Chapter 5: Flow Control, Exceptions and Assertions

Question 1

To which IF belongs the else statement?

if(exam.done()) //1
if(exam.getScore() < 0.61) //2
System.out.println(“Try again”)
else System.out.println(“java master”)

Answer 1

2

The most inner if.

Question 2

What is is the result of the following expression?
int y = 5;
int x = 2;
if((x > 3) &amp;&amp; (y < 2) | true) {
System.out.println("true")
}

Answer 2

It will print nothing.

x is not greather than 3, && is a short circuit operator so it will not evaluate the rest of the statements.

Question 3

What is the result of the following?

if(b = true) {} // 1
if(x = 2) {} // 2

Answer 3

1, evaluates to true. An assingment wil return the assigned value.
2, compiler error 2 cannot be evaluated to a boolean

Question 4

What are valid evaluation types for a switch statement?

Answer 4

Primitive types that can be implicitly cast to int (char, byte, short, int) or Enum.

Question 5

What is the result of the following code?
int b = 2;
int x = 0;
switch (x) {
   case b:
   System.out.println("b");
   break;
   case 2 > x:
   break;
}

Answer 5

Compiler error!

A case constant must evaluate to the same type and must be a compile time constant! (mark it final or use a direct literal).

2 > x is not valid in a switch. It can only check for equality.

Question 6

What is the result?
byte b = 2;
switch(b) {
   case 128:
}

Answer 6

Compiler error!

128 doesn’t fit into a byte.

Question 7

What is the result?
int x = 10;
switch(x) {
   case 20:
   case 40:
   case 40:
}

Answer 7

Compiler error!

You can’t define the same case statements multiple times.

Question 8

What does the break keyword do in a switch statement?

Answer 8

It will break the fall through of the switch cases. The first case evaulated to be equal en will execute all following cases until a break; is found.

Question 9

What is the result?
int x = 2;
switch(x) {
   case 2: System.out.println("2");
   default: System.out.println("default");
   case 3: System.out.println("3");
}

Answer 9

2, default, 3

The default case works just like any other case for fall through. The only difference is, the case itself will only match with the values that are not provided as cases.

Question 10

What is the result?
do {
System.out.println(“loop”);
} while (false);

Answer 10

It will print loop once.

The do-while executes once and checks the expression afterwards. If evaluated to true it will execute again.

Question 11

What is the result?

for(int x = 0, y = 1; (x > 0), (y > 5); x++, System.out.println(“loop”)) {
System.out.println(“test”);
}

Answer 11

Compiler error!

The for-loop contains three parts (seperated by a ;). The 1st (Declaration and Initialization) may contain multiple statements seperated by a ,. The same applies for the third part, the Iteration Expression. The third part can conain any piece of code, not just increment operations. The second part may only contain a single expression!

Question 12

What can a loop cause to stop (apart from the condition conditioning to false)?

Answer 12

break, return, System.exit(); or an Exception.

Question 13

What is the result?
for( ; ; ) {
System.out.println(“test”);
}

Answer 13

An endless loop.

The three parts are all optional.

Question 14

What does the continue keyword do (unlabeled context)?

Answer 14

It will move on to the next iteration of the inner most loop.

Question 15

What is the result?
outer:
   for(int i = 0; i < 5; i++) {
      while(true) {
         someLabel: System.out.println("inner");
         break outer;
      }
      System.out.println("outer");
   }
System.out.prinln("end");

Answer 15

It will print “inner end”

The break statement is a labelled break. And will break out of the outer loop instead of the inner loop if used without a label.

someLabel points to a statement. But can’t be called upon, but is perfectly valid (but is has no functional use).

Question 16

What is wrong?
try {
// Do Stuff
}

Answer 16

A try always needs atleast one catch or finally block.

Question 17

Describe the exception hierachy.

Answer 17

Object
Throwable
Exception - Error
RunTimeExceptoion

Question 18

What is wrong?
try {
// Do Risky Stuff
} 
catch (IOException e) {}
catch(FileNotFoundException e) {}

Answer 18

Compiler error!

The specific exception catch should always be defined before the more generic catch.

Question 19

How do you throw an exception?

Answer 19

throw new MyException();

Question 20

What is the catch or declare rule?

Answer 20

Each method must either handle all checked exceptions by supplying a catch clause or list each unhandeled checked exception as a thrown exception.

Question 21

What is wrong?
public void doStuff() throws MyException1, MyException2 {
   try { throw new IOException();}
   catch(IOException e) { throw e; }
}

Answer 21

The method doStuff() does not declare the throws clause for IOException.

Question 22

What are assert statements in Java?

Answer 22

Validating method input parameters using assert statements during the development phase of an program.

private void methodA(int num) {
assert(num >= 0); // Throws an AssertionError
}

Question 23

Describe a assert with a String message.

Answer 23

assert(b) : “The value of b is “ + b;

Question 24

What are valid and invalid assert statements?
assert(x == 1); // 1
assert(true); // 2
assert 0; // 3
assert(b = true); // 4
assert(x == 1) : 1; // 5
assert(x == 1) : ; //6
assert true; // 7

Answer 24

Valid: 1, 2, 4, 5, 7
Invalid: 3, 6

Question 25

What is wrong?
int assert = 5;
private void assert() {}

Answer 25

assert is a keyword since java 1.4.

Question 26

How to compile a class using a specific java version?

Answer 26

javac -source OldCode.java
javac -source 1.4 OldCode.java
javac -source 4 OldCode.java

If you want to use the assert as an identifier instead of a keyword you need to compile using 1.3.

Question 27

How do you enable/disable assertions at runtime?

Answer 27

java -ea TestClass // Enable
java -enableassertions TestClass // Enable
java -da TestClass //Disable, redundant because default
java -disableassertions TestClass //Disable, redundant because default

Question 28

What do the following run statements?

1. java -ea
2. java -da
3. java -ea:com.foo.Bar
4. java -ea -da:com.foo…
5. java -ea -dsa

Answer 28

1. Enable assertions
2. Disable assertions
3. Enable assertions in class Bar
4. Enable assertions in application,, except the classes in package com.foo.
5. Enable assertions in application, disable assertions for system classes.

Question 29

What are the do’s and dont’s for using assertions?

Answer 29

Don’t:
Use assertions to validate arguments to public methods.
Use assertions to validate command line arguments.
Use Assert expressions that can cause side effects.

Do’s:
Use Assertions to validate arguments to a private method.
Use assertions to check for cases that you know are never, ever supposed to happen.

Question 30

What will happen?
public class TestClass
{
   public static void main(String args[])
   {
      try
      {
         RuntimeException re = null;
         throw re;
      }
      catch(Exception e)
      {
         System.out.println(e);
      }
   }
}

Answer 30

The program will compile without error and will print java.lang.NullPointerException when run.

Question 31

What is wrong?
try
        {
            m2();
        }
        finally
        {
            m3();
        }
        catch (NewException e){}

Answer 31

Compile error. The correct order is Try, optional catch blocks, optional finally block.

Question 32

Which of the following code snippets will compile without any errors?
while (false) { x=3; } // 1
if (false) { x=3; } // 2
do{ x = 3; } while(false); // 3
for( int i = 0; i< 0; i++) x = 3; // 4

Answer 32

2,3,4

1: while (false) { x=3; } is a compile-time error because the statement x=3; is not reachable;