Java substring.equals versus ==

Question

Using the standard loop, I compared a substring to a string value like this:

if (str.substring(i, i+3) == "dog" ) dogcount++;

and it broke the iteration. After the first increment, no further instances of "dog" would be detected.

So I used substring.equals, and that worked:

if (str.substring(i, i+3).equals("dog")) dogcount++;

My question is, why? Just seeking better understand, thx.

this has been asked/answered so many times http://stackoverflow.com/questions/767372/java-string-equals-versus — Nikita Ignatov, May 25 '12 at 18:16
disagree with downvoters. @Cshift3iLike this is such a basic question that it's not surprising that one would have hard time finding the answer. — MK., May 25 '12 at 18:19
It seems some of the members are having fun with the people who are answering. Why all the unnecessary downvotes?! It's stupid! — Kazekage Gaara, May 25 '12 at 18:26

Alex Lockwood · Accepted Answer · 2012-05-25T18:33:21.277

10

You should use equals() instead of == to compare two strings.

s1 == s2 returns true if both strings point to the same object in memory. This is a common beginners mistake and is usually not what you want.

s1.equals(s2) returns true if both strings are physically equal (i.e. they contain the same characters).

edited May 25 '12 at 18:33

answered May 25 '12 at 18:17

Alex Lockwood

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Thx much. But why does using == break the iteration? Even if the count variable isn't incremented, I'd expect the iteration to continue. – Jacob Stevens May 25 '12 at 21:49
You never mentioned what the "loop" was in your question, so I can't be sure. The loop could be `while(true) { }` for all I know. – Alex Lockwood May 25 '12 at 21:54
It's from http://codingbat.com/prob/p111624: public boolean catDog(String str) { int dogcount = 0; int catcount = 0; for (int i=0; i – Jacob Stevens May 25 '12 at 22:09

score 8 · Answer 2 · edited May 27 '12 at 23:38

8

== compares references. You want to compare values using equals() instead.

edited May 27 '12 at 23:38

Alex Lockwood

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answered May 25 '12 at 18:16

Oleksi

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score 4 · Answer 3 · edited May 28 '12 at 16:07

4

== compares references(storage location of strings) of strings

and

.equals() compares value of strings

edited May 28 '12 at 16:07

Alex Lockwood

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answered May 25 '12 at 18:17

Samir Mangroliya

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@downvoter care to say why downvoted ? –is it wrong kind of answer? – Samir Mangroliya May 25 '12 at 18:23
Some idiots are down-voting all of the answers. Maybe they feel you are encouraging poor questions like this one by giving an answer? – erickson May 25 '12 at 18:27
Exactly! This should be reported I feel. – Kazekage Gaara May 25 '12 at 18:28

Kazekage Gaara · Answer 4 · 2012-05-25T18:23:18.680

4

For String comparison, always use equals() method. Because comparing on == compares the references.

Here's the link for further knowledge.

You might also want to read this for understanding difference between == and equals() method in a better way along with code examples.

edited May 25 '12 at 18:23

answered May 25 '12 at 18:17

Kazekage Gaara

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score 3 · Answer 5 · answered May 25 '12 at 18:24

"String" is an object in Java, so "==" compares the references, as stated. However, code like

String str1 = "dog";
String str2 = "dog";
if(str1==str2)
    System.out.println("Equal!");

will actually print out "Equal!", which might get you confused. The reason is that JVM optimizes your code a little bit when you assign literals directly to String objects, so that str1 and str2 actually reference the same object, which is stored in the internal pool inside JVM. On the other hand, code

String str1 = new String("dog");
String str2 = new String("dog");
if(str1==str2)
    System.out.println("Equal!");

will print out nothing, because you explicitly stated that you want two new objects.
If you want to avoid complications and unexpected errors, simply never use "==" with strings.

Java substring.equals versus ==

5 Answers5