scjp考题1（详尽答案）(2)

class MyParent {
int x, y;
MyParent(int x, int y){
this.x = x;
this.y = y;
}
public int addMe(int x, int y){
return this.x x y this.y;
}
public int addMe(MyParent myPar){
return addMe(myPar.x, myPar.y);
}
}

class MyChild extends MyParent{
int z;
MyChild (int x, int y, int z){
super(x,y);
this.z = z;
}
public int addMe(int x, int y, int z){
return this.x x this.y y this.z z;
}
public int addMe(MyChild myChi){
return addMe(myChi.x, myChi.y, myChi.z);
}
public int addMe(int x, int y){
return this.x x this.y y;
}
}
public class MySomeOne{
public static void main(String args[]){
MyChild myChi = new MyChild(10, 20, 30);
MyParent myPar = new MyParent(10, 20);
int x = myChi.addMe(10, 20, 30);
int y = myChi.addMe(myChi);
int z = myPar.addMe(myPar);
System.out.println(x y z);
}
}
A.300
B.240
C.120
D.180
E.compile error
F.none of the above
A is the correct choice. In the above code, MyChild class overrides the addMe(int x, int y) method of the MyParent class. And in both the MyChild and MyParent class, addMe() method is overloaded. There is no compilation error anywhere in the above code.
On execution, first, the object of MyChild class will be constructed. Please note that there is a super() call from the constructor of MyChild class, which will call the constructor of MyParent class. This will cause the value of z variable of MyChild class to be 30 and x, y variables of MyParent class will become 10 and 20 respectively. The next statement will again call the constructor of MyParent class with same x and y values. This is followed by execution of addMe() method of MyChild class with x as 10, y as 20 and z as 30. Also x and y are inherited by MyChild class from the MyParent class. Thus in the addMe() method of the MyChild class, the value of this.x will be 10, this.y will be 20 and this.z will be 30. The return value of this method will be "10 10 20 20 30 30", which is equal to 120. Thus x will become 120.
This is followed by the invocation of the other addMe() method which takes object reference of the MyChild class. From this method, the method which was called earlier is invoked. This call is exactly the same as the earlier one. Thus the value of y will also be 120 like x.
Now the addMe() method of MyParent class is invoked. This method invokes another addMe() method of the same class. Its equivalent to the invocation of addMe(int x, int y) method with x as 10 and y as 20. Also the value of instance variables x and y of My Parent class is 10 and 20 respectively. The value of z will be evaluated to "10 10 20 20", which is equal to 60. Thus the value of x, y and z after all the invocations will be 120, 120 and 60 respectively. As a result of this finally, "120 120 60" which is equal to 300 will be printed. Thus A is the correct choice.

5. The class AssertionError has "is -a" relationship with these classes (choose two)
A.RuntimeException
B.Error
C.VirtualMachineError
D.IllegalAccessException
E.Throwable
B and E are correct. The class AssertionError is an Error, which denotes an “incorrect condition” as opposed to an “unusual condition” (Exception). Since, the class Error descends from Throwable, AssertionError also has “is-a” relationship with Throwable. Here is the hierarchy –java.lang.Object
|
-java.lang.Throwable
|
-java.lang.Error Exception
| |
-java.lang.AssertionError RuntimeException IOException
Want to know more?
You can find more information about this as an answer to the question - “Why is AssertionError a subclass of Error rather than RuntimeException?” at - http://java.sun.com/J2SE/1.4/docs/guide/lang/assert.html #design-faq-error

6. What will be the result of executing the following code?
1. boolean a = true;
2. boolean b = false;
3. boolean c = true;
4. if (a == true)
5. if (b == true)
6. if (c == true) System.out.println("Some things are true in this world");
7. else System.out.println("Nothing is true in this world!");
8. else if (a && (b = c)) //这里是赋值，不是比较
System.out.println("It's too confusing to tell what is true and what is false");
9. else System.out.println("Hey this won't compile");
A.The code won’t compile.
B.“some things are true in this world” will be printed
C.“hey this won’t compile” will be printed
D.None of these
D is correct. This is a very good question to test the concepts of execution flow in case of if conditions. The rule for attaching else statements with if conditions is the same as attaching close brackets with open brackets. A close bracket attaches with the closest open bracket, which is not already closed. Similarly an else statement attaches with the closest if statement, which doesn't have an else statement already, attached to it. So the else statement at line 7 attaches to the if statement at line 6. The else statement at line 8 attaches to the if statement at line 5. The else statement at line 9 attaches to the if statement at line 8.

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