Can you explain what the term 'immutable object' means and what its key features are?

An immutable object is an object whose state cannot be changed after creation.

Key features:

• All fields of the object are declared as final and are initialized only in the constructor.
• There are no methods that modify the internal state of the object (no setters).
• If the object contains references to mutable objects, they are copied upon creation (deep copying) or copies are returned to prevent modification of the internal state.

Advantages:

• Thread safety without additional synchronization.
• Simplicity of understanding and debugging.

Example of an immutable class in Java:

public final class Person {
    private final String name;
    private final int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

After creating a Person object, its name and age cannot be changed.