java - String is immutable. What exactly is the meaning?

I wrote the following code on immutable Strings.

public class ImmutableStrings {

    public static void main(String[] args) {
        testmethod();
    }

    private static void testmethod() {
        String a = "a";

        System.out.println("a 1-->" + a);
        a = "ty";
        System.out.println("a 2-->" + a);
    }
}

Output:

a 1-->a  

a 2-->ty

Here the value of variable a has been changed (while many say that contents of the immutable objects cannot be changed). But what exactly does one mean by saying String is immutable? Could you please clarify this topic for me?

source : https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

Answer

Before proceeding further with the fuss of immutability, let's just take a look into the String class and its functionality a little before coming to any conclusion.

This is how String works:

String str = "knowledge";

This, as usual, creates a string containing "knowledge" and assigns it a reference str. Simple enough? Lets perform some more functions:

 String s = str;     // assigns a new reference to the same string "knowledge"

Lets see how the below statement works:

  str = str.concat(" base");

This appends a string " base" to str. But wait, how is this possible, since String objects are immutable? Well to your surprise, it is.

When the above statement is executed, the VM takes the value of String str, i.e. "knowledge" and appends " base", giving us the value "knowledge base". Now, since Strings are immutable, the VM can't assign this value to str, so it creates a new String object, gives it a value "knowledge base", and gives it a reference str.

An important point to note here is that, while the String object is immutable, its reference variable is not. So that's why, in the above example, the reference was made to refer to a newly formed String object.

At this point in the example above, we have two String objects: the first one we created with value "knowledge", pointed to by s, and the second one "knowledge base", pointed to by str. But, technically, we have three String objects, the third one being the literal "base" in the concat statement.

What if we didn't have another reference s to "knowledge"? We would have lost that String. However, it still would have existed, but would be considered lost due to having no references.
Look at one more example below

String s1 = "java";
s1.concat(" rules");
System.out.println("s1 refers to "+s1);  // Yes, s1 still refers to "java"

What's happening:

1. The first line is pretty straightforward: create a new String "java" and refer s1 to it.


2. Next, the VM creates another new String "java rules", but nothing
   refers to it. So, the second String is instantly lost. We can't reach
   it.

The reference variable s1 still refers to the original String "java".

Almost every method, applied to a String object in order to modify it, creates new String object. So, where do these String objects go? Well, these exist in memory, and one of the key goals of any programming language is to make efficient use of memory.

As applications grow, it's very common for String literals to occupy large area of memory, which can even cause redundancy. So, in order to make Java more efficient, the JVM sets aside a special area of memory called the "String constant pool".

When the compiler sees a String literal, it looks for the String in the pool. If a match is found, the reference to the new literal is directed to the existing String and no new String object is created. The existing String simply has one more reference. Here comes the point of making String objects immutable:

In the String constant pool, a String object is likely to have one or many references. If several references point to same String without even knowing it, it would be bad if one of the references modified that String value. That's why String objects are immutable.

Well, now you could say, what if someone overrides the functionality of String class? That's the reason that the String class is marked final so that nobody can override the behavior of its methods.