为什么当相等值时Long equals返回false？

Question

Running the following code I expect true as result, but the output I get is false .

Long value = new Long(0);
System.out.println(value.equals(0));

Why does the equals comparison of Long return false ?

Answer 1

Long.equals return true only if the argument is also a Long .

The javadoc says:

Compares this object to the specified object. The result is true if and only if the argument is not null and is a Long object that contains the same long value as this object .

In fact the following code gets true as output.

Long value = new Long(0);
System.out.println(value.equals(new Long(0)));
System.out.println(value.equals((long) 0));
System.out.println(value.equals(0L);

Answer 2

looking inside in the implemented compare method you will find the critical criteria:

if (obj instanceof Long) 



public boolean equals(Object obj) {
    if (obj instanceof Long) {
        return value == ((Long)obj).longValue();
    }
    return false;
}

so passing any other numeric type will return false, even if the hold the same value...

Integer i = 0;

and

Long l = 0L;

are not the same in that context.

Answer 3

You compared an Long with an int!
the .equals method also is checking the type of the Variable.
Here is a code to campare an int with an long:

int i = 0;
long l = 0L;

//v1
System.out.println(i == l);
//v2
Long li = new Long(i);
Long ll = new Long(l);
System.out.println(li.eqauls(ll));
//v3
System.out.println(((long)i) == l);