This question already has answers here:
Comparing strings with == which are declared final in Java
(6 answers)
How do I compare strings in Java?
(23 answers)
Closed 4 years ago.
Please follow the code below:
String s="helloworld";
String ss="hello";
String sss=ss+"world";
System.out.print(sss==s);
The output is false. Don't they get checked with the string pool rule for String? And what if we make them final?
A little explanation of internal working will help. Thanks in Advance.
String literals points to the same location if the content of them is same, that's what I got from different sources, am I right? If yes, then what's happening here? I'm a little confused about it.
EDIT:-
I think I didn't phrase it correctly. Let me rephrase it a little(Sorry for earlier attempt):-
String ss="hello";
System.out.print(ss+"world"=="helloworld");
This returns false. However these are String literals and as I have read they don't create two different objects for same value. They are just reference to a same value. Here, "helloworld" is the value for both sides of ==. I hope that I'm able to communicate it well.
Because String is an object, it is comparing that the two objects are the same with ==, which will equate to false.
Using the object ss to concat into sss will not make s = sss.
If you set ss to s, then using == will equate to true since they are now the same object.
If you set a second String object with a string literal, using == will equate the true.
If you use the String object's function .equals(String), you will find that it equates to true.
If you compare two string literals, i.e. "helloworld" == "helloworld" or "helloworld" == "hello" + "world", these will also equate to true.
As lealceldeiro pointed out, strings should always be compared with .equals().
EDIT
A good thing to look at is this answer. It has good references and explanation.
Other resources:
JournalDev
Baeldung
Related
This question already has answers here:
Comparing strings with == which are declared final in Java
(6 answers)
Closed 7 years ago.
I was doing some java tests to practice and I came across a question that I don't understand. I created a small program to test it:
The question was to say what would be the output of System.out.println(ab==abc);
I answered 'true' thinking that String literals are not objects, so the can be seen as a kind of primitive type so the comparation == would compare the values and nothing to do with references. But actually the answer in "false";
Then I did this test and I even print the outputs and as you can see ab and abc are exactly the same, however the comparation is returning false , but if I do the comparation directly without doing any concatenation (as I did at the end of the program) the comparation is returning true. So it seems clear the reason has to be with the concatenation, I know that Strings are inmutable so when concatenating then we are getting another String literal with exactly same value.
Can someone please explain me who't going on here?
For those telling me that String literals are objects, why then this code returns true?
String p="meowdeal";
String o="meowdeal";
System.out.println(o == p);
//output true
Of course I would understand that this code
String o=new String("meowdeal");
String p=new String("meowdeal");
System.out.println(o==p);
returns false because in that case they are really objects but not when they are String literal, am I right?
Thank you for your time
public static void main(String ads[] ){
String a="meow";
String ab=a+"deal";
String abc="meowdeal";
System.out.println(a);
System.out.println(ab);
System.out.println(abc);
System.out.println(ab == abc);
//output
//meow
//meowdeal
//meowdeal
//false
String p="meowdeal";
String o="meowdeal";
System.out.println(o == p);
//output
//true
}
ab and abc are objects so .equals() is used to see if they have the same contents and == is used to see if they are the same object.
The last test is only true due to a compile optimization known as string interning
(Your second comment below is correct)
In Java you should compare Strings with .equals(String s) because the '==' returns only true if it's the exact same String in the memory and doesn't compare if the content is the same. In your first example you've got two different strings in your memory. In the second, java saw, that these would be the same and allocated only space for one string and your p and o are only a reference to the same object.
String literals are Objects. User equals method instead of == operator.
Just to explain what happen here,
String p="meowdeal"; //creates "meowdeal" in heap
String o="meowdeal"; // o also refer to same object in heap.
This is called pooling of String. Since both refer to same memory location == operator returns true.
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 8 years ago.
my case:
I try to get a value from my MySQL database. This data is defined as a VARCHAR.
This is done in java with a prepared statement and resultset.
String add1 =rs.getString("include");
according to this website table 5.1 states "Can always be converted to these Java types".
therefore a VARCHAR should be able to be converted in a java.lang.string.
When i check the data type in java it indeed gives java.lang.string. So for so good.
My issue is when i try to compare this obtained "String" value to a in java defined String value they don't seem to be equal.
Let's give a simple example.
I get the (VARCHAR) value "1" from the database and store in in String add1
I want to check if this value is indeed "1" and therefore I use the following statement
if(add1=="1")
{
/* do something */
}
Only the values are , according to java, not the same. So I was curious about why this is.
I started searching why those two "Strings" are not equal.
First of I checked the length of both strings. Both came out to be 1 in length as expected.
Afterwards I checked the value of both. To see if both values where indeed the same i wanted to check their hexadecimal representance.
Both came out with the value 0x31. Still as expected.
But then why isn't my if statement accepted by java if both seem to represent the same string-hex-bin-you name it value.
In short: Why is my VARCHAR "String" obtained from a MySQL databse in java not the same as a String defined in java.
Compare values, not memory references:
if(add1.equals("1"))
{
/* do something */
}
Please see this post best answer.
You must always compare strings with the .equals method in your case it will be
add1.equals("1")
"==" or equality operator in Java is a binary operator It is good for compairing primitives like boolean, int, float "==" works fine but when it comes to compare objects it creates confusion with equals method in Java. "==" compare two objects based on memory reference. so "==" operator will return true only if two object reference it is comparing represent exactly same object otherwise "==" will return false.
String comparison is a common scenario of using both == and equals method. Since java.lang.String class override equals method, It return true if two String object contains same content but == will only return true if two references are pointing to same object.
Read more: http://javarevisited.blogspot.com/2012/12/difference-between-equals-method-and-equality-operator-java.html#ixzz3PdLbOre0
You need to compare the Strings with .equals() method. because == checks for object reference.
What is the difference between == vs equals() in Java?
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 8 years ago.
I understand and know why you typically have to use == to compare strings in java, but for some reason I am able to do it in Eclipse. My code is
Code:
public class Test{
public static void main(String [] args){
String str1 = "string";
if(str1 == "string"){
System.out.println("wtf");
}
}
}
Why does this print "wtf" yet using javac from command line does not?
Eclipse allows you to compare references because it is a legitimate comparison. Just probably not the one you really want.
Because of String interning it will sometimes appear to work, but you should not rely upon it unless you know the strings you're comparing have been interned. The correct way to compare Strings for equal value is to use .equals.
It allow cause it probably faster to compare two address than comparing 2 string (but use it with caution, you probably never have to compare two String address).
It is sometime usefull to compare Object memories address, and as long as String is an object, eclipse allow you to compare using ==
This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 9 years ago.
Can you explain me why do I have "false" output? If I understand correctly, references point to the same object!
public class mainC {
String str1,str2;
public static void main(String [] args){
mainC m=new mainC();
m.str1="a";
m.str2="b";
System.out.print("m.str1 == m.str2: "+m.str1 == m.str2);
}
}
Thank you.
m.str1 and m.str2 point to different String objects, which is why you get false. The == compares str1 and str2, not m.
Side note: Now, if you had:
m.str1="a";
m.str2="a"; // Same series of characters, e.g., "a"
...you'd be getting true, but it would be misleading. == compares object references. So you can have two different String objects that have the same characters in them, but they would not be == to each other (in fact, that's quite common). To compare strings, you use equals. The reason my example above returns true is that both strings are initialized pointing to literals, and String literals in Java are intern'd by default, so that literals with the same characters are mapped to the same object.
A string in Java is implemented as a reference type and not a value type. Since this is the case, their pointers in memory aren't equal. To get around this, you can use their equals function to compare them.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How do I compare strings in Java?
Java String.equals versus ==
I am new to Selenium and Java.
I have tried the below to compare the field value of last name to the one which i supplied.
String lastname=selenium.getValue("//*[#id='lastName']");
System.out.println(lastname);
assertTrue (lastname == "xxx");
It is keep on failing.
Just literally tried to change the last line with help of Eclipse (just trial and error)
assertTrue("lastname.equals("xxx"));
It is working fine... Why it is failed in first case? == is not allowed to compare strings?
Short answer: == checks for same object .equals checks for the same value.
More info in How do I compare strings in Java?
equals function checks the actual contents of the lastname. == operator checks whether the references to the objects are equal.
You must use equals() to compare strings with Java, as you guessed.
You're actually comparing pointers (or really reference equality) when you use == with strings. In other words you're testing if two objects are the same, rather than the content of the objects.
For comparing equality of string ,We Use equals() Method. There are two ways of comparison in java. One is "==" operator and another "equals()" method . "==" compares the reference value of string object whereas equals() method is present in the java.lang.Object class. This method compares content of the string object.