java.util.String

What is the o/p of the code blocks below and why is the difference in o/p's?

  String helloWorld = "Hello, world!";
  if(helloWorld=="Hello, world!"){
   System.out.println("== is true");
  }else{
   System.out.println("== is false");
  }
  
  if(helloWorld.equals("Hello, world!")){
   System.out.println(".equals is true");
  }else{
   System.out.println(".equals is false");
  }

  String helloWorld = new String("Hello, world!");
  if(helloWorld=="Hello, world!"){
   System.out.println("== is true");
  }else{
   System.out.println("== is false");
  }
  
  if(helloWorld.equals("Hello, world!")){
   System.out.println(".equals is true");
  }else{
   System.out.println(".equals is false");
  }