Java - Any way to make variable unmodifiable after being set in the constructor?

Question

I have a member variable that in a class that I want to make sure nothing changes it after it is set in the constructor. So, it would look something like this:

public class MyClass
{
    private String myVar = null;

    public MyClass(DataObj dataObj)
    {
        myVar = dataObj.getFinalVarValue();
        //at this point I don't want anything to be able change the value of myVar
    }

    ....
}

My first thought was to see I could just add the final modifier to the variable, but the compiler complains about assigning a value to the final field. Is it possible to accomplish this?

Answer 1

The compiler complains (in the constructor), because you already provide an initialization by writing

private String myVar = null;

remove the ' = null ' part. Add final and assign the value in the constructor.

public class MyClass
{
    private final String myVar;

    public MyClass(DataObj dataObj)
    {
        myVar = dataObj.getFinalVarValue(); // the only assignment/init happens here.
    }

    ....
}

Answer 2

Make it private , and then don't include a setter. As long as you never change it in within the class methods, it's immutable.

Answer 3

String objects are already immutable. So adding the private final modificator is enough.