Comparing Object with null

[Asad Khan]

public boolean equals(Object o) {
if (!(o.equals(null))) { // *
if (o instanceof foo) {
if (this.equals(o)) {
return true;
}
}
}
return false;
}

I have the above method in a class foo. When I run the following code:

f.equals(f)

where f is an object of class foo, then I get a nullpointerexception in this
method. The reason is that since f is of type foo, in line * it recursively
calls the equals method and then o becomes null, and of course I can't
dereference null.

So how can I check if an object is null or not?

Thanks.

 

[Adam]

public boolean equals(Object o) {
if (!(o.equals(null))) { // *
if (o instanceof foo) {
if (this.equals(o)) {
return true;
}
}
}
return false;
}

I have the above method in a class foo. When I run the following code:

f.equals(f)

where f is an object of class foo, then I get a nullpointerexception in this
method. The reason is that since f is of type foo, in line * it recursively
calls the equals method and then o becomes null, and of course I can't
dereference null.

So how can I check if an object is null or not?

Thanks.

if (o != null) ....

or to check it is null:
o==null

Adam

 

[xarax]

public boolean equals(Object o) {
if (!(o.equals(null))) { // *
if (o instanceof foo) {
if (this.equals(o)) {
return true;
}
}
}
return false;
}

I have the above method in a class foo. When I run the following code:

f.equals(f)

where f is an object of class foo, then I get a nullpointerexception in this
method. The reason is that since f is of type foo, in line * it recursively
calls the equals method and then o becomes null, and of course I can't
dereference null.

So how can I check if an object is null or not?

Thanks.

// Your attempted implementation of equals(Object)
// is merely:
public boolean equals(Object obj)
{
return (this == obj);
}
// which is the default implementation in Object class.


If you have a special case of equals that compares
other stuff, then you probably want something like:

public boolean equalsFoo(Foo foo)
{
// perform special case testing here.
return (this == foo); // whatever else you want to test
}

public boolean equals(Object obj)
{
return (null != obj)
&& ((this == obj)
|| ((obj instanceof Foo)
&& equalsFoo((Foo) obj)));
}

HTH

 

[Roedy Green]

o.equals(null)

you might as well do that more directly with
o == null

 

[Roedy Green]

if (!(o.equals(null))) { // *
if (o instanceof foo) {
if (this.equals(o)) {
return true;

the first two lines are unnecessary. You can shorten that to

return this.equals( o );

But that is equivalent to having no method at all, just using the
default equals method.

Fresh Fish For Sale

Obviously, they are for sale, why put up a sign otherwise

Fresh Fish

Obviously they are fresh. Otherwise you could smell them.

Fish

Obviously they are fish. Any fool can see that.

 

[Gary Labowitz]

public boolean equals(Object o) {
if (!(o.equals(null))) { // *
if (o instanceof foo) {
if (this.equals(o)) {
return true;
}
}
}
return false;
}

I have the above method in a class foo. When I run the following code:

f.equals(f)

where f is an object of class foo, then I get a nullpointerexception in this
method. The reason is that since f is of type foo, in line * it recursively
calls the equals method and then o becomes null, and of course I can't
dereference null.

So how can I check if an object is null or not?

I think you don't mean to "check if an object is null" or else I don't
understand what you mean. If you want to see if the reference variable does
not have a reference to an object, that would be "check if the reference is
null."
And you do that with
f == null
This confusion comes about from saying things like: "where f is an object of
class foo." While that is commonly said, it really is "where f is a
reference to an object of class foo."
Say the wrong thing, but think the right thing.

 

[Guest]

If you have a special case of equals that compares other stuff, then you
probably want something like:

public boolean equalsFoo(Foo foo)
{
// perform special case testing here.
return (this == foo);
//whatever else you want to test
}

public boolean equals(Object obj)
{
return (null != obj)
&& ((this == obj)
|| ((obj instanceof Foo)
&& equalsFoo((Foo) obj)));
}

HTH

hashCode() and equals() work in tandem, so if you override one, so should
override the other. In particular, equals() should check any state used
in computing the hashCode().

-- La'ie Techie