java.util.Arrays helpers

Figure 406. Arrays.toString(...) and Arrays.sort(...) Slide presentation
Code 
final String[] names = {"Eve", "Aaron", "Paul", "Mandy"}

IO.println("Original list: " + Arrays.toString(names));

Arrays.sort(names);

IO.println("  Sorted list: " + Arrays.toString(names));
Result 
Original list: [Eve, Aaron, Paul, Mandy]
  Sorted list: [Aaron, Eve, Mandy, Paul]

Figure 407. Arrays.binarySearch(...) Slide presentation
Code 
final String[] names = {"Aaron", "Eve", "Mandy", "Paul"};

// Precondition: Array values must be in ascending order
...println("find(Mand): " + Arrays.binarySearch(names, "Mand"));
...println("find(Mandy): " + Arrays.binarySearch(names, "Mandy"));
...println("find(Mandyer): " + Arrays.binarySearch(names, "Mandyer"));
Result 
find(Mand): -3
find(Mandy): 2
find(Mandyer): -4

exercise No. 150

Understanding search results

Q:

In Figure 407, “Arrays.binarySearch(...) ” the result reads: 

sort|find(Mand): -3
sort|find(Mandy): 2
sort|find(Mandyer): -4

  1. What does the positive value 2 in return to "Mandy" indicate?

  2. Why do we get negative values -3 and -4 for "Mand" and "Mandyer" respectively?

  3. What is the exact meaning of -3 and -4 besides being just negative?

Tip

Read Arrays.binarySearch(...).

A:

The input array reads {"Aaron", "Eva", "Mandy", "Paul"}.

Array element Index
"Aaron" 0
"Eve" 1
"Mandy" 2
"Paul" 3

  1. A positive value indicates an exact match: The string "Mandy" does exist within the array at index position 2.

  2. A negative value indicates a value not being found: Both "Mand" and "Mandyer" do not exist in the array

  3. The exact value of a negative integer being returned defines the hypothetical insertion position keeping the resulting array sorted. Regarding "Mandyer" the insertion index would be 3 squeezing "Paul" one step to the right: 

       {"Aaron", "Eva", "Mandy", "Mandyer", "Paul"} 
/* Index 0      1        2          3      4 /*

    The observed return value of -4 when searching for "Mandyer" thus corresponds to insertion index 3. Hence we have the following rule:

    Negate the return value and subsequently subtract 1.

    Ratio: Subtracting 1 is being required for conveying the «not in list» information under all circumstances. Otherwise not finding and hypothetically inserting e.g. "A" at the list's beginning would result in 0. This could not be disambiguated from an array featuring an existing value"A" at its leftmost start position.
Figure 408. Arrays.fill(...) Slide presentation
Code 
final String[] names = {"Eve", "Aaron", "Paul", "Mandy"};

IO.println("toString: " + Arrays.toString(names));

Arrays.fill(names, "N.N.");

IO.println("toString: " + Arrays.toString(names));
Result 
toString: [Eve, Aaron, Paul, Mandy]
toString: [N.N., N.N., N.N., N.N.]

Figure 409. Arrays.copyOfRange(...) Slide presentation
Code 
final String[] names = {"Eve", "Aaron", "Paul", "Mandy"};

final String[] lastTwoNamesPlusTwo =
     Arrays.copyOfRange(names, 2, 6); // Index values 2, 3, 4 and 5

IO.println(Arrays.toString(lastTwoNamesPlusTwo));
Result 
[Paul, Mandy, null, null]

Figure 410. Arrays.equals(...) Slide presentation
Code 
final String[]
  l1 = {"Eve", "Aaron", "Paul", "Mandy"},
  l2 = {"Eve", "Aaron", "Paul", "Mandy"},
  l3 = {"Eve", "Aaron", "Paul", "Mobile"};

IO.println("l1.equals(l2): " + Arrays.equals(l1, l2));

IO.println("l1.equals(l3): " + Arrays.equals(l1, l3));
Result 
l1.equals(l2): true
l1.equals(l3): false

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