JAVA系列 - equal的String不==

結果很機車耶...
String s1 = "Hello world!";
String s2 = "Hello world!";
System.out.println("s1: " + s1);
System.out.println("s2: " + s2);
System.out.println("s1==s2: " + (s1 == s2));
s1 = "Hello world!Hello world!".substring("Hello world!".length());
System.out.println("s1: " + s1);
System.out.println("s2: " + s2);
System.out.println("s1==s2: " + (s1 == s2));

大概是因為substring出來的String是放在正常的heap
而不是Pool