Immutable String in Java

A String is an unavoidable type of variable while writing any application program. String references are used to store various attributes like username, password, etc. In Java, String objects are immutable. Immutable simply means unmodifiable or unchangeable.

Once a String object is created, its data or state can't be changed, but a new String object is created.

Example: Understanding Immutability

Testimmutablestring.java

class Testimmutablestring {  
 public static void main(String args[]) {  
   String s = "Sachin";  
   s.concat(" Tendulkar"); // concat() method appends the string at the end  
   System.out.println(s); // will print Sachin because strings are immutable objects  
 }  
}

Output:

Sachin

Now it can be understood by the diagram given below. Here Sachin is not changed, but a new object is created with Sachin Tendulkar. That is why String is known as immutable.

Assigning New Reference

As seen in the example above, two objects are created, but the s reference variable still refers to "Sachin", not "Sachin Tendulkar".

However, if we explicitly assign it to the reference variable, it will refer to "Sachin Tendulkar".

Testimmutablestring1.java

class Testimmutablestring1 {  
 public static void main(String args[]) {  
   String s = "Sachin";  
   s = s.concat(" Tendulkar");  
   System.out.println(s);  
 }  
}

Output:

Sachin Tendulkar

In this case, s points to "Sachin Tendulkar". Please note that the Sachin object is still not modified.

Why String Objects Are Immutable in Java?

Java uses the concept of String literal. Suppose there are 5 reference variables, all referring to one object "Sachin". If one reference variable changes the value of the object, it will affect all reference variables. That is why String objects are immutable in Java.

Features Making String Objects Immutable:

1. ClassLoader:

A ClassLoader in Java uses a String object as an argument. If the String object were modifiable, its value might change, causing the class that is supposed to be loaded to be different. To avoid this misinterpretation, String is immutable.

2. Thread Safety:

As String objects are immutable, we don't have to handle synchronization when sharing an object across multiple threads.

3. Security:

Immutable String objects prevent unauthorized modifications, making applications more secure. For example, in banking software, the username and password cannot be modified by an intruder because String objects are immutable.

4. Efficient Heap Space Usage:

The immutability of String minimizes memory usage in the heap. When declaring a new String object, the JVM checks whether the value already exists in the String pool. If it does, the same value is assigned to the new object, optimizing heap space utilization.

Why Is String Class Final in Java?

The String class is declared final to prevent overriding its methods. This ensures that new String objects retain the same behavior and features as existing ones.