Strategy for Defining Immutable Objects
An object is considered immutable if its state cannot change after it is constructed.
Maximum reliance on immutable objects is widely accepted as a sound strategy for creating simple, reliable code.
Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state
The following rules define a simple strategy for creating immutable objects
Final Class: The class is declared as final to prevent subclassing.
Private and Final Fields: All fields of the class are declared private and final to ensure they are assigned only once.
No Setter Methods: There are no methods that modify the fields of the class after the object is created.
All Fields Initialized in Constructor: The class has a constructor that initializes all fields.
Defensive Copies for Mutable Fields: If the class has fields that refer to mutable objects, the constructor and getter methods make defensive copies of these objects to prevent external modification.
public List<String> getMutableList() {
return new ArrayList<>(this.mutableList);
}
Here’s a simple example of a class whose Object will be Immutable:
public final class ImmutableRGB {
// Values must be between 0 and 255.
final private int red;
final private int green;
final private int blue;
final private String name;
private void check(int red,
int green,
int blue) {
if (red < 0 || red > 255
|| green < 0 || green > 255
|| blue < 0 || blue > 255) {
throw new IllegalArgumentException();
}
}
public ImmutableRGB(int red,
int green,
int blue,
String name) {
check(red, green, blue);
this.red = red;
this.green = green;
this.blue = blue;
this.name = name;
}
public int getRGB() {
return ((red << 16) | (green << 8) | blue);
}
public String getName() {
return name;
}
public ImmutableRGB invert() {
return new ImmutableRGB(255 - red,
255 - green,
255 - blue,
"Inverse of " + name);
}
}
In this example class ImmutableRGB, All the fields are private and final, and they are initialized only once through the constructor. There are no methods that allow modification of these fields after the object is created.
#Importance of immutable Objects
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Thread Safety: Immutable objects are inherently thread-safe. Since their state cannot be changed after creation, there are no synchronization issues, making them ideal for concurrent programming.
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Simplicity and Predictability: Immutable objects are easier to understand and work with. Since their state doesn’t change, you don’t need to worry about the object’s state being altered unexpectedly, leading to fewer bugs and more predictable behavior.
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Security: Immutable objects are more secure. Since their state can’t be modified, they can safely be shared without risk of unintended changes.
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Hashing: Immutable objects are excellent candidates for use as keys in hash-based collections like HashMap and HashSet. Their hash codes are consistent and won’t change over time, ensuring reliable behavior in these collections.
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Caching: Since immutable objects can’t change, they can be safely cached and reused. This can lead to performance improvements by reducing the need for repeated object creation.
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Fail-Fast Principle: When you use immutable objects, failures due to unexpected state changes are minimized. This aligns well with the fail-fast principle in programming, where errors are caught early in the development cycle.
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Easy to Test: Testing immutable objects is straightforward because you don’t need to account for different states of the object. This simplifies unit testing and ensures more robust test coverage.
In summary, immutable objects provide a robust foundation for building reliable, maintainable, and secure code. They simplify concurrent programming, reduce bugs, and improve the overall quality of your software.
Author: Mohammad J Iqbal
