Friday, November 14, 2014

String immutable in Java: What does it really mean?

This is one of the favourite questions at the fresher level that interviewer asks. Most of the candidates tend to gloss over this. If interviewer tends to go into this deep, most of the candidates tend to falter. At least that if what I have observed having interviewed hundreds of candidates in the past.

We will try and make this article single point of reference for Strings in Java. Hopefully we cover something that might be helpful to you. Read here, for knowing Why It was made Immutable.

Let's try and go in depth on String immutability.

Let's start by Source Code of I always say that source code is the best resource to learn a language.

How String is Immutable?
This question is self answered by the above code snippet. Member variable data type is final. Hence String Object once created becomes immutable in Java.

How come then we are able to assign same variable, a different value?
Now we are talking. Valid question. Let's check it via code,

So the question stands. What does immutability means?

Well, it can be explained by what happens internally. Variable immutable refers to a memory space which is allocated to "javaonjava. This memory allocation is what we call immutable. Same variable can point to different memory spaces.

String literals are stored separately in a String Pool. Once a String is Created, say "javaonjava". Whenever a new variable needs "javaonjava" String, a new String is not created. The old one is referenced. This is what we mean by immutability. So String Object is not immutable, String itself is immutable.

Compare it with any User defined object. When that object is updated, content is re-written on the same memory space. Hope things are getting clear now.

How to check String is immutable in Java

1. == (equals operator), is the best check.
As we read above StringPool contains single copy of a given String. So if this operator returns true on two string variables, it means they are pointing to same memory and content inside the StringPool

Thus we see, they both point to the same content.

2. No operations modify String

Thus we see neither concat, nor substring modifies the String. They simply create a new String, which is returned as we see below.

In the last example, what happens to "javaonjava" String. Well it remains in the StringPool but unreferenced. If not reused for considerable time, it could be Garbage Collected.

See the diagram below for better understanding

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