java - Is there a way with concurrency, that a same Object.equals(Object) returns false? -

that's pestered mind 1 of days, looking concrete answer.

(please, bear me, not looking make beautiful code. au contraire, i'm lookng problem code smell)

imagine have stateful object comming class foo

public class foo {      public int attribute = 0;       // hashcode implemented :p      @override      public boolean equals(object o) {          if (o instanceof foo) {             foo = (foo) o;              return this.attribute == that.attribute;          }          return false;      } } 

and have workers on foo

public class doombringer implements runnable {      private final foo foo;      public doombringer(foo foo) {         this.foo = foo;     }     @override     public void run() {        this.foo.attribute++;     } } 

and 1 prints result of #equals object passed parameter constructor.

public class selfequalitytestprinter implements runnable {      private final foo foo;      public selfequalitytestprinter(foo foo) {         this.foo = foo;     }      @override     public void run() {         system.out.println(foo.equals(foo));     } } 

is possible false printed 1 day if have concurrent threads modifying same foo instance?

my guess it's possible. don't think equals synchronized unless make so. way avoid testing this == o @ object#equals(object o) method, same problem may rise comparing different instances should equal.

yes, possible. variables not synchronized can changed @ moment thread given access variables.

if access foo.attribute synchronized, , .equals synchronized when referencing attribute, not case.

more surprising, if 1 thread changed attribute, thread read afterwards , old value! last piece can fixed through use of volatile keyword. here's explanation of volatile keyword.

as brian , yshavit have said, problems in not declaring variable volatile include race conditions. here, example, 2 threads calling attribute++ result in attribute being incremented once. second thread increment use unincremented value attribute stored in cached memory instead of incremented value written first thread.