I'm surprised I haven't seen anybody mention the "public final fields" approach yet. I use this myself for immutable objects and I quite like it. Granted, you still need to implement equals and hashcode, but these can be generated easily and verified with tools like EqualsVerifier [2].

[1]

  public class MyImmutable {
    public final String field1;
    public final int field2;

    public MyImmutable(String field1, int field2) {
        // assign
    }

     // equals, hashcode, toString
  }

[2] https://github.com/jqno/equalsverifier