Wiki**3

Problema

1. Considere a tabela da Aula Prática 05. Redefina a classe como sendo comparável. Diz-se que uma tabela é menor/igual/maior que outra quando se verificar a relação menor/igual/maior entre as somas dos elementos de cada uma.
2. Considere ainda o caso da alínea anterior. Defina agora dois comparadores (devem ser implementados como classes internas da tabela) que estabelecem as relações de comparação relativamente (i) aos máximos das tabelas e (ii) ao número de elementos (independentemente dos valores).
3. Considere agora, em lugar de tabelas de inteiros, que as tabelas contêm gatos (classe definida acima). Altere o código da alínea 2(i) para que funcione com as tabelas de gatos.

Solução

1. Tabela comparável

Para este caso, é necessário definir a classe Table como implementando Comparable.

Ficheiro Table.java

/** * A table holding a fixed number of integers. * * It is possible to verify certain predicates against the table's contents. */ public class Table implements Comparable<Table> { /** * Space for a fixed number of integers. */ int _vector[]; /** * @param nInts * number of integers to store. */ public Table(int nInts) { _vector = new int[nInts]; } /** * FIXME: insert checks to ensure position is within range. * * @param position * position to define * @return value at position */ public int getValue(int position) { return _vector[position]; } /** * FIXME: insert checks to ensure position is within range. * * @param position * position to define * @param value * value to set */ public void setValue(int position, int value) { _vector[position] = value; } /** * Set all positions to the same value. * * @param value * value to set */ public void setAll(int value) { for (int position = 0; position < _vector.length; position++) _vector[position] = value; } /** * @param predicate * the predicate to validate. * @return true, if the predicate is valid for at least one position; false, * otherwise. */ public boolean contains(SelectionPredicate predicate) { for (int position = 0; position < _vector.length; position++) if (predicate.ok(_vector[position])) return true; return false; } /** * This method makes it easy to get the sum of all elements. As with the * predicate, this type of algorithm could also be provided from outside: * instead of a selector, a collector would have to be provided. * * @return sum of all elements. */ public int getSum() { int sum = 0; for (int i : _vector) sum += i; return sum; } /** * @see java.lang.Comparable#compareTo(java.lang.Object) */ @Override public int compareTo(Table other) { return getSum() - other.getSum(); } }

2. Comparadores de Máximo e Comprimento

Diagrama de classes
(por lapso, o diagrama UML omite o método contains, mas o resto está correcto)

Notar que as classes internas são static e que apenas estão contidas na classe Table por conveniência de ocultação de código.

Ficheiro Table.java

import java.util.Comparator; /** * A table holding a fixed number of integers. * * It is possible to verify certain predicates against the table's contents. */ public class Table implements Comparable<Table> { /** * A comparator for tables: based on maximum value. * * Use as: Table.MAX_COMPARATOR */ public final static Comparator<Table> MAX_COMPARATOR = new MaxComparator(); /** * A comparator for tables: based on length. * * Use as: Table.LENGTH_COMPARATOR */ public final static Comparator<Table> LENGTH_COMPARATOR = new LengthComparator(); /** * This is a private class implementing the comparator. */ private static class MaxComparator implements Comparator<Table> { /** * @see java.util.Comparator#compare(java.lang.Object, java.lang.Object) */ @Override public int compare(Table table1, Table table2) { return table1.getMax() - table2.getMax(); } } /** * This is a private class implementing the comparator. */ private static class LengthComparator implements Comparator<Table> { /** * @see java.util.Comparator#compare(java.lang.Object, java.lang.Object) */ @Override public int compare(Table table1, Table table2) { return table1.getLength() - table2.getLength(); } } /** * Space for a fixed number of integers. */ int _vector[]; /** * @param nInts * number of integers to store. */ public Table(int nInts) { _vector = new int[nInts]; } /** * FIXME: insert checks to ensure position is within range. * * @param position * position to define * @return value at position */ public int getValue(int position) { return _vector[position]; } /** * FIXME: insert checks to ensure position is within range. * * @param position * position to define * @param value * value to set */ public void setValue(int position, int value) { _vector[position] = value; } /** * Set all positions to the same value. * * @param value * value to set */ public void setAll(int value) { for (int position = 0; position < _vector.length; position++) _vector[position] = value; } /** * @param predicate * the predicate to validate. * @return true, if the predicate is valid for at least one position; false, * otherwise. */ public boolean contains(SelectionPredicate predicate) { for (int position = 0; position < _vector.length; position++) if (predicate.ok(_vector[position])) return true; return false; } /** * This method makes it easy to get the sum of all elements. As with the * predicate, this type of algorithm could also be provided from outside: * instead of a selector, a collector would have to be provided. * * @return sum of all elements. */ public int getSum() { int sum = 0; for (int i : _vector) sum += i; return sum; } /** * This method makes it easy to get the maximum of all elements. As with the * predicate, this type of algorithm could also be provided from outside: * instead of a selector, a collector would have to be provided. * * @return maximum value in the table. */ public int getMax() { int max = _vector[0]; for (int i : _vector) if (i > max) max = i; return max; } /** * This method makes it easy to get the number of elements. As with the * predicate, this type of algorithm could also be provided from outside: * instead of a selector, a collector would have to be provided. * * In this case, we are not actually iterating, since it is much easier and * efficient to access the length attribute. * * @return length of table. */ public int getLength() { return _vector.length; } /** * @see java.lang.Comparable#compareTo(java.lang.Object) */ @Override public int compareTo(Table other) { return getSum() - other.getSum(); } }

3. Comparador de Tabelas de Gatos

(a publicar)

Exemplo de Aplicação

Ficheiro Application.java

public class Application { /** * @param args */ public static void main(String[] args) { Table t1 = new Table(3); // table with 3 integers Table t2 = new Table(3); // table with 3 integers t1.setAll(3); t2.setAll(90); System.out.println(t1.compareTo(t2)); // <0 System.out.println(Table.MAX_COMPARATOR.compare(t1, t2)); // <0 System.out.println(Table.MAX_COMPARATOR.compare(t1, t1)); // 0 System.out.println(Table.MAX_COMPARATOR.compare(t2, t2)); // 0 System.out.println(Table.MAX_COMPARATOR.compare(t2, t1)); // >0 System.out.println(Table.LENGTH_COMPARATOR.compare(t1, t2)); // 0 (same length) } }