以上弄懂，下面这些就能搞明白啦

String s1 = String s1 = "a";
String s2 = "b"; 
String s3 = "a" + "b"; 
String s4 = s1 + s2; 
String s5 = "ab"; 
String s6 = s4.intern();
//  System.out.println(s3 == s4); //false
System.out.println(s3 == s5);   //true
System.out.println(s3 == s6);
String x2 = new String("c") + new String("d"); 
String x1 = "cd"; 
x2.intern();
// 如果调换了【最后两行代码】的位置呢，如果是jdk1.6呢 
System.out.println(x1 == x2);