AP计算机教程3-6：简单多选题

Given the following code segment, what is in the string referenced by s1?

String s1 = "xy";
String s2 = s1;
s1 = s1 + s2 + "z";

What is the value of len after the following executes?

String s1 = "Hey, buddy!";
int len = s1.length();

What is the value of pos after the following code executes?

String s1 = "ac ded ca";
int pos = s1.indexOf("d");

What is the value of s1 after the following code executes?

String s1 = "Hey";
String s2 = s1.substring(0,1);
String s3 = s2.toLowerCase();