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- Question
Which statement is true about assertions in the Java programming language?
Options- A. Assertion expressions should not contain side effects.
- B. Assertion expression values can be any primitive type.
- C. Assertions should be used for enforcing preconditions on public methods.
- D. An AssertionError thrown as a result of a failed assertion should always be handled by the enclosing method.
- Correct Answer
- Assertion expressions should not contain side effects.
ExplanationOption A is correct. Because assertions may be disabled, programs must not assume that the boolean expressions contained in assertions will be evaluated. Thus these expressions should be free of side effects. That is, evaluating such an expression should not affect any state that is visible after the evaluation is complete. Although it is not illegal for a boolean expression contained in an assertion to have a side effect, it is generally inappropriate, as it could cause program behaviour to vary depending on whether assertions are enabled or disabled.Assertion checking may be disabled for increased performance. Typically, assertion checking is enabled during program development and testing and disabled for deployment.
Option B is wrong. Because you assert that something is "true". True is Boolean. So, an expression must evaluate to Boolean, not int or byte or anything else. Use the same rules for an assertion expression that you would use for a while condition.
Option C is wrong. Usually, enforcing a precondition on a public method is done by condition-checking code that you write yourself, to give you specific exceptions.
Option D is wrong. "You're never supposed to handle an assertion failure"
Not all legal uses of assertions are considered appropriate. As with so much of Java, you can abuse the intended use for assertions, despite the best efforts of Sun's Java engineers to discourage you. For example, you're never supposed to handle an assertion failure. That means don't catch it with a catch clause and attempt to recover. Legally, however, AssertionError is a subclass of Throwable, so it can be caught. But just don't do it! If you're going to try to recover from something, it should be an exception. To discourage you from trying to substitute an assertion for an exception, the AssertionError doesn't provide access to the object that generated it. All you get is the String message.
More questions
- 1. What will be the output of the program?
public class Test { public static void main(String [] args) { signed int x = 10; for (int y=0; y<5; y++, x--) System.out.print(x + ", "); } }
Options- A. 10, 9, 8, 7, 6,
- B. 9, 8, 7, 6, 5,
- C. Compilation fails.
- D. An exception is thrown at runtime. Discuss
Correct Answer: Compilation fails.
Explanation:
The word "signed" is not a valid modifier keyword in the Java language. All number primitives in Java are signed. Hence the Compilation will fails.- 2. Which two of the following statements, inserted independently, could legally be inserted into missing section of this code?
- boolean test = (Component instanceof t);
- boolean test = (t instanceof Ticker);
- boolean test = t.instanceof(Ticker);
- boolean test = (t instanceof Component);
import java.awt.*; class Ticker extends Component { public static void main (String [] args) { Ticker t = new Ticker(); /* Missing Statements? */ } }
Options- A. 1 and 4
- B. 2 and 3
- C. 1 and 3
- D. 2 and 4 Discuss
Correct Answer: 2 and 4
Explanation:
(2) is correct because class type Ticker is part of the class hierarchy of t; therefore it is a legal use of the instanceof operator. (4) is also correct because Component is part of the hierarchy of t, because Ticker extends Component.(1) is incorrect because the syntax is wrong. A variable (or null) always appears before the instanceof operator, and a type appears after it. (3) is incorrect because the statement is used as a method (t.instanceof(Ticker);), which is illegal.
- 3. What will be the output of the program?
class Test { public static void main(String [] args) { int x=20; String sup = (x < 15)? "small" : (x < 22)? "tiny" : "huge"; System.out.println(sup); } }
Options- A. small
- B. tiny
- C. huge
- D. Compilation fails Discuss
Correct Answer: tiny
Explanation:
This is an example of a nested ternary operator. The second evaluation (x < 22) is true, so the "tiny" value is assigned to sup.- 4. Which statement, inserted at line 10, creates an instance of Bar?
class Foo { class Bar{ } } class Test { public static void main (String [] args) { Foo f = new Foo(); /* Line 10: Missing statement? */ } }
Options- A. Foo.Bar b = new Foo.Bar();
- B. Foo.Bar b = f.new Bar();
- C. Bar b = new f.Bar();
- D. Bar b = f.new Bar(); Discuss
Correct Answer: Foo.Bar b = f.new Bar();
Explanation:
Option B is correct because the syntax is correct-using both names (the enclosing class and the inner class) in the reference declaration, then using a reference to the enclosing class to invoke new on the inner class.Option A, C and D all use incorrect syntax. A is incorrect because it doesn't use a reference to the enclosing class, and also because it includes both names in the new.
C is incorrect because it doesn't use the enclosing class name in the reference variable declaration, and because the new syntax is wrong.
D is incorrect because it doesn't use the enclosing class name in the reference variable declaration.
- 5. What line of code should replace the missing statement to make this program compile?
/* Missing Statement? */ public class foo { public static void main(String[]args)throws Exception { java.io.PrintWriter out = new java.io.PrintWriter(); new java.io.OutputStreamWriter(System.out,true); out.println("Hello"); } }
Options- A. No statement required.
- B. import java.io.*;
- C. include java.io.*;
- D. import java.io.PrintWriter; Discuss
Correct Answer: No statement required.
Explanation:
The usual method for using/importing the java packages/classes is by using an import statement at the top of your code. However it is possible to explicitly import the specific class that you want to use as you use it which is shown in the code above. The disadvantage of this however is that every time you create a new object you will have to use the class path in the case "java.io" then the class name in the long run leading to a lot more typing.- 6. When is the B object, created in line 3, eligible for garbage collection?
void start() { A a = new A(); B b = new B(); a.s(b); b = null; /* Line 5 */ a = null; /* Line 6 */ System.out.println("start completed"); /* Line 7 */ }
Options- A. after line 5
- B. after line 6
- C. after line 7
- D. There is no way to be absolutely certain. Discuss
Correct Answer: There is no way to be absolutely certain.
- 7. Which is a reserved word in the Java programming language?
Options- A. method
- B. native
- C. subclasses
- D. reference
- E. array Discuss
Correct Answer: native
Explanation:
The word "native" is a valid keyword, used to modify a method declaration.Option A, D and E are not keywords. Option C is wrong because the keyword for subclassing in Java is extends, not 'subclasses'.
- 8. Which statement is true for the class java.util.ArrayList?
Options- A. The elements in the collection are ordered.
- B. The collection is guaranteed to be immutable.
- C. The elements in the collection are guaranteed to be unique.
- D. The elements in the collection are accessed using a unique key. Discuss
Correct Answer: The elements in the collection are ordered.
Explanation:
Yes, always the elements in the collection are ordered.- 9. What will be the output of the program?
public class Test { public static void main(String[] args) { final StringBuffer a = new StringBuffer(); final StringBuffer b = new StringBuffer(); new Thread() { public void run() { System.out.print(a.append("A")); synchronized(b) { System.out.print(b.append("B")); } } }.start(); new Thread() { public void run() { System.out.print(b.append("C")); synchronized(a) { System.out.print(a.append("D")); } } }.start(); } }
Options- A. ACCBAD
- B. ABBCAD
- C. CDDACB
- D. Indeterminate output Discuss
Correct Answer: Indeterminate output
Explanation:
It gives different output while executing the same compiled code at different times.- 10. Which is true about a method-local inner class?
Options- A. It must be marked final.
- B. It can be marked abstract.
- C. It can be marked public.
- D. It can be marked static. Discuss
Correct Answer: It can be marked abstract.
Explanation:
Option B is correct because a method-local inner class can be abstract, although it means a subclass of the inner class must be created if the abstract class is to be used (so an abstract method-local inner class is probably not useful).Option A is incorrect because a method-local inner class does not have to be declared final (although it is legal to do so).
C and D are incorrect because a method-local inner class cannot be made public (remember-you cannot mark any local variables as public), or static.
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- 1. What will be the output of the program?