java.util.Arrays helpers

Figure 426. Arrays.toString(...) and Arrays.sort(...) Slide presentation 
final String[] names = {"Eve", "Aaron", "Paul", "Mandy"};

System.out.println("     toString: " + Arrays.toString(names));

Arrays.sort(names);

System.out.println("sort|toString: " + Arrays.toString(names));

Result:

     toString: [Eve, Aaron, Paul, Mandy]
sort|toString: [Aaron, Eve, Mandy, Paul]

Figure 427. Arrays.binarySearch(...) Slide presentation 
final String[] names = {"Aaron", "Eve", "Mandy",  "Paul"};

// Precondition: Array must be ordered!
...println("sort|find(Mand): " + Arrays.binarySearch(names, "Mand"));
...println("sort|find(Mandy): " + Arrays.binarySearch(names, "Mandy"));
...println("sort|find(Mandyer): " + Arrays.binarySearch(names, "Mandyer"));

Result:

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

exercise No. 147

Understanding search results

Q:

Read the Javadoc™ of Arrays.binarySearch(...)and explain the three integer search result values in Figure 427, “Arrays.binarySearch(...) ”.

In particular give a detailed account explaining the two negative values -3 and -4.

A:

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

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

A positive return value conveys:

  1. The value is present within the array.

  2. The return value is equal to the array element's index.

Searching for "Mandy" thus returns the index value 2 corresponding to the third array element.

Negative return values actually also provide two types of information:

  1. The value is not present in the given array.

  2. The position where the element would be inserted to become part of an ordered list.

In the given example hypothetically inserting the string "Mand" retaining a sorted list results in: 

{"Aaron", "Eva", "Mand", "Mandy", "Paul"}

This insertion point's index is 2 corresponding to the third array element. According to the binarySearch(...) method's documentation this corresponds to a return value of -2 - 1 == -3.

The observed return value of -4 when searching for "Mandyer" corresponds to insertion index 3: 

{"Aaron", "Eva", "Mandy", "Mandyer", "Paul"}

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 "A" at its front position.
Figure 428. Arrays.fill(...) Slide presentation 
final String[] names = 
  {"Eve", "Aaron", "Paul", "Mandy"};

System.out.println("toString: " +
   Arrays.toString(names));

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

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

Figure 429. Arrays.copyOfRange(...) Slide presentation 
final String[] names = {"Eve", "Aaron", "Paul", "Mandy"};

final String[] lastTwoNames = Arrays.copyOfRange(names, 2, 6);

System.out.println("toString: " + Arrays.toString(lastTwoNames));

Result:

toString: [Paul, Mandy, null, null]

Figure 430. Arrays.equals(...) Slide presentation 
final String[]
  l1 = {new String("Eve"), new String("Aaron"), 
        new String("Paul"), new String("Mandy")},

  l2 = {new String("Eve"), new String("Aaron"), 
        new String("Paul"), new String("Mandy")},

  l3 = {new String("Eve"), new String("Aaron"), 
        new String("Paul"), new String("Mobile")};

System.out.println("l1.equals(l2):" + Arrays.equals(l1, l2));
System.out.println("l1.equals(l3):" + Arrays.equals(l1, l3));

Result:

l1.equals(l2):true
l1.equals(l3):false

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