Question

Java question, Please answer everything. Thank you

Answer the following questions as briefly (but completely) as possible:

1. What is a checked exception, and what is an unchecked exception?
2. What is NullPointerException?
3. Which of the following statements (if any) will throw an exception? If no exception is thrown, what is the output?

   1: System.out.println( 1 / 0 );
   2: System.out.println( 1.0 / 0 );

4. Point out the problem in the following code. Does the code throw any exceptions?

   1: long value = Long.MAX_VALUE + 1;
   2: System.out.println( value );

5. What are the differences between constructors and methods?
6. What is wrong with each of the following programs?

   1. 1: public class ShowErrors {
      2:    public static void main ( String [] args ) {
      3:       ShowErrors t = new ShowErrors( 5 );
      4:    }
      5: }

   2. 1: public class ShowErrors {
      2:    public static void main ( String [] args ) {
      3:       ShowErrors t = new ShowErrors();
      4:       t.x();
      5:    }
      6: }

   3. 1: public class ShowErrors {
      2:    public void method1 () {
      3:       Circle c;
      4:       System.out.println( "What is radius "
      5:          + c.getRadius() );
      6:       c = new Circle();
      7:    }
      8: }

   4.  1: public class ShowErrors {
       2:    public static void main(String[] args) {
       3:       C c = new C(5.0);
       4:       System.out.println(c.value);
       5:    }
       6: }
       7: 
       8: class C {
       9:    int value = 2;
      10: }

7. Which of the following statements are valid?
   1. int i = new int(30);
   2. double d[] = new double[30];
   3. char[] r = new char(1..30);
   4. int i[] = (3, 4, 3, 2);
   5. float f[] = {2.3, 4.5, 6.6};
   6. char[] c = new char();
8. Given an array of doubles, write Java statements to do the following:
   1. Assign the value 5.5 to the last element in the array.
   2. Display the sum of the first two elements of the array.
   3. Write a loop that computes the sum of all elements in the array.
   4. Write a loop that finds the minimum element in the array.
   5. Randomly generate an index and display the element of this index in the array.
   6. Use an array initializer to create another array with the initial value 3.5, 5.5, 4.52, and 5.6.
9. Use the following illustration as an example, show how to apply the binary search approach to a search first for key 10 and then key 12, in the list:
   [2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79].

   key is 11
            0   1   2   3   4   5   6   7   8   9  10  11  12
   11<50  [ 2,  4,  7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79]
          low=0                   mid=6                   hi=12
   
   11>7   [ 2,  4,  7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79]
          low=0    mid=2      hi=5
   
   11=11  [ 2,  4,  7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79]
                      low=3    hi=5
                          mid=4

   (Note how binary search eliminates half of the list from further consideration after each comparison.)

10. What types of array can be sorted using the Java.util.Arrays.sort method? Does this sort method create a new array?
11. Which of the following statements are valid?
    1. int[][] r = new int[2];
    2. int[] x = new int[];
    3. int[][] y = new int [3][];
    4. int[][] z = {{1, 2}};
    5. int[][] m = {{1, 2}, {2, 3}};
    6. int[][] n = {{1, 2}, {2, 3}, };
12. How do you do the following tasks?
    1. Create an ArrayList for storing double values?
    2. Append an object to a List?
    3. Insert an object at the beginning of a List?
    4. Find the number of objects in a List?
    5. Remove a given object from a List?
    6. Remove the last object from a List?
    7. Check whether a given object is in a List?
    8. Retrieve an object at a specified index from a List?
13. Identify the errors in the following code fragment:

    1: ArrayList<String> list = new ArrayList<String>();
    2: list.add( "Denver" );
    3: list.add( "Austin" );
    4: list.add( new java.util.Date() );
    5: String city = list.get( 0 );
    6: list.set( 2, "Dallas" );
    7: System.out.println( list.get(2) );

14. Explain why the following code fragment displays [1, 3] rather than [2, 3].

    1: ArrayList<Integer> list = new ArrayList<Integer>();
    2: list.add(1);
    3: list.add(2);
    4: list.add(3);
    5: list.remove(1);
    6: System.out.println( list );

15. Describe the difference between passing a parameter of a primitive type and passing a parameter of a reference type. Then show the output of the following program:

     1: class Test {
     2:     public static void main ( String [] args ) {
     3:         Count myCount = new Count();
     4:         int times = 0;
     5:         for ( int i = 0; i < 100; i++ )
     6:             increment( myCount, times );
     7:         System.out.println( "count is " + myCount.count );
     8:         System.out.println( "times is " + times );
     9:     }
    10:     public static void increment ( Count c, int times ) {
    11:         c.count++;
    12:         times++;
    13:     }
    14: }
    15: 
    16: class Count {
    17:     public int count;
    18:     public Count ( int c ) {
    19:         count = c;
    20:     }
    21:     public Count () {
    22:         count = 1;
    23:     }
    24: }

16. What is wrong in the following code?

    1: public class Test {
    2:    public static void main ( String [] args ) {
    3:       java.util.Date[] dates = new java.util.Date[10];
    4:       System.out.println( dates[0] );
    5:       System.out.println( dates[0].toString() );
    6:    }
    7: }

17. If a class contains only private data fields and no “set” methods, is the class considered to be immutable?
18. If a class contains only data fields that are both private and primitive, and no “set” methods, is the class considered to be immutable?
19. What is wrong in the following code?

     1: public class C {
     2:     private int p;
     3: 
     4:     public C () {
     5:         System.out.println( "C's no-arg constructor invoked" );
     6:         this(0);
     7:     }
     8: 
     9:     public C ( int p ) {
    10:         p = p;
    11:     }
    12: 
    13:     public void setP ( int p ) {
    14:         p = p;
    15:     }
    16: }

20. What is wrong in the following code?

    1: public class Test {
    2:     private int id;
    3:     public void m1 () {
    4:         this.id = 45;
    5:     }
    6:     public void m2 () {
    7:         Test.id = 45;
    8:     }
    9: }

Answer 1

Checked exceptions are checked at compile-time. It means if a method is throwing a checked exception then it should handle the exception using try-catch block or it should declare the exception using throws keyword, otherwise the program will give a compilation error.

Unchecked exceptions are not checked at compile time. It means if your program is throwing an unchecked exception and even if you didn’t handle/declare that exception, the program won’t give a compilation error. Most of the times these exception occurs due to the bad data provided by user during the user-program interaction. It is up to the programmer to judge the conditions in advance, that can cause such exceptions and handle them appropriately. All Unchecked exceptions are direct sub classes of RuntimeException class.

Class NullPointerException

Thrown when an application attempts to use null in a case where an object is required. These include: ... Accessing or modifying the field of a null object. Taking the length of null as if it were an array. Accessing or modifying the slots of null as if it were an array.

System.out.println(1 / 0); // Throws an exception 
System.out.println(1.0 / 0); // Will not throw an exception

Adding 1 to Long.MAX_VALUE exceeds the maximum value allowed by a long value. But the current versions of Java does not report this as an exception.

Constructor is used to initialize an object whereas method is used to exhibits functionality of an object. Constructors are invoked implicitly whereas methods are invoked explicitly. Constructor does not return any value where the method may/may not return a value.

When you pass a primitive, you actually pass a copy of value of that variable. ... When you pass an object you don't pass a copy, you pass a copy of 'handle' of that object by which you can access it and can change it. This 'handle' is a 'reference'. In this changes will be reflected in original.