Simple question but having some confusion

Question

4.00/5 (1 vote)

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Java

String str1 ="abc";
 
String str2 ="abc";
 
 
if(str1 == str2)
System.out.println("Equal 1");

why we don't prefer this..?

i googled it find == compares only the identity of the objects.but it print right answer why?

and equal method compare the content .

Posted 23-Oct-12 3:26am

ravi1989h

Updated 23-Oct-12 3:42am

Nelek

v2

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2 solutions

Solution 1

Java

== 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).

Java

== 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.

Posted 23-Oct-12 3:37am

psychic6000

Comments

ravi1989h 23-Oct-12 9:45am

can you please explain more about reference equality.?

ravi1989h 23-Oct-12 9:45am

i understand equal method

ravi1989h 23-Oct-12 9:46am

interning..? mean?

ravi1989h 23-Oct-12 9:49am

// ... but these are because literals are interned by
// the compiler and thus refer to the same object

what is the meaning?
if this true what about this
"test" == "est" ==> false // why it is false

ravi1989h 23-Oct-12 9:51am

interned strings meaning..>?

psychic6000 23-Oct-12 10:10am

string Interning refers to store only one copy of string
here is link for more details.
http://en.wikipedia.org/wiki/String_interning

if it solves your confusion, then select the solution as answer

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Nagy Vilmos · Accepted Answer · 2012-10-23T03:53:00

Further to the previous answer.

Taking your example:

Java

String one = "abcd";
String two = "abcd";

System.out.println(one == two);

This will print out true as both reference a single constant. However changing it thus:

Java

String one = new String("abcd");
String two = new String("abcd");

System.out.println(one == two);

will result in an output of false. The two objects contain the same values [content is equal] but are different objects [occupy different memory addresses].

It is an important habit to get into in java, using .equals(Object) rather then ==.