More Data Types and Operators

Question 1

Show two ways to declare a one-dimensional array of 12 doubles.

Answer 1

double x[] = new double[12];
double[] x = new double[12];

Question 2

Show how to initialize a one-dimensional array of integers to the values 1 through 5.

Answer 2

int[] x = { 1, 2, 3, 4, 5 };

Question 3

Write a program that uses an array to find the average of ten double values. Use any ten
values you like.

Answer 3

// Average 10 double values.
class Avg {
public static void main(String[] args) {
double[] nums = { 1.1, 2.2, 3.3, 4.4, 5.5,
6.6, 7.7, 8.8, 9.9, 10.1 };
double sum = 0;

for(int i=0; i < nums.length; i++)
  sum += nums[i];

System.out.println("Average: " + sum / nums.length);   } }

Question 4

Change the sort in Try This 5-1 so that it sorts an array of strings. Demonstrate that it works.

Answer 4

// Demonstrate the Bubble sort with strings.
class StrBubble {
public static void main(String[] args) {
String[] strs = {
“this”, “is”, “a”, “test”,
“of”, “a”, “string”, “sort”
};
int a, b;
String t;
int size;

size = strs.length; // number of elements to sort

// display original array
System.out.print("Original array is:");
for(int i=0; i < size; i++)
  System.out.print(" " + strs[i]);
System.out.println();

// This is the bubble sort for strings.
for(a=1; a < size; a++)
  for(b=size-1; b >= a; b--) {
    if(strs[b-1].compareTo(strs[b]) > 0) { // if out of order
      // exchange elements
      t = strs[b-1];
      strs[b-1] = strs[b]; 
      strs[b] = t; 
    }
  } // display sorted array
System.out.print("Sorted array is:");
for(int i=0; i < size; i++)
  System.out.print(" " + strs[i]);
System.out.println();   } }

Question 5

What is the difference between the String methods indexOf( ) and lastIndexOf( )?

Answer 5

The indexOf( ) method finds the first occurrence of the specified substring. lastIndexOf( ) finds the
last occurrence.

Question 6

Since all strings are objects of type String, show how you can call the length( ) and charAt( ) methods on this string literal: “I like Java”.

Answer 6

As strange as it may look, this is a valid call to length( ):
System.out.println(“I like Java”.length());

The output displayed is 11. charAt( ) is called in a similar fashion.

Question 7

Expanding on the Encode cipher class, modify it so that it uses an eight-character string as
the key.

Answer 7

// An improved XOR cipher.
class Encode {
public static void main(String[] args) {
String msg = “This is a test”;
String encmsg = “”;
String decmsg = “”;
String key = “abcdefgi”;
int j;

System.out.print("Original message: ");
System.out.println(msg);

// encode the message
j = 0;
for(int i=0; i < msg.length(); i++) {
  encmsg = encmsg + (char) (msg.charAt(i) ^ key.charAt(j));
  j++;
  if(j==8) j = 0;
}

System.out.print("Encoded message: ");
System.out.println(encmsg); 
 
// decode the message
j = 0;
for(int i=0; i < msg.length(); i++) {
  decmsg = decmsg + (char) (encmsg.charAt(i) ^ key.charAt(j)); j++;
  if(j==8) j = 0;
}

System.out.print("Decoded message: ");
System.out.println(decmsg);   } }

Question 8

Can the bitwise operators be applied to the double type?

Answer 8

No.

Question 9

Show how this sequence can be rewritten using the ? operator.

if(x < 0) y = 10;
else y = 20;

Answer 9

Here is the answer:
y = x < 0 ? 10 : 20;

Question 10

In the following fragment, is the & a bitwise or logical operator? Why?

boolean a, b;
// …
if(a & b) …

Answer 10

It is a logical operator because the operands are of type boolean.

Question 11

Is it an error to overrun the end of an array?

Is it an error to index an array with a negative value?

Answer 11

Yes.

Yes. All array indexes start at zero.

Question 12

What is the unsigned right-shift operator?

Answer 12

> > >

Question 13

Rewrite the MinMax class shown earlier in this chapter so that it uses a for-each style for loop.

Answer 13

// Find the minimum and maximum values in an array.
class MinMax {
public static void main(String[] args) {
int[] nums = new int[10];
int min, max;

nums[0] = 99;
nums[1] = -10;
nums[2] = 100123;
nums[3] = 18;
nums[4] = -978;
nums[5] = 5623; 
nums[6] = 463;
nums[7] = -9;
nums[8] = 287;
nums[9] = 49;

min = max = nums[0];
for(int v : nums) {
  if(v < min) min = v;
  if(v > max) max = v; 
} 
System.out.println("min and max: " + min + " " + max);   } }

Question 14

Can the for loops that perform sorting in the Bubble class shown in Try This 5-1 be converted into for-each style loops? If not, why not?

Answer 14

No, the for loops in the Bubble class that perform the sort cannot be converted into for-each style loops. In the case of the outer loop, the current value of its loop counter is needed by the inner loop. In the case of the inner loop, out-of-order values must be exchanged, which implies assignments. Assignments to the underlying array cannot take place when using a for-each style loop.

Question 15

Can a String control a switch statement?

Answer 15

Yes.

Question 16

What keyword is reserved for use with local variable type inference?

Answer 16

The context-sensitive keyword var is reserved for use with local variable type inference.

Question 17

Show how to use local variable type inference to declare a boolean variable called done that has an initial value of false.

Answer 17

var done = false;

Question 18

Can var be the name of a variable? Can var be the name of a class?

Answer 18

Yes, var can be the name of a variable. No, var cannot be the name of a class.

Question 19

Is the following declaration valid? If not, why not.

var[] avgTemps = new double[7];

Answer 19

No, it is not valid because array brackets are not allowed after var. Remember, the complete type is
inferred from the initializer.

Question 20

Is the following declaration valid? If not, why not?

var alpha = 10, beta = 20;

Answer 20

No, only one variable at a time can be declared when type inference is used.

Question 21

In the show( ) method of the ShowBits class developed in Try This 5-3, the local variable
mask is declared as shown here:

long mask = 1;

Change this declaration so that it uses local variable type inference. When doing so, be sure that mask is of type long (as it is here), and not of type int.

Answer 21

var mask = 1L; // Notice that the initial value is explicitly
// specified as long so that mask will be
// inferred to be long.