Which is a valid identifier？（）  

public class Mycircle {  public double radius；  public ...

单项选择题

public class Mycircle { 
public double radius； 
public double diameter； 
public void setRadius（double radius） 
this.radius = radius； 
this.diameter= radius * 2； 
} 
public double getRadius（）   { 
return radius； 
} 
Which statement is true？（）  

class BaseClass{  private float x= 1.0f；  protected voi...

多项选择题

class BaseClass{ 
private float x= 1.0f； 
protected void setVar （float f） {x = f；} 
} 
class SubClass extends BaseClass   { 
private float x = 2.0f； 
//insert code here 
}  
Which two are valid examples of method overriding？（）     
  

1. class A implements Runnable (  2. int i;  3. public ...

单项选择题

1. class A implements Runnable ( 
2. int i; 
3. public void run () ( 
4. try ( 
5. thread.sleep(5000); 
6. i= 10; 
7. ) catch(InterruptedException e) {} 
8. ) 
9. ) 
10.   
11. public class Test { 
12. public static void  main (string args[]) (
13. try ( 
14. A a = new A (); 
15. Thread t = new Thread (a); 
16. t.start(); 
17.   
18. int j= a.i; 
19.   
20. ) catch (Exception e) {} 
21. ) 
22. }  
Which statement al line 17 will ensure that j=10 at line 19？（）