ETS Test 2 回顾

Enthuware Test Studio Test 2 错题回顾，题目编号为测试系统的编号。

2.Given:

  
package loops;
public class JustLooping {
    private int j;
    void showJ(){
        while(j<=5){
            for(int j=1; j <= 5;){
                System.out.print(j+" ");
                j++;
            }
            j++;
        }
    }
    public static void main(String[] args) {
        new JustLooping().showJ();
    }
}

What is the result?
You had to select 1 option

✨Explanation 2

The point to note here is that the j in for loop is different from the instance member j. Therefore, j++ occuring in the for loop doesn’t affect the while loop. The for loop prints 1 2 3 4 5.
The while loop runs for the values 0 to 5 i.e. 6 iterations. Thus, 1 2 3 4 5 is printed 6 times. Note that after the end of the while loop the value of j is 6.

6.Given:

  
//in file Movable.java
package p1;
public interface Movable {
  int location = 0;
  void move(int by);
  public void moveBack(int by);
}

//in file Donkey.java
package p2;
import p1.Movable;
public class Donkey implements Movable{
    int location = 200;
    public void move(int by) {
        location = location+by;
    }
    public void moveBack(int by) {
        location = location-by;
    }
}

//in file TestClass.java
package px;
import p1.Movable;
import p2.Donkey;
public class TestClass {
    public static void main(String[] args) {
        Movable m = new Donkey();
        m.move(10);
        m.moveBack(20);
        System.out.println(m.location);
    }
}

Identify the correct statement(s).
You had to select 1 option

Donkey.java will not compile.
TestClass.java will not compile.
Movable.java will not compile.
It will print 190 when TestClass is run.
It will print 0 when TestClass is run.

✨Explanation 6

There is no problem with the code. All variables in an interface are implicitly public, static, and final. All methods in an interface are public.
There is no need to define them so explicitly. Therefore, the location variable in Movable is public and static and the move() method is public.
Now, when you call m.move(10) and m.moveBack(20), the instance member location of Donkey is updated to 190 because the reference m refers to a Donkey at run time and so move and moveBack methods of Donkey are invoked at runtime. However, when you print m.location, it is the Movable’s location (which is never updated) that is printed.

8.What will be the result of trying to compile and execute the following program?

  
public class TestClass{
   public static void main(String args[] ){
      int i = 0 ;
      int[] iA = {10, 20} ;
      iA[i] = i = 30 ;
      System.out.println(""+ iA[ 0 ] + " " + iA[ 1 ] + "  "+i) ;
    }
}

You had to select 1 options

It will throw ArrayIndexOutOfBoundsException at Runtime
Compile time Error.
It will print 10 20 30
It will print 30 20 30

✨Explanation 8

The statement iA[i] = i = 30; will be processed as follows:
iA[i] = i = 30; 👉 iA[0] = i = 30; 👉 i = 30; iA[0] = i ; 👉 iA[0] = 30;

Here is what JLS says on this:
1 Evaluate Left-Hand Operand First
2 Evaluate Operands before Operation
3 Evaluation Respects Parentheses and Precedence
4 Argument Lists are Evaluated Left-to-Right

For Arrays: First, the dimension expressions are evaluated, left-to-right. If any of the expression evaluations completes abruptly, the expressions to the right of it are not evaluated.

11.Consider the following class :

  
public class Test{
   public static void main(String[] args){
      if (args[0].equals("open"))
         if (args[1].equals("someone"))
            System.out.println("Hello!");
      else System.out.println("Go away "+ args[1]);
    }
}

Which of the following statements are true if the above program is run with the command line :
java Test closed
You had to select 1 options

It will throw ArrayIndexOutOfBoundsException at runtime.
It will end without exceptions and will print nothing.
It will print Go away
It will print Go away and then will throw ArrayIndexOutOfBoundsException.
None of the above.

✨Explanation 11

As in C and C++, the Java if statement suffers from the so-called “dangling else problem,” The problem is that both the outer if statement and the inner if statement might conceivably own the else clause. In this example, one might be tempted to assume that the programmer intended the else clause to belong to the outer if statement.

The Java language, like C and C++ and many languages before them, arbitrarily decree that an else clause belongs to the innermost if so as the first if() condition fails (args[0] not being “open”) there is no else associated to execute. So, the program does nothing. The else actually is associated with the second if.
So had the command line been :
java Test open, it would have executed the second if and thrown ArrayIndexOutOfBoundsException.
If the command line had been:
java Test open xyz, it would execute the else part(which is associated with the second if) and would have printed “Go away xyz”.

14.Which of the following code snippets will compile without any errors?
(Assume that the statement int x = 0; exists prior to the statements below.)
You had to select 3 options

while (false) { x=3; }
if (false) { x=3; }
do{ x = 3; } while(false);
for( int i = 0; i< 0; i++) x = 3;

✨Explanation 14

while (false) { x=3; } is a compile-time error because the statement x=3; is not reachable;
Similarly, for( int i = 0; false; i++) x = 3; is also a compile time error because x = 3; is unreachable.

In if(false){ x=3; }, although the body of the condition is unreachable, this is not an error because the JLS explicitly defines this as an exception to the rule. It allows this construct to support optimizations through the conditional compilation. For example,
if(DEBUG){ System.out.println("beginning task 1"); }
Here, the DEBUG variable can be set to false in the code while generating the production version of the class file, which will allow the compiler to optimize the code by removing the whole if statement entirely from the class file.

20.Given the following code, which of these statements are true?

  
class TestClass{
   public static void main(String args[]){
      int k = 0;
      int m = 0;
      for ( int i = 0; i <= 3; i++){
         k++;
         if ( i == 2){
            // line 1
         }
         m++;
      }
      System.out.println( k + ", " + m );
   }
}

You had to select 3 options

It will print 3, 2 when line 1 is replaced by break;
It will print 3, 2 when line 1 is replaced by continue.
It will print 4, 3 when line 1 is replaced by continue.
It will print 4, 4 when line 1 is replaced by i = m++;
It will print 3, 3 when line 1 is replaced by i = 4;

✨Explanation 20

This is a simple loop. All you need to do is execute each statement in your head. For example, if line 1 is replaced by break:

① k=0, m=0
② iteration 1: i=0
⇨ k = 1
⇨ i == 2 is false
⇨ m = 1
③ iteration 2: i = 1
⇨ k=2
⇨ i==2 is false
⇨ m = 2
④ iteration 3: i = 2
⇨ k=3
⇨ i==2 is true
⇨ break
⑤ print 3, 2

25.Which of these statements are true?
You had to select 2 options

A static method can call other non-static methods in the same class by using the ‘this’ keyword.
A calss may contain both static and non-static variables and both static and non-static methods.
Each object of a class has its own copy of each non-static member variable.
Instance methods of a class has it own copy of each non-static member variable.
Instance methods may access local variables of static methods.
All methods in a class are implicitly passed a ‘this’ parameter when called.

✨Explanation 25

‘this’ is assigned a reference to the current object automatically by the JVM. Thus, within an instance method foo, calling this.foo(); is same as calling foo();
Since there is no current object available for a static method, ‘this’ reference is not available in static methods and therefore it can only be used within instance methods. For the same reason, static methods cannot access non static fields or methods of that class directly i.e. without a reference to an instance of that class.
Note : you can’t reassign ‘this’ like this: this = new Object();

28.Which of the following comparisons will yield false?
You had to select 3 options

Boolean.parseBoolean("true") == true
Boolean.parseBoolean("TrUe") == new Boolean(null);
new Boolean("TrUe") == new Boolean(true);
new Boolean() == false;
new Boolean("true") == Boolean.TRUE
new Boolean("no") == false;

✨Explanation 28

You need to remember the following points about Boolean:
1. Boolean class has two constructors - Boolean(String) and Boolean(boolean) The String constructor allocates a Boolean object representing the value true if the string argument is not null and is equal, ignoring case, to the string “true”. Otherwise, allocate a Boolean object representing the value false. Examples: new Boolean("True") produces a Boolean object that represents true. new Boolean("yes") produces a Boolean object that represents false. The boolean constructor is self explanatory.
2. Boolean class has two static helper methods for creating booleans - parseBoolean and valueOf. Boolean.parseBoolean(String ) method returns a primitive boolean and not a Boolean object (Note - Same is with the case with other parseXXX methods such as Integer.parseInt - they return primitives and not objects). The boolean returned represents the value true if the string argument is not null and is equal, ignoring case, to the string “true”.
Boolean.valueOf(String ) and its overloaded Boolean.valueOf(boolean ) version, on the other hand, work similarly but return a reference to either Boolean.TRUE or Boolean.FALSE wrapper objects. Observe that they dont create a new Boolean object but just return the static constants TRUE or FALSE defined in Boolean class.
3. When you use the equality operator ( == ) with booleans, if exactly one of the operands is a Boolean wrapper, it is first unboxed into a boolean primitive and then the two are compared (JLS 15.21.2). If both are Boolean wrappers, then their references are compared just like in the case of other objects. Thus, new Boolean("true") == new Boolean("true") is false, but new Boolean("true") == Boolean.parseBoolean("true") is true.

29.Identify the valid for loop constructs assuming the following declarations:

  
Object o = null;
Collection c = //valid collection object.
int[][] ia = //valid array

You had to select 2 options

for(o : c){ }
Cannot use an existing/predefined variable in the variable declaration part.
for(final Object o2 :c){ }
final is the only modifier (excluding annotations) that is allowed here.
for(int i : ia) { }
Each element of ia is itself an array. Thus, they cannot be assigned to an int.
for(Iterator it : c.iterator()){ }
c.iterator() does not return any Collection. Note that the following would have been valid:
Collection<Iterator> c = //some collection that contains Iterator objects
for(Iterator it : c){ }
for(int i : ia[0]){ }
Since ia[0] is an array of ints, this is valid. (It may throw a NullPointerException or ArrayIndexOutOfBoundsException at runtime if ia is not appropriately initialized.)

✨Explanation 29

see above 👆

33.Which of these assignments are valid?
You had to select 3 options

short s = 12;
This is valid since 12 can fit into a short and an implicit narrowing conversion can occur.
long g = 012;
012 is a valid octal number.
int i = (int) false;
Values of type boolean cannot be converted to any other types.
float f = -123;
Implicit widening conversion will occur in this case.
float d = 0 * 1.5;
double cannot be implicitly narrowed to a float even though the value is representable by a float.

✨Explanation 33

Note that
float d = 0 * 1.5f; and float d = 0 * (float)1.5; are OK
An implicit narrowing primitive conversion may be used if all of the following conditions are satisfied:
1. The expression is a compile time constant expression of type byte, char, short, or int.
2. The type of the variable is byte, short, or char.
3. The value of the expression (which is known at compile time, because it is a constant expression) is representable in the type of the variable.
Note that implicit narrowing conversion does not apply to long or double. So, char ch = 30L; will fail even though 30 is representable in char.

44.What will the following code print?

  
public class TestClass{
        int x = 5;
        int getX(){ return x; }

        public static void main(String args[]) throws Exception{
            TestClass tc = new TestClass();
            tc.looper();
            System.out.println(tc.x);
        }
        
        public void looper(){
            int x = 0;
            while( (x = getX()) != 0 ){
                for(int m = 10; m>=0; m--){
                    x = m;
                }
            }
            
       }     
}

You had to select 1 option

It will not compile.
It will throw an exception at runtime.
It will print 0.
It will print 5.
None of these.
This program will compile and run but will never terminate.

✨Explanation 44

Note that looper() declares an automatic variable x, which shadows the instance variable x. So when x = m; is executed, it is the local variable x that is changed not the instance field x. So getX() never returns 0. If you remove int x = 0; from looper(), it will print 0 and end.

48.What will the following program print?

  
class Test{
   public static void main(String args[]){
      int var = 20, i=0;
      do{
         while(true){
         if( i++ > var) break;
         }
      }while(i<var--);
      System.out.println(var);
   }
}

You had to select 1 option

✨Explanation 48

When the first iteration of outer do-while loop starts, var is 20. Now, the inner loop executes till i becomes 21. Now, the condition for outer do-while is checked, while( 22 < 20 ), [i is 22 because of the last i++>var check], thereby making var 19. And as the condition is false, the outer loop also ends. So, 19 is printed.

56.Consider the following code:

  
class A{
   A() {  print();   }
   void print() { System.out.println("A"); }
}
class B extends A{
   int i =   4;
   public static void main(String[] args){
      A a = new B();
      a.print();
   }
   void print() { System.out.println(i); }
}

What will be the output when class B is run ?
You had to select 1 option

✨Explanation 56

Note that method print() is overridden in class B. Due to polymorphism, the method to be executed is selected depending on the class of the actual object.
Here, when an object of class B is created, first B’s default constructor (which is not visible in the code but is automatically provided by the compiler because B does not define any constructor explicitly) is called. The first line of this constructor is a call to super(), which invokes A’s constructor. A’s constructor in turn calls print(). Now, print is a non-private instance method and is therefore polymorphic, which means, the selection of the method to be executed depends on the class of actual object on which it is invoked. Here, since the class of actual object is B, B’s print is selected instead of A’s print. At this point of time, variable i has not been initialized (because we are still in the middle of initializing A), so its default value i.e. 0 is printed.
Finally, 4 is printed.

58.What will be the result of attempting to compile and run the following program?

  
class TestClass{
   public static void main(String args[]){
      int i = 0;
      loop :         // 1
      {
         System.out.println("Loop Lable line");
         try{
            for (  ;  true ;  i++ ){
               if( i >5) break loop;       // 2
            }
         }
         catch(Exception e){
            System.out.println("Exception in loop.");
         }
         finally{
            System.out.println("In Finally");      // 3
         }
      }
   }
}

You had to select 1 option

Compilation error at line 1 as this is an invalid syntax for defining a label.
You can apply a label to any code block or a block level statement (such as a for statement) but not to declarations. For example: loopX : int i = 10;
Compilation error at line 2 as ‘loop’ is not visible here.
No compilation error and line 3 will be executed.
Even if the break takes the control out of the block, the finally clause will be executed.
No compilation error and line 3 will NOT be executed.
Only the line with the label loop will be printed.

✨Explanation 58

A break without a label breaks the current loop (i.e. no iterations any more) and a break with a label tries to pass the control to the given label. ‘Tries to’ means that if the break is in a try block and the try block has a finally clause associated with it then it will be executed.

65.Consider the following code snippet:

  
XXXX m ;
//other code
  switch( m ){
     case 32  : System.out.println("32");   break;
     case 64  : System.out.println("64");   break;
     case 128 : System.out.println("128");  break;
  }

What type can ‘m’ be of so that the above code compiles and runs as expected ?
You had to select 3 options

int m;
m can hold all the case values.
long m;
long, float, double, and boolean can never be used as a switch variable.
char m;
m can hold all the case values.
byte m;
m will not be able to hold 128. a byte’s range is -128 to 127.
short m;
m can hold all the case values.

✨Explanation 65

Here are the rules for a switch statement:
1. Only String, byte, char, short, int, (and their wrapper classes Byte, Character, Short, and Integer), and enums can be used as types of a switch variable. (String is allowed only since Java 7).
2. The case constants must be assignable to the switch variable. For example, if your switch variable is of class String, your case labels must use Strings as well.
3. The switch variable must be big enough to hold all the case constants. For example, if the switch variable is of type char, then none of the case constants can be greater than 65535 because a char’s range is from 0 to 65535.
4. All case labels should be COMPILE TIME CONSTANTS.
5. No two of the case constant expressions associated with a switch statement may have the same value.
6. At most one default label may be associated with the same switch statement.

69.Consider the following code:

  
interface Flyer{ String getName(); }

class Bird implements Flyer{
    public String name;
    public Bird(String name){
        this.name = name;
    }
    public String getName(){ return name; }
}

class Eagle extends Bird {
    public Eagle(String name){
        super(name);
    }
}

public class TestClass {
    public static void main(String[] args) throws Exception {
        Flyer f = new Eagle("American Bald Eagle");
        //PRINT NAME HERE
   }
}

Which of the following lines of code will print the name of the Eagle object?
You had to select 3 options

System.out.println(f.name);
System.out.println(f.getName());
System.out.println(((Eagle)f).name);
System.out.println(((Bird)f).getName());
System.out.println(Eagle.name);
name is not a static field in class Eagle.
System.out.println(Eagle.getName(f));
This option doesn’t make any sense.

✨Explanation 69

While accessing a method or variable, the compiler will only allow you to access a method or variable that is visible through the class of the reference.
When you try to use f.name, the class of the reference f is Flyer and Flyer has no field named “name”, thus, it will not compile. But when you cast f to Bird (or Eagle), the compiler sees that the class Bird (or Eagle, because Eagle inherits from Bird) does have a field named “name” so ((Eagle)f).name or ((Bird)f).name will work fine.
f.getName() will work because Flyer does have a getName() method.

🔚

ETS Test 2 回顾

✨Explanation 2

✨Explanation 6

✨Explanation 8

✨Explanation 11

✨Explanation 14

✨Explanation 20

✨Explanation 25

✨Explanation 28

✨Explanation 29

✨Explanation 33

✨Explanation 44

✨Explanation 48

✨Explanation 56

✨Explanation 58

✨Explanation 65

✨Explanation 69

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