Checking if strings are equal in java using ==

when i execute the code below, the output is "false"

String string1 = new String("ABC");
String string2= new String("ABC");
System.out.println(string1==string2);

However the output when I don't use the string class's constructor is "true"

String string1;
String string2;
string1="ABC";
string2= "ABC";
System.out.println(string1==string2);

I get that its better to use the .equals() methods but why the difference in output?