contains problem with own class

Programming Questions

clankill3r

contains problem with own class

in Programming Questions • 5 months ago

contains uses the equals method of a object.

PVector has it's own for example so this will print true:

void setup() {
ArrayList<PVector> pvl = new ArrayList<PVector>();
pvl.add(new PVector());
PVector v = new PVector();
println(pvl.contains(v));
}

Notice i didn't add v to the list.

I want this as well on my own class.

This is my attempt:

void setup() {
Test t1 = new Test();
Test t2 = new Test();
println("equal: "+t1.equals(t2));
ArrayList<Test> ta = new ArrayList<Test>();
ta.add(t1);
println("contains: "+ta.contains(t2));
}
class Test {
int a = 0;
Test() {
}
boolean equals(Test t) {
return (a == t.a);
}
int hashCode() {
int result = 1;
result = 31 * result;
return result;
}
}

The equals work, the contains does not.

I readed on stackoverflow that hascode has to be overwritten as well but it doesn't seem to help.

What is it i'm doing wrong?

Replies(6)

GoToLoop

Re: contains problem with own class

5 months ago

What about extending your custom class w/ PVector?!

Then you got x, y, & z and all PVector's methods, like constains()!

[EDIT] -> Oops. Testing here and paying more attention, contains() is a method from List interface!

Nothing to do w/ PVector class!

I still don't get how contains() works. Anyways, here's a simple test:

import java.util.List;

List<PVector> pvl = new ArrayList();

pvl.add( new PVector(1, 2) );

PVector v = new PVector(1, 2);
println( pvl.contains(v) );

v.set(1, 3, 0);
println( pvl.contains(v) );

v.set(1, 2, 0);
println( pvl.contains(v) );

exit();

PhiLho

Re: contains problem with own class

5 months ago

Your hashCode() function doesn't match the equals() function! Actually, it is a constant...
In your simple test, since you use a single field for equals(), you should use the same field for hashCode(), and its value can be the the hashCode of the variable, or, here, just the value of the variable itself:
int hashCode() { return a; }

[EDIT] I just noticed the signature of your equals() function is incorrect too, so you actually don't override the real method.
It should be:
boolean equals(Object t) {
    return a == ((Test) t).a;
}
Or, more exactly:
boolean equals(Object t) {
    if (!(t instanceof Test) return false;
    return a == ((Test) t).a;
}
because apparently, equals() can be called with something else than a Test (or can be called with a null).

clankill3r

Re: contains problem with own class

5 months ago

thx!

And it also work without overwriting the hashcode. So why is it needed?

quarks

Re: contains problem with own class

5 months ago

The ArrayList contains method does not use the hashCode method so you don't need to use it. Also there are very special requirements when implementing your own hashCode method so it is best avoided.

The problem is in your implementation of the equals method.

All class in Java including ones that you create yourself ultimately inherit from the Object class and that class has an equals method - source code shown here

public boolean equals(Object obj) {
return (this == obj);
}

Although you have an equals method it is not used by the ArrayList class because its parameter is of type Test and not of type Object so it ignores it.

Now the contains method of ArrayList will look for this method in your class and if it can't find it will use the one provided in the Object class and that method will return false if they are NOT the same object (even if their attributes are equal). So in your code t1 and t2 are different objects so Java says that the array list does not contaion t2 so returns false.

The solution is to get rid of the hashCode method and replace your method with the one provided by PhiLho i.e.

boolean equals(Object t) {
if (!(t instanceof Test) return false;
return a == ((Test)t).a;
}

clankill3r

Re: contains problem with own class

5 months ago

yeah allready did but thx :D

PhiLho

Re: contains problem with own class

5 months ago

Well, it is good practice to define both equals() and hashCode() at the same time.
Because here, you put the class in an ArrayList, but if you ever use it as key in a HashMap, or in a TreeMap, etc., hashCode() can be used.
Note also, since I know you use Eclipse, that it can generate these methods for you!

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