为什么如果比较在java中不起作用

Question

I am making a Hash Table in java. In searching function, I am doing some comparison in IF statement. but it is not doing any comparison.

here's is some part of my code.

---

while (table[pos]!=null) {
        if (table[pos]==key) {
            System.out.println("SEARCH "+key+" at INDEX "+home);
            return;
        }
        else {pos=h(home+p(i));
        i++;
        }
    }
    System.out.println("Failed to find "+key+".");
    return;
}

---

It doesn't work even when table[pos] and key are the same! but I add very simple assigning variable to another one. It work! I don't know why it works. I wanna know it xD

   while (table[pos]!=null) {
        int x = table[pos];
        if (x==key) {
            System.out.println("SEARCH "+key+" at INDEX "+home);
            return;
        }
        else {pos=h(home+p(i));
        i++;
        }
    }
    System.out.println("Failed to find "+key+".");
    return;
}

Answer 1

Well, if table[pos] and key are both Integer (and table[pos] must be a reference type, since you are comparing it to null in the while statement), they should be compared with equals , not with == , since two different Integer objects may have the same int value.

When you assign table[pos] to the int variable x , it is un-boxed to a primitive value.

Now, when you compare the int x to the Integer key , the key is also un-boxed to an int , and int comparison works with == .

This can be demonstrated by the following short example:

Integer i1 = 300;
Integer i2 = 300;
System.out.println (i1 == i2);
int i3 = i1;
System.out.println (i3 == i2);

which outputs:

false
true

The code code would be:

while (table[pos] != null) {
    if (table[pos].equals(key)) {
        System.out.println("SEARCH "+key+" at INDEX "+home);
        return;
    } else {
        pos = h(home + p(i));
        i++;
    }
}
System.out.println("Failed to find "+key+".");

Answer 2

When comparing two objects with == , you check if both of these references point to the same place in the memory, while using == with primitives simply checks if values are the same. To correctly check equation of values inside two Integers you should use equals() method.

In your second example you used unboxing from Integer to int so it checked values as you expected it to do. In the first one you compared if both values point to the same place in memory.

Answer 3

Actually the correct way is to use both just like HashMap does for example, this way we will always be sure. This is for example how HashMap does it internally:

if(((k = first.key) == key || (key != null && key.equals(k)))) ...

So in your case it would be:

if ((table[pos] == key) || (key != null && (table[pos].equals(key)))) {