Can any one explain why the output of this code is '0' (zero)?

Question

public class Program
{
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

== compare references and not valued, use. equals() instead