java string concatenation and interning

Question 1

String a1 = "I Love" + " Java";
String a2 = "I Love " + "Java";
System.out.println( a1 == a2 ); // true

String b1 = "I Love";
b1 += " Java";
String b2 = "I Love ";
b2 += "Java";
System.out.println( b1 == b2 ); // false

In the first case, I understand that it is a concatenation of two string literals, so the result "I Love Java" will be interned, giving the result true. However, I'm not sure about the second case.

Question 2

String a1 = "I Love" + " Java"; // line 1
String a2 = "I Love " + "Java"; // line 2

String b1 = "I Love";
b1 += " Java";
String b2 = "I Love ";
b2 += "Java";
String b3 = b1.intern();
System.out.println( b1 == b3 ); // false

The above returns false, but if I comment out lines 1 and 2, it returns true. Why is that?