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Java String.equals versus == [duplicate]

Posted by: admin November 2, 2017 Leave a comment

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Use the string.equals(Object other) function to compare strings, not the == operator.

The function checks the actual contents of the string, the == operator checks whether the references to the objects are equal. Note that string constants are usually “interned” such that two constants with the same value can actually be compared with ==, but it’s better not to rely on that.

if (usuario.equals(datos[0])) {
    ...
}

NB: the compare is done on ‘usuario’ because that’s guaranteed non-null in your code, although you should still check that you’ve actually got some tokens in the datos array otherwise you’ll get an array-out-of-bounds exception.

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Meet Jorman

Jorman is a successful businessman and has 2 houses.

enter image description here

But others don’t know that.

Is it the same Jorman?

When you ask neighbours from either Madison or Burke streets, this is the only thing they can say:

enter image description here

Using the residence alone, it’s tough to confirm that it’s the same Jorman. Since they’re 2 different addresses, it’s just natural to assume that those are 2 different persons.

That’s how the operator == behaves. So it will say that datos[0]==usuario is false, because it only compares the addresses.

An Investigator to the Rescue

What if we sent an investigator? We know that it’s the same Jorman, but we need to prove it. Our detective will look closely at all physical aspects. With thorough inquiry, the agent will be able to conclude whether it’s the same person or not. Let’s see it happen in Java terms.

Here’s the source code of String’s equals() method:

enter image description here

It compares the Strings character by character, in order to come to a conclusion that they are indeed equal.

That’s how the String equals method behaves. So datos[0].equals(usuario) will return true, because it performs a logical comparison.

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It’s good to notice that in some cases use of “==” operator can lead to the expected result, because the way how java handles strings – string literals are interned (see String.intern()) during compilation – so when you write for example "hello world" in two classes and compare those strings with “==” you could get result: true, which is expected according to specification; when you compare same strings (if they have same value) when the first one is string literal (ie. defined through "i am string literal") and second is constructed during runtime ie. with “new” keyword like new String("i am string literal"), the == (equality) operator returns false, because both of them are different instances of the String class.

Only right way is using .equals() -> datos[0].equals(usuario). == says only if two objects are the same instance of object (ie. have same memory address)

Update: 01.04.2013 I updated this post due comments below which are somehow right. Originally I declared that interning (String.intern) is side effect of JVM optimization. Although it certainly save memory resources (which was what i meant by “optimization”) it is mainly feature of language

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equals() function is a method of Object class which should be overridden by programmer. String class overrides it to check if two strings are equal i.e. in content and not reference.

== operator checks if the references of both the objects are the same.

Consider the programs

String abc = "Awesome" ;
String xyz =  abc;

if(abc == xyz)
     System.out.println("Refers to same string");

Here the abc and xyz, both refer to same String "Awesome". Hence the expression (abc == xyz) is true.

String abc = "Hello World";
String xyz = "Hello World";

if(abc == xyz)
    System.out.println("Refers to same string");
else
    System.out.println("Refers to different strings");

if(abc.equals(xyz))
     System.out.prinln("Contents of both strings are same");
else
     System.out.prinln("Contents of strings are different");

Here abc and xyz are two different strings with the same content "Hello World". Hence here the expression (abc == xyz) is false where as (abc.equals(xyz)) is true.

Hope you understood the difference between == and <Object>.equals()

Thanks.

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Instead of

datos[0] == usuario

use

datos[0].equals(usuario)

== compares the reference of the variable where .equals() compares the values which is what you want.

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== tests for reference equality.

.equals() tests for value equality.

Consequently, if you actually want to test whether two strings have the same value you should use .equals() (except in a few situations where you can guarantee that two strings with the same value will be represented by the same object eg: String interning).

== is for testing whether two strings are the same Object.

// These two have the same value
new String("test").equals("test") ==> true 

// ... but they are not the same object
new String("test") == "test" ==> false 

// ... neither are these
new String("test") == new String("test") ==> false 

// ... but these are because literals are interned by 
// the compiler and thus refer to the same object
"test" == "test" ==> true 

// concatenation of string literals happens at compile time resulting in same objects
"test" == "te" + "st"  ==> true

// but .substring() is invoked at runtime, generating distinct objects
"test" == "!test".substring(1) ==> false

It is important to note that == is much cheaper than equals() (a single pointer comparision instead of a loop), thus, in situations where it is applicable (i.e. you can guarantee that you are only dealing with interned strings) it can present an important performance improvement. However, these situations are rare.

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== operator check if the two references point to the same object or not.
.equals() check for the actual string content(value).

Note that .equals() method belongs to Class Object(Super class of all classes). You need to override it as per you class requirement but for String it is already implemented and it checks whether two string have same value or not.

Case1)
String s1 = "StackOverflow";
String s2 = "StackOverflow";
s1 == s1;      //true
s1.equals(s2); //true
Reason:  String literals created without null are stores in String pool in permgen area of heap. So both s1 and s2 point to same object in the pool.
Case2)
String s1 = new String("StackOverflow");
String s2 = new String("StackOverflow");
s1 == s2;      //false
s1.equals(s2); //true
Reason: If you create String object using new keyword separate space is allocated to it on heap.

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It will also work if you call intern() on the string before inserting it into the array.
Interned strings are reference-equal (==) if and only if they are value-equal (equals().)

public static void main (String... aArguments) throws IOException {

String usuario = "Jorman";
String password = "14988611";

String strDatos="Jorman 14988611";
StringTokenizer tokens=new StringTokenizer(strDatos, " ");
int nDatos=tokens.countTokens();
String[] datos=new String[nDatos];
int i=0;

while(tokens.hasMoreTokens()) {
    String str=tokens.nextToken();
    datos[i]= str.intern();            
    i++;
}

//System.out.println (usuario);

if(datos[0]==usuario) {  
     System.out.println ("WORKING");    
}

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Let’s analyze the following Java, to understand the identity and equality of Strings:

public static void testEquality(){
    String str1 = "Hello world.";
    String str2 = "Hello world.";

    if (str1 == str2)
        System.out.print("str1 == str2\n");
    else
        System.out.print("str1 != str2\n");

    if(str1.equals(str2))
        System.out.print("str1 equals to str2\n");
    else
        System.out.print("str1 doesn't equal to str2\n");

    String str3 = new String("Hello world.");
    String str4 = new String("Hello world.");

    if (str3 == str4)
        System.out.print("str3 == str4\n");
    else
        System.out.print("str3 != str4\n");

    if(str3.equals(str4))
        System.out.print("str3 equals to str4\n");
    else
        System.out.print("str3 doesn't equal to str4\n");
}

When the first line of code String str1 = "Hello world." executes, a string \Hello world."
is created, and the variable str1 refers to it. Another string "Hello world." will not be created again when the next line of code executes because of optimization. The variable str2 also refers to the existing ""Hello world.".

The operator == checks identity of two objects (whether two variables refer to same object). Since str1 and str2 refer to same string in memory, they are identical to each other. The method equals checks equality of two objects (whether two objects have same content). Of course, the content of str1 and str2 are same.

When code String str3 = new String("Hello world.") executes, a new instance of string with content "Hello world." is created, and it is referred to by the variable str3. And then another instance of string with content "Hello world." is created again, and referred to by
str4. Since str3 and str4 refer to two different instances, they are not identical, but their
content are same.

Therefore, the output contains four lines:

Str1 == str2

Str1 equals str2

Str3! = str4

Str3 equals str4

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You should use string equals to compare two strings for equality, not operator == which just compares the references.

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== operator compares the reference of an object in java. You can use string’s equals method .

String s = "Test";
if(s.equals("Test"))
{
    System.out.println("Equal");
}

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The == operator is a simple comparison of values.
For object references the (values) are the (references). So x == y returns true if x and y reference the same object.

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I know this is an old question but here’s how I look at it (I find very useful):


Technical explanations

In Java, all variables are either primitive types or references.

(If you need to know what a reference is: “Object variables” are just pointers to objects. So with Object something = ..., something is really an address in memory (a number).)

== compares the exact values. So it compares if the primitive values are the same, or if the references (addresses) are the same. That’s why == often doesn’t work on Strings; Strings are objects, and doing == on two string variables just compares if the address is same in memory, as others have pointed out. .equals() calls the comparison method of objects, which will compare the actual objects pointed by the references. In the case of Strings, it compares each character to see if they’re equal.


The interesting part:

So why does == sometimes return true for Strings? Note that Strings are immutable. In your code, if you do

String foo = "hi";
String bar = "hi";

Since strings are immutable (when you call .trim() or something, it produces a new string, not modifying the original object pointed to in memory), you don’t really need two different String("hi") objects. If the compiler is smart, the bytecode will read to only generate one String("hi") object. So if you do

if (foo == bar) ...

right after, they’re pointing to the same object, and will return true. But you rarely intend this. Instead, you’re asking for user input, which is creating new strings at different parts of memory, etc. etc.

Note: If you do something like baz = new String(bar) the compiler may still figure out they’re the same thing. But the main point is when the compiler sees literal strings, it can easily optimize same strings.

I don’t know how it works in runtime, but I assume the JVM doesn’t keep a list of “live strings” and check if a same string exists. (eg if you read a line of input twice, and the user enters the same input twice, it won’t check if the second input string is the same as the first, and point them to the same memory). It’d save a bit of heap memory, but it’s so negligible the overhead isn’t worth it. Again, the point is it’s easy for the compiler to optimize literal strings.

There you have it… a gritty explanation for == vs. .equals() and why it seems random.

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@Melkhiah66 You can use equals method instead of ‘==’ method to check the equality.
If you use intern() then it checks whether the object is in pool if present then returns
equal else unequal. equals method internally uses hashcode and gets you the required result.

public class Demo
{
  public static void main(String[] args)
  {
              String str1 = "Jorman 14988611";
    String str2 = new StringBuffer("Jorman").append(" 14988611").toString();
    String str3 = str2.intern();
    System.out.println("str1 == str2 " + (str1 == str2));           //gives false
    System.out.println("str1 == str3 " + (str1 == str3));           //gives true
    System.out.println("str1 equals str2 " + (str1.equals(str2)));  //gives true
    System.out.println("str1 equals str3 " + (str1.equals(str3)));  //gives true
  }
}

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If You are going to compare any assigned value of the string ie)primitive string, Both “==” and .equals will work, but for the new string object you should use only .equals, here “==” will not work

Example:

String a = "name";

String b = "name";

if(a == b) and (a.equals(b)) will return true.

but

String a = new String("a");

in this case if(a == b) will return false

so its better to use .equals operator….

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The .equals() will check if the two strings have the same value and return the boolean value where as the == operator checks to see if the two strings are the same object.

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Generally .equals is used for Object comparison, where you want to verify if two Objects have an identical value.

== for reference comparison (are the two Objects the same Object on the heap) & to check if the Object is null. It is also used to compare the values of primitive types.

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Use Split rather than tokenizer,it will surely provide u exact output
for E.g:

string name="Harry";
string salary="25000";
string namsal="Harry 25000";
string[] s=namsal.split(" ");
for(int i=0;i<s.length;i++)
{
System.out.println(s[i]);
}
if(s[0].equals("Harry"))
{
System.out.println("Task Complete");
}

After this I am sure you will get better results…..

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Someone said on a post higher up that == is used for int and for checking nulls.
It may also be used to check for Boolean operations and char types.

Be very careful though and double check that you are using a char and not a String.
for example

    String strType = "a";
    char charType = 'a';

for strings you would then check
This would be correct

    if(strType.equals("a")
        do something

but

    if(charType.equals('a')
        do something else

would be incorrect, you would need to do the following

    if(charType == 'a')
         do something else

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a==b

Compares references, not values. The use of == with object references is generally limited to the following:

  1. Comparing to see if a reference is null.

  2. Comparing two enum values. This works because there is only one object for each enum constant.

  3. You want to know if two references are to the same object

"a".equals("b")

Compares values for equality. Because this method is defined in the Object class, from which all other classes are derived, it’s automatically defined for every class. However, it doesn’t perform an intelligent comparison for most classes unless the class overrides it. It has been defined in a meaningful way for most Java core classes. If it’s not defined for a (user) class, it behaves the same as ==.