Java: String reference getting unexpected output

Question

My code is :

String str = "Hello";
String str1;
str1 = str;
str1 = str1 + " World";
System.out.println(str1);
System.out.println(str);

The output I get is :

Hello World
Hello

The output I was expecting is Hello World for both the cases because according to my understanding after str1 = str both objects are referencing to same location so if I change the content of one object other should also get affected.

So, is str1 = str1 + " World"; creating a new string object at different memory loction.?

betomontejo · Accepted Answer · 2012-09-28 07:55:35Z

7

Strings are immutable. When you do str1 = str1 + " World";, you're creating a totally new String and updating str1's reference to it. You're never re-asigning str so this IS the expected behaviour.

answered Sep 28, 2012 at 7:55

betomontejo

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Comments

Rudolf Mühlbauer · Accepted Answer · 2012-09-28 08:01:29Z

3

The point is that the + operator creates a new String object, effectively concatenating the operands. It does not modify the object str.

You might want to read http://javarevisited.blogspot.co.at/2010/10/why-string-is-immutable-in-java.html, which explains the concept of immutable strings quite nicely.

The class StringBuffer might also be worth noting, which is optimized for long, cascaded concatenations.

edited Sep 28, 2012 at 8:01

answered Sep 28, 2012 at 7:56

Rudolf Mühlbauer

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2 Comments

RanRag Over a year ago

Interesting blog post. Thanks for it.

Rudolf Mühlbauer Over a year ago

It was really the first google hit for java string immutable ;)

Subhrajyoti Majumder · Accepted Answer · 2012-09-28 07:55:12Z

2

In Java String is immutable.

str1 = str1 + " World"; It will create a new instance and assign to str1.

answered Sep 28, 2012 at 7:55

Subhrajyoti Majumder

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Comments

turtledove · Accepted Answer · 2012-09-28 07:57:30Z

2

java.lang.String is immutable

str1 = str1 + " World";

This code will make str1 reference to a new created String object "Hello World", and str still reference to the "Hello" object.

answered Sep 28, 2012 at 7:57

turtledove

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Jochem Gruter · Accepted Answer · 2012-09-28 07:58:21Z

2

Reference to primitive datetypes creates always a new Object

answered Sep 28, 2012 at 7:58

Jochem Gruter

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NoNaMe · Accepted Answer · 2012-09-28 07:57:41Z

1

No, this is the right out put as in the code you had init the string

String str = "Hello";
 String str1;

Then you say

str1 = str;

it means str1 contains "Hello"

str1 = str1 + " World";

The above line will print "Hello World" because str1 having "Hello" and " World" is given there as a new string

and when you print str it is containing the word "Hello"

System.out.println(str);

answered Sep 28, 2012 at 7:57

NoNaMe

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Comments

titogeo · Accepted Answer · 2012-09-28 08:36:06Z

What is so special about String in here, It's the common behavior for all. It's about the assignment operator (in line number 4 you assigning it again). Your code is similar to below.

Person p1 = new Person("Stack");
Person p2;
p2 = p1;
p1 = new Person("OverFlow");

What would this Print? Would both print "Overflow"?

From your words.

The output I was expecting is Hello World for both the cases because according to my understanding after str1 = str both objects are referencing to same location so if I change the content of one object other should also get affected.

Line 3: str1 = str;

As per Line number 3 you are correct.

Line 4: str1 = str1 + " World";

But in line number 4 you are not changing the content, you are reassigning str1 to a different object.

Here str, str1 and p1 are Object references. Not real objects. You would get same content if you are changing the object. Like

p1 = p1.setName("Overflow");

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