• Arrays

    Multiple values of common type.

    Loop style handling.

final String
      karen = "Karen Smith",
      john = "John Duncan",
      paul = "Paul Jacobs",
      suzanne = "Suzanne Enders",
      peter = "Peter Phillips";  // 10 more to come ...

    IO.println(karen);
    IO.println(john);
...

  • Generate Comma separated list:

    Karen Smith, John Duncan, Paul Jacobs, Suzanne Enders, Peter Phillips

  • Generate HTML list emphasizing family names:

    <ul>
  <li>Karen <emph>Smith</emph></li>
  <li>John <emph>Duncan</emph></li>
  <li>Paul <emph>Jacobs</emph></li>
  <li>Suzanne <emph>Enders</emph></li>
  <li>Peter <emph>Phillips</emph></li>
</ul>
final int[] primes ❶ = new int[5]; ❷

primes[0] = 2; ❸
primes[1] = 3;
primes[2] = 5;
primes[3] = 7;
primes[4] = 11;
  1. Assignment to final variable?
for (int i = 0; i < 5; i++) {
  IO.println("At index " + i + ": value == " + primes[i]);
}

Result:

At index 0: value == 2
At index 1: value == 3
At index 2: value == 5
At index 3: value == 7
At index 4: value == 11
for (int i = 0; i < 6; i++) {
  IO.println("At index " + i + ": value == " + primes[i]);
}

Result:

At index 0: value == 2
At index 1: value == 3
At index 2: value == 5
At index 3: value == 7
At index 4: value == 11
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
  at Primes.main(Primes.java:5)
IO.println("primes.length == " + primes.length);
for (int i = 0; i < primes.length; i++) {
  IO.println("At index " + i + ": value == " + primes[i]);
}
Result: 
primes.length == 5
At index 0: value == 2
At index 1: value == 3
At index 2: value == 5
At index 3: value == 7
At index 4: value == 11
 
for (final int p: primes) {
  IO.println("value == " + p);
}
Result: 
value == 2
value == 3
value == 5
value == 7
value == 11
 
final int[] primes = new int[5]; // Last index is 4 rather than 5!

primes[0] = 2;
primes[1] = 3;
primes[2] = 5;
primes[3] = 7;
primes[4] = 11;
primes[5] = 13; // Excessing array limit
Result: 
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
  at Primes.main(Primes.java:25)

Combining array allocation and value assignment:

final int[]
  primes = {2, 3, 5, 7, 11};
 
final int[] primes = new int[5];
primes[0] = 2;
primes[1] = 3;
primes[2] = 5;
primes[3] = 7;
primes[4] = 11;

  • Series of objects having identical type.

  • Array consists of array elements.

  • Element access by index value.

  • Holding either primitive types or object references (Class instance or array).

  • Contiguous storage in memory.

  • Arbitrary array dimensions by virtue of nesting: One-dimensional, two-dimensional etc. 

  1. type[] arrayName; // preferred
  2. type arrayName[];
void main() {
  // Error: Array initializer is not allowed here
  double average1 = getAverage({1, 5, 20, 4});

  // Works!
  double average2 = getAverage(new int[]{1, 5, 20, 4});
}

/**
 * Compute an array's average of its values
 *
 * @param values Values to be considered
 * @return The average of all values
 */
double getAverage(int[] values) {
   ...
   return ...;
}
  1. Implementing getAverage(...).
  • Arrays
    • ➟ Multi-dimensional arrays

      Multiple dimensions by nesting of arrays.

Code Result 
// Allocation
final int[][] MATRIX = new int[2][3];

// Initialization
for (int row = 0; row < 2; row++) {
  for (int col = 0; col < 3; col++) {
    MATRIX[row][col] = col + row;
  }
}

// Output
for (int row = 0; row < 2; row++) {
  IO.println(Arrays.toString(MATRIX[row]));         
}
 
[0, 1, 2]
[1, 2, 3]
 
final int[][] MATRIX = new int[2][]; // Array containing two int arrays
MATRIX[0] = new int[3];              // first int array
MATRIX[1] = new int[3];              // second int array

Conclusions:

  • Java only supports one-dimensional arrays.

  • By means of nesting we get the illusion of multi- dimensional arrays.

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  1. 2-dimensional arrays and .length
final int[][] MATRIX = new int[][] {
  {0, 1, 2},
  {1, 2, 3}
};
Code Result 
final String[][] GROUPS = new String[][] {
   {"Jill", "Tom"},
   {"Jane", "Smith", "Joe"},
   {"Jeff"}
};

for (final String[] GROUP: GROUPS) {
   IO.print('[');
   for (final String s: GROUP) {
      IO.print(s + ' ');
   }
   IO.println(']');
}
 
[Jill Tom ]
[Jane Smith Joe ]
[Jeff ]
  1. Representing revenues by quarter and department
  2. Summing up revenues
  3. External array and string exercises