public class TestString 1 { 
public static void main（String[] args） { 
String str = “420”； 
str += 42； 
System.out.print（str）； 
} 
} 
What is the output？（）

1. public class Boxer1 { 
2. Integer i； 
3. int x； 
4. public Boxer1（int y） { 
5. x=i+y； 
6. System.out.println（x）； 
7. } 
8. public static void main（String[] args） { 
9. new Boxer1（new Integer（4））； 
10. } 
11. } 
What is the result？（） 

10. public class ClassA { 
11. public void count（int i） { 
12. count（++i）； 
13. } 
14. } 
And： 
20. ClassA a = new ClassA（）； 
21. a.count（3）； 
Which exception or error should be thrown by the virtual machine？（） 

public class Foo { 
static int[] a； 
static { a[0]=2； } 
public static void main（ String[] args） {} 
} 
Which exception or error will be thrown when a programmer attempts to run this code？（）