I asked the interviewee to write me an example of an immutable class. Here was his answer.

ImmutableExample.java

public class ImmutableExample {
    private int i;
    
    public ImmutableExample ( int i ) {
        this.i = i;
    }
    
    public int get() {
        return this.i;
    }
    
    public int set() {
        
    }
}