New to Java

Please can someone explain why the output is so for the foll code

StringBuffer sb1 = new StringBuffer("Amit");
StringBuffer sb2= new StringBuffer("Amit");
String ss1 = "Amit";
System.out.println(sb1==sb2);
System.out.println(sb1.equals(sb2)); _//WHY DOES THIS LINE OUTPUT FALSE WHEN BOTH CONTENTS ARE THE SAME??_
System.out.println(sb1.equals(ss1));
System.out.println("Poddar".substring(3));

Ans:
a) false
false
false
dar