Substring is not equal to a string of the same value

Question

I am writing a code that checks if String codon, a substring of String dna, is equal to "ATG". The code is as follows:

public class Test{
  public void test() {
      String dna = "ATGCTGTAG";
      String codon = dna.substring(0,3);
      System.out.println(codon); // should print ATG
      System.out.println(codon == "ATG"); // should print true
  }

  public static void main(String[] args) {
    Test t = new Test();
    t.test();
  }
}

However, when I run the quote, line 6 would print out false.

Another weird thing, when I changed a parameter of line 4: String codon = dna.substring(0,9); and line 6: System.out.println(codon == "ATGCTGTAG"); line 6 would print out true.

I need to know if I need to change anything in my code to fix this bug.