How to Create a Once-Settable Variable in Java Without Using Final

How can I create a variable in Java that can only be set once and is not declared as final?

public class Example {
    private long id = 0;

    public long getId() {
        return id;
    }

    public void setId(long id) throws Exception {
        if (this.id == 0) {
            this.id = id;
        } else {
            throw new Exception("Can't change id once set");
        }
    }
}

In Java, you can create a variable that can only be set once by implementing a method that restricts changing its value after the initial assignment. Your current approach using a setter is a valid solution. However, there are alternative patterns that may be more elegant and maintainable, such as using a custom setter with a boolean flag or leveraging the Optional class.

public class Example {
    private Long id;
    private boolean isIdSet = false;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        if (!isIdSet) {
            this.id = id;
            this.isIdSet = true;
        } else {
            throw new IllegalStateException("ID can only be set once.");
        }
    }
}

Causes

• Trying to use a final variable while ensuring it can be set outside of the constructor.
• Creating instances without initializing certain properties.

Solutions

• Use a private boolean flag to track if the variable has been set.
• Consider using an Optional type for more flexible handling of uninitialized variables.
• Implement a Builder pattern for creating complex objects without requiring all properties to be set upon construction.

Mistake: Not checking if the ID is already set before assigning a new value.

Solution: Always check a flag or the current state before modifying the value.

Mistake: Using a default value for the ID variable (like 0) that may be valid as input.

Solution: Use a nullable type (e.g., Long) to differentiate between 'not set' and 'set to zero'.