Latest revision as of 21:11, 8 November 2018

Problema

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.
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).
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)
        
    }

}

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