0Snippet of an Immutable Class:
package com.stringhr;
/*Declare the class as 'final'. This would prevent any other class from extending it
and hence from overriding any method from it which could modify instance variable values.*/
final class ImmutableClass {
// private variables, thus invisible beyond current class...
private final int salary;
private final String name;
// private constructor, so that child class object can't be instantiated...
private ImmutableClass(int salary, String name) {
this.salary = salary;
this.name = name;
}
// Only Getters, no Setter method so that no field can be modified
public int getSalary() {
return salary;
}
public String getName() {
return name;
}
}
Why is String immutable in Java?
The string is immutable for several reasons, here is a summary:
- Security: Parameters are typically represented as String in network connections, database connection URLs, usernames/passwords etc. If it were mutable, these parameters could be easily changed.
- String pool: If String weren’t immutable, then we never had a String pool great feature in String class. Let say:
String a=“Paras”;
String b=“Paras”
We all know that a and b points to the same Object in a heap if a changed to ”Chawla”, b will still point to “Paras” which is not possible if String weren’t immutable.
String a="Paras";
String b=a;
System.out.println(a==b);
a="Chawla";
System.out.println(a +b);
Output:
true
- Synchronization and Concurrency: Making String immutable automatically makes them thread safe thereby solving the synchronization issues. Since String is immutable it can safely share between many threads which are very important for multithreaded programming and to avoid any synchronization issues in Java, Immutability also makes String instance thread-safe in Java, means you don't need to synchronize String operation externally.
- Caching: When compiler optimizes your String objects, it seems that if two objects have the same value (a="test", and b=" test") and thus you need only one string object (for both a and b, these two will point to the same object). A string is immutable, no one can change its contents once created which guarantees hashCode of String to be same on multiple invocations.
- Class loading: String is used as arguments for class loading. If mutable, it could result in the wrong class being loaded (because mutable objects change their state). Had String been mutable, a request to load "java.io.Writer" could have been changed to load "mil.vogoon.DiskErasingWriter"
If I have a class with all static members is it immutable?
If your class has only static members, then objects of this class are immutable, because you cannot change the state of that object (you probably cannot create it either).
