Abstract Class Java

Question

I need to write abstract class, which looks like this.

public abstract class Value {

public abstract String toString();
public abstract Value add(Value v);
public abstract Value sub(Value v);
public abstract boolean eq(Value v);
public abstract boolean lte(Value v);
public abstract boolean gte(Value v);
public abstract boolean neq(Value v);
public abstract boolean equals(Object other);
public abstract int hashCode();
public abstract Value create(String s);

}

Now I need to make few classe, which inherit from that one. I started from Int class and implemented it like this:

public class Int extends Value {

int val;

public String toString() {
    String toStr = Integer.toString(val);
    return toStr;
}

public Int add(Value v) {
    Int result = new Int();
   if(v instanceof Int) {
        Int temp = (Int) v;
        result.val = val + temp.val;
    }

    return result;
}

public Int sub(Value v) {
    Int result = new Int();
    if(v instanceof Int) {
        Int temp = (Int) v;
        result.val = val - temp.val;
    }
    return result;
}

public boolean eq(Value o) {
    if(this == o) return true;
    if(this == null) return false;
    if(getClass() != o.getClass()) return false;
    Int other = (Int) o;
    return toString() == other.toString();
}

public boolean lte(Value v) {
    if(v instanceof Int) {
        Int temp = (Int) v;
        return this.val < temp.val;
    }
    return false;
}

public boolean gte(Value v) {
    if(v instanceof Int) { 
        Int temp = (Int) v;
        return this.val > temp.val;
    }
    return false;
}

public boolean neq(Value v) {
    if(v instanceof Int) {
        Int temp = (Int) v;
        return !eq(temp);
    }
    return true;
}

public boolean equals(Object o) {
    if(this == o) return true;
    if(this == null) return false;
    if(getClass() != o.getClass()) return false;
    Int other = (Int) o;
    return toString() == other.toString();
}

public int hashCode() {
    Integer hash = val;
    return hash.hashCode();
}

public Int create(String s) {
    val = Integer.parseInt(s);
    return this;
}

}

Everything is compiling and working, but I have no clue if my hashcode() function and equals() are good. Furthermore i want to use create() to make objects like this:

getInstance().create("1234");

Is my method also sufficient?

So you allow to add `Value` but you do nothing it it is not `Int` - kind of yyy wrong. Make it generic so int would only allow ints. — Antoniossss, Oct 16 '18 at 19:54
`equals()` should compare state (not toString()): `return val == other.val` — Andrew S, Oct 16 '18 at 19:56
`Value` is abstract but has not state and does not provide any implementation for any method - why not make it an interface? — Andrew S, Oct 16 '18 at 19:59
I am not asking what is being returned, I am asking about the variable? field? or whatever named `result` is. I can not find any `result` being declared in above code — user85421, Oct 16 '18 at 19:59
`if(this == null)` (in `equals()`), this is a pointless comparison. Maybe you meant to check if `o` is null. — Joakim Danielson, Oct 16 '18 at 20:04
and better use just `return Integer.hashCode(val);` instead of creating an `Integer` just to get its hash code. The `create` method is a bit *strange* - being an instance method. Should be `static` and as such no need to e overriden. — user85421, Oct 16 '18 at 20:07
In my opinion the create method should be static, and I don't understand where `getInstance` came from honestly. — Bashar Ali Labadi, Oct 16 '18 at 20:11

Mureinik · Answer 1 · 2018-10-17T02:23:45.220

The hashCode() method is fine (although I'd add an @Override annotation, just to make the code easier to maintain and avoid mistakes), but the equals(Object) definitely isn't.

Following the logic you have in place, == isn't the right way to compare strings. You should use equals instead (see, e.g., How do I compare strings in Java?). In addition, as Joakim Danielson noted in the comments, this can never be null - you should check if o is null instead:

public boolean equals(Object o) {
    if (this == o) {
        return true;
    }
    if (o == null) {
        return false;
    }
    if(getClass() != o.getClass()) {
        return false;
    }
    Int other = (Int) o;
    return toString().equals(other.toString()); // Here!
}

But in all fairness, there's no reason to use toString - you could just compare the internal val:

public boolean equals(Object o) {
    if (this == o) {
        return true;
    }
    if (o == null) {
        return false;
    }
    if(getClass() != o.getClass()) {
        return false;
    }
    Int other = (Int) o;
    return val ==  other.val; // Here!
}

@JoakimDanielson nice catch, thanks for noticing. Copy-and-bugged from the OP :-) I've edited my answer to address this. — Mureinik, Oct 17 '18 at 02:24

syntagma · Answer 2 · 2018-10-16T20:12:52.153

Everything is compiling and working, but I have no clue if my hashcode() function and equals() are good.

Your equals() should compare int val and not result of toString() of compared objects (this.val == other.val).

Your hashCode() looks good, though I would add @Override to it (same with equals()).

Furthermore i want to use create() to make objects like this: getInstance().create("1234");

Looking at its implementation, it looks fine (i.e. would work according to your needs):

public Int create(String s) {
    val = Integer.parseInt(s);
    return this;
}

though I don't think you really want to use it with getInstance(). Simply Int.create() would be enough:

public static Int create(String s) {
    val = Integer.parseInt(s);
    return new Int(val);
}

Note that you would need a private constructor.

Also, as someone noted in the comments, consider using generics instead of inheritance.

score 0 · Answer 3 · answered Oct 16 '18 at 19:57

0

First when you override Methods please do it with @Override Annotation. Then i would implement your equals method in another way. Just do return this.val == other.val instead of doing this.toString() == other.toString(). Your toString() method implementation is ok. Your hashCode is good as well. But please remove that create method. Use a constructor instead.

answered Oct 16 '18 at 19:57

onCC

71
2
10

Thanks I will add annotations. I need to make that create function, but dont really know how. Its part of my homework. Later I have to create ArrayList and create specific value type – Grzegorz Kunc Oct 16 '18 at 20:03
@GrzegorzKunc And what is the method supposed to do? – onCC Oct 16 '18 at 20:05

score 0 · Answer 4 · answered Oct 16 '18 at 20:19

0

Can I implement equals() method using eq() like this ?

public boolean equals(Object o) {
    Value compare = (Value) o;
    return eq(compare);
}

answered Oct 16 '18 at 20:19

Grzegorz Kunc

1
2

Abstract Class Java

4 Answers4