Explain me

Question

class MyClass {
public static void main(String[ ] args) {
String a="a";
String b="b";
String c=a+b;
String d="ab";
if(c==d){
System.out.println(1);
}
else
System.out.println(0);
}
}

Answer 1

thnx

Answer 2

then why the answer is 0 I'm asking this