Using the equals() method with String and Object in Java
equals
for Object
compares memory references.
That is why it is false since they are different Object
sequals
for String
is overridden to compare based on characters.
You have 2 empty String
objects that is why equals
returns true
.
Because equals() for String compares the content, not the object itself.
public boolean equals(Object anObject)
Compares this string to the specified object. The result is true if and only if the argument is not null and is a
String
object that represents the same sequence of characters as this object.
/* String.equals() */
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = count;
if (n == anotherString.count) {
char v1[] = value;
char v2[] = anotherString.value;
int i = offset;
int j = anotherString.offset;
while (n-- != 0) {
if (v1[i++] != v2[j++])
return false;
}
return true;
}
}
return false;
}
(Link to the source of String.equals())
Versus the equals for Object:
The equals method for class
Object
implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference valuesx
andy
, this method returns true if and only ifx
andy
refer to the same object (x == y
has the valuetrue
).
/* Object.equals() */
public boolean equals(Object obj) {
return (this == obj);
}
(Link to the source of Object.equals())
Also, don't forget the contract of the equals()
function:
The equals method implements an equivalence relation on non-null object references:
- It is reflexive: for any non-null reference value
x
,x.equals(x)
should return true.- It is symmetric: for any non-null reference values
x
andy
,x.equals(y)
should return true if and only ify.equals(x)
returns true.- It is transitive: for any non-null reference values
x
,y
, andz
, ifx.equals(y)
returnstrue
andy.equals(z)
returnstrue
, thenx.equals(z)
should returntrue
.- It is consistent: for any non-null reference values
x
andy
, multiple invocations ofx.equals(y)
consistently returntrue
or consistently returnfalse
, provided no information used in equals comparisons on the objects is modified.- For any non-null reference value
x
,x.equals(null)
should returnfalse
.
Also recommended reading:
- Object.hashCode()
- Effective Java (Bloch)