Example when two strings compared by == and equals() will give different results?

Question

When is a == b is true but a.equals(b) is false, or vice versa?

I know that equals() is used to compare the values of the Strings and the == to check if two variables point at the same instance of a String object. But in which case will these two be different ?

"when a==b is true but a.equals(b) is false" no, properly implemented `equals` method always returns true for `x.equals(x)`, "or vice versa" read two same strings from file and compare them. — Pshemo, Jan 05 '16 at 18:49

Mandeep Rajpal · Accepted Answer · 2016-01-05T18:58:30.167

7

Consider this code,

String str1 = "ABC";
String str2 = new String("ABC");
System.out.println(str1==str2); // false
System.out.println(str1.equals(str2)); //true

The reason for this is that when you write String str1="ABC" this string is kept in string pool and String str2 = new String("ABC") creates the new object in heap and it won't check the string pool if it is already present. But as contents of the both string are same, equals method returns true.

edited Jan 05 '16 at 18:58

answered Jan 05 '16 at 18:50

Mandeep Rajpal

1,667
2
11
16

1

You might add that if a==b, then a.equals(b) should always return true since they are actually the same object. – JustinKSU Jan 05 '16 at 18:51
1

Maybe you should also explain the why of the results, IMO. – nbro Jan 05 '16 at 18:52
@nbro Edited and tried to explain. – Mandeep Rajpal Jan 05 '16 at 19:03
@MandeepRajpal, thats not the reason, please correct it. If you create both strings in the heap then also it will give the same result as above. – MagicBeans Jan 05 '16 at 19:06

score 2 · Answer 2 · answered Jan 05 '16 at 18:52

While most compilers will intern strings so that

a = "Foo";
b = "Foo";
System.out.println(a == b);

will generally print True, you can always construct strings that you know to be identical but the compiler does not:

a = "Foo";
b = "Fo";
c = "o";
System.out.println((a == b + c));

This will just about always print False, while a.equals(b + c) will always be True. Basically, there is no guarantee that strings with the same content will point to the same object.

score 0 · Answer 3 · answered Jan 05 '16 at 18:55

Strings are immutable in nature. So, when we do this,

String a1 = "a";
String a2 = "a";

Actually in memory, only one instance of String "a" is created. Both a1 and a2 will point to the same instance.

That's why, a1 == a2 and a1.equals(a2) are both true.

But, in this,

String a1 = "a";
String a2 = new String("a");

We created a new String instance explicitly. So, a1.equals(a2) will be true. But a1 == a2 will not.

Example when two strings compared by == and equals() will give different results?

3 Answers3