Why doesn’t == work on String?

Question

I just started Java programming. I love it so far, but I have been stuck on this problem for a while.

When I run this code, whenever I type in “boy” it will just respond with GIRL:

import java.util.Scanner;

public class ifstatementgirlorboy {
    public static void main(String args[]) {
        System.out.println("Are you a boy or a girl?");
        Scanner input = new Scanner(System.in);
        String gender = input.nextLine();

        if(gender=="boy") { 
            System.out.println("BOY");
        } 
        else { 
            System.out.println("GIRL"); 
        }
    }
}

Why?

use equals instead of == for string in java. Check other questions on this site for more precisions about this. — benzonico, Jul 03 '13 at 08:31
This has been asked a lot of time in SO. Please take a while to explore before posting http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java — sanbhat, Jul 03 '13 at 08:34
whenever someone compares Java `String`s with `==` a Developer cries... — Marco Forberg, Jul 03 '13 at 09:27
Oh god... you know you haven’t done Java in a while when you’re suddenly get flashbacks to pointer exam questions the second == is brought up — jscul, Oct 24 '18 at 02:14
"Why doesn't == work on String`? It _does_ work! It just **doesn't do what you normally want** when you're asking if one string is "equal to" another. — Kevin Anderson, Oct 24 '18 at 02:27
None of these answers explains the deeper "why can't you compare strings using ==?" As in the foundation of the decision in the language specification. For someone who has no experience of C, it's entirely counterintuitive. — jarmod, Nov 22 '20 at 17:16
This is the answer you were looking for: https://stackoverflow.com/a/14123474/912829 — ACV, Jan 13 '21 at 22:36

score 30 · Answer 1 · edited Oct 24 '18 at 02:12

Use the String.equals(String otherString) function to compare strings, not the == operator.

This is because the == operator only compares object references, while the String.equals() method compares both String's values i.e. the sequence of characters that make up each String.

equals() method from Source code of String:

        public boolean equals(Object anObject) {
1013        if (this == anObject) {
1014            return true;
1015        }
1016        if (anObject instanceof String) {
1017            String anotherString = (String)anObject;
1018            int n = count;
1019            if (n == anotherString.count) {
1020                char v1[] = value;
1021                char v2[] = anotherString.value;
1022                int i = offset;
1023                int j = anotherString.offset;
1024                while (n-- != 0) {
1025                    if (v1[i++] != v2[j++])
1026                        return false;
1027                }
1028                return true;
1029            }
1030        }
1031        return false;
1032    }

So you should write

if(gender.equals("boy")){

}

or to comapre with regardless of case

if(gender.equalsIgnoreCase("boy")){

}

and for null safety

if("boy".equals(gender)){

}

Future reference:

String s1 = "Hello";              // String literal
String s2 = "Hello";              // String literal
String s3 = s1;                   // same reference
String s4 = new String("Hello");  // String object
String s5 = new String("Hello");  // String object

Here s1 == s2 == s3 but s4 != s5

Where as

anyOfAbove.equals(anyOtherOfAbove); //true

Or, even better, `"boy".equals(gender)` because this works even if `gender == null`. — , Jul 03 '13 at 08:32
@Tichodroma Sometimes if something is `null`, you should take care of it.. So you actually want to see the NPE in order to prevent this.. However, if you **can** set some String to `null` in purpose.. then this attitude is very good. — Maroun, Jul 03 '13 at 09:04
@MarounMaroun: If null is expected and to be treated differently then checking for `null` would be better to catch the NPE — Marco Forberg, Jul 03 '13 at 09:29

score 7 · Answer 2 · edited Dec 01 '14 at 21:21

7

When comparing objects of type String, you should use the equals method instead of operator ==. equals will compare the values of String objects while == checks to see if they are the same object in memory.

So instead of:

if(gender=="boy")

use

if(gender.equals("boy"))

edited Dec 01 '14 at 21:21

Jimmy Pelton

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answered Jul 03 '13 at 08:32

Kevin Bowersox

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score 1 · Answer 3 · answered Jul 03 '13 at 08:32

1

Use equals instead of ==

if("boy".equals(gender)){

}

use equals To compare the value. while == is compare object reference.

answered Jul 03 '13 at 08:32

bNd

7,512
7
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score 1 · Answer 4 · edited Jul 22 '13 at 11:57

1

String a,b;

a==b;

here references(addresses) of both string objects are compared

a.equals(b);

here contents of both strings are compared

edited Jul 22 '13 at 11:57

RAS

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answered Jul 03 '13 at 08:35

claw

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wassup · Answer 5 · 2013-07-03T09:16:02.103

In Java, strings are objects (String). Variables which contain objects are references. If you compare two objects with == operator, true is returned only if they are the same objects (in memory). But in your code they aren't ("boys" is an instantialized String on the fly).

There is, however, a method String.equals(), which compares two strings and returns true if they have the same characters in the same order, not if they are the same objects.

Correct code here:

import java.util.Scanner;

public class ifstatementgirlorboy {
    public static void main(String args[]) {
        System.out.println("Are you a boy or a girl?");
        Scanner input = new Scanner(System.in);
        String gender = input.nextLine();

        if (gender.equals("boy")) {
            System.out.println("BOY");
        } 

        else {
            System.out.println("GIRL");
        }
    }
}

You can also swap the two strings (the advantage is that it prevents NullPointerException from being thrown if gender == null):

"boy".equals(gender)

To ignore the case of letters in comparison, use equalsIgnoreCase() instead.

score 0 · Answer 6 · answered Jul 03 '13 at 08:32

0

you cant compare strings like that in java, You need to use

if (gender.equals("boy"))

for it to work. Your way is comparing the object rather than the content

answered Jul 03 '13 at 08:32

exussum

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Thanks for these quick answers. First time here on stackoverflow. – user2545642 Jul 03 '13 at 08:34

score 0 · Answer 7 · edited May 23 '17 at 12:18

0

You're using reference equality there. The == is literally comparing the 2 references. i.e. are they the same object.

You need to use the equals() method, which (in this case) will compare the contents of the two strings. See here for more info.

edited May 23 '17 at 12:18

Community

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answered Jul 03 '13 at 08:32

Brian Agnew

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correct, but should his code not work, too? strings are internalized, so when reading "boy" the ref should both point to the same "boy" string object? – AlexWien Jul 03 '13 at 08:36

Why doesn’t == work on String?

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