How can an immutable object, with non-final fields, be thread unsafe?

Question

say we have this

// This is trivially immutable.
public class Foo {
    private String bar;
    public Foo(String bar) {
        this.bar = bar;
    }
    pu         


        

Answer 1

From the link posted in comments:

class FinalFieldExample { 
    final int x;
    int y; 
    static FinalFieldExample f;

    public FinalFieldExample() {
        x = 3; 
        y = 4; 
    } 

    static void writer() {
        f = new FinalFieldExample();
    } 

    static void reader() {
        if (f != null) {
            int i = f.x;  // guaranteed to see 3  
            int j = f.y;  // could see 0
        } 
    } 
}

One thread may call writer() and and another thread may call reader(). The if condition in reader() could evaluate to true, but becuase y is not final the object initalizion may not have completely finished (so the object has not been safely published yet), and thus int j = 0 could happen as it has not been initialized.