isn't reflexive - x.equals(x) never true if x reference instance of doc.)

basically, shouldn't try make instance of 1 type equal instance of unrelated type. end badly. it's hard enough make equality work nicely within type hierarchy - unrelated types it's really bad idea.