Comparing Java with == two strings is false?

Question

The string parts are the string [6]:

 ["231", "CA-California", "Sacramento-155328", "aleee", "Customer Service Clerk", "Alegra Keith.doc.txt"]

But when I compare parts[0] with "231" :

 "231" == parts[0] 

the result is incorrect

I'm confused, so can someone tell me why?

Answer 1

The == operator compares object references, not the String s value.

To compare String s values, use the String.equals method:

 "231".equals(parts[0]); 

This is true for any other object in Java - when comparing values, always use the equals method instead of using the == operator.

The equals method is part of Object and must be overridden by classes that will be compared one way or another.

Answer 2

If the strings are not interned, then == checks the reference identifier. Using:

  "231".equals(parts[0]); 

instead.

Answer 3

== in Java compares the address of objects (strings in this case).

You want parts[0].equals("231")

Answer 4

The following prints out "true",

 String s = "231"; if(s == "231") { System.out.println("true"); } else { System.out.println("false"); } 

This is because the lines are not changed, and java will try to save as much space as possible, therefore it points to the same memory link.

However, the following produces false:

 String s = new String("231"); if(s == "231") { System.out.println("true"); } else { System.out.println("false"); } 

new will force it to save the string in a new memory location.

By the way, you should ALWAYS use .equals() to compare strings (for cases like this)

Answer 5

Use the equals: parts [0] .equals ("231") method. == operator compares object references.

Answer 6

"==" compares object references, in your case "231" is a different object than the parts [0].

You want to use String.equals .

 parts[0].equals("231") 

Answer 7

The answer is very simple: when comparing strings using the == operator, you actually compare if two different variables refer to the same String object. And they don’t do it, the string in the array and the newly created “231” are different String objects with the same contents.

The correct thing is to use the following expression: "231".equals(parts[0]) or "231".equalsIgnoreCase(parts[0]) . This will give you what you need and return true if these String objects contain the same values.

Answer 8

I thought it would be useful to express the answer in a test example:

 public class String231Test extends TestCase { private String a; private String b; protected void setUp() throws Exception { a = "231"; StringBuffer sb = new StringBuffer(); sb.append("231"); b = sb.toString(); } public void testEquals() throws Exception { assertTrue(a.equals(b)); } public void testIdentity() throws Exception { assertFalse(a == b); } } 

Answer 9

You can also use the compareTo (String) method :

 String str = "test"; if( str.compareTo("test") == 0) //the argument string is equal to str; else //the argument string is not equal to str; 

Answer 10

Use the equals method to compare objects:

 String[] test = {"231", "CA-California", "Sacramento-155328", "aleee", "Customer Service Clerk", "Alegra Keith.doc.txt"}; System.out.println("231".equals(test[0])); 

Comparison '==' compares links, not values.

Answer 11

Here is a really good example. The '==' statement with a string can be really complicated in Java.

 class Foo { public static void main(String[] args) { String a = "hello"; String b = "hello"; String c = "h"; c = c + "ello"; String operator = null; if(a == b) { operator = " == "; } else { operator = " != "; } System.out.println(a + operator + b); if(a == c) { operator = " == "; } else { operator = " != "; } System.out.println(a + operator + c); if(a == "hello") { operator = " == "; } else { operator = " != "; } System.out.println(a + operator + "hello"); if(c == "hello") { operator = " == "; } else { operator = " != "; } System.out.println(c + operator + "hello"); } } 

Which will produce the following output:

 hello == hello hello != hello hello == hello hello != hello 

Answer 12

As many others have already explained, you are trying to compare with the equality operator, but then it will rely on Object.equals () instead of String.equals ().

So you can do the job by explicitly calling String.equals (), but instead of writing

 parts[0].equals("blahblah") 

I would prefer:

 "blahblah".equals(parts[0]) 

Since it avoids testing the potential invalidity of parts [0] (but be careful that the part variable itself can be null ...)

Another way is to use String.intern ():

 if (parts[0].intern() == "blahblah") ... 

See http://java.sun.com/j2se/1.4.2/docs/api/java/lang/String.html#intern () for more details.