HashMap and reference

Question

Hi,
In HashMap or any other collection framework in general,
when I look up something, in order for that something to be
in the collection, I need to look it up by an object with the
same reference, not an object that is "equal" by "equals()" method.
Let me give an example.
==========================
class test {
public static void main(String args[]) {
HashMap<MyObject, String> someMap =
new HashMap<MyObject, String>();
MyObject abc = new MyObject();
abc.a = 7;
someMap.put(abc, new String("roses"));
if (someMap.get(abc) != null) {
System.out.println((String) someMap.get(abc));
MyObject def = new MyObject();
def.a = 7;
if (someMap.get(def) != null) {
System.out.println((String) someMap.get(def));
class MyObject {
int a;
===================================
If the above example, the first println will print "roses",
because I have put <abc, "roses"> in the HashMap.
However, if I look up "roses" by def, which is an identical
copy of abc (but, difference reference), the HashMap
someMap will not be able to look it up and find "roses".
This is not desired in my situation.
And I notice that if "MyObject" is replaced by an immutable
class such as String or Integer, the program works as
I inteded, that is the second println also prints "roses".
How could I make the lookup in the collection (HashMap,
LinkedList, .... ) work with recognizing "equals()" (or any
mechanism to tell that two different objects are identical
copy of each other) and lookup succeeds if the key is
identical (not the same reference)?
If I could work around this problem, by simply make
"MyObject" class be something similar to those immutable
class such as String or Integer, how do I do that?
for example, could I use some kind of keyword to specify
that "MyObject" is an immutable object and thus, lookup
succeeds if two different MyObject are identical copies of each other?
Thank you very much for your help!
My question ends here.
But, just to clarify things more,
let me give 2 more examples.
This example is similar to the 1st example,
but since I use Integer instead of "MyObject",
the 2nd println in the program also prints "roses".
==========================
class test {
public static void main(String args[]) {
HashMap<Integer, String> someMap =
new HashMap<Integer, String>();
Integer abc = new Integer(7);
someMap.put(abc, new String("roses"));
if (someMap.get(abc) != null) {
System.out.println((String) someMap.get(abc));
Integer def = new Integer(7);
if (someMap.get(def) != null) {
System.out.println((String) someMap.get(def));
===================================
My third example shows why I kind of need the functionality
that I want (look up by recognizing identical copy, not just
the reference).
==========================
class test {
public static void main(String args[]) {
HashMap<MyIdentity, String> phonebook =
new HashMap<MyIdentity, String>();
MyIdentity abc = new MyIdentity();
abc.name = "heloise";
abc.age = "16";
someMap.put(abc, new String("342-5259"));
if (someMap.get(abc) != null) {
System.out.println((String) someMap.get(abc));
MyIdentity def = new MyIdentity();
def.name = "heloise";
def.age = "16";
if (someMap.get(def) != null) {
System.out.println((String) someMap.get(def));
// for the sake of argument, let's just say that
// a person is uniquely defined by name and age,
// by not by name alone.
class MyIdentity {
String name;
int age;
===================================
Since I want to look up the phonebook, possibly in
different process or different context from where I
made input to the phone book,
I won't be able to keep the reference, but I should be
able to make the identical MyIdentity object and want
to look up.
But it fails in the example.

Answer 1

Thank you!
I actually tried implementing equals().
But, it seems I failed to implement the correct interface of equals()
previously.
I tried something like
public boolean equals(MyObject a) {
Now that i changed it to
public boolean equals(Object a) {
it just works!
Big thanks!