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Revision as of 17:04, 23 October 2013
Problema
- Considere a tabela da Aula Prática 04. 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.
<java5>
/**
* 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
{
/**
* 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();
}
}
</java5>
2. Comparadores de Máximo e Comprimento
Notar que as classes internas são static e que apenas estão contidas na classe Table por conveniência de ocultação de código.
<java5>
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 {
/**
* A comparator for tables: based on maximum value.
*
* Use as: Table.MAX_COMPARATOR
*/
public final static Comparator MAX_COMPARATOR = new MaxComparator();
/**
* A comparator for tables: based on length.
*
* Use as: Table.LENGTH_COMPARATOR
*/
public final static Comparator LENGTH_COMPARATOR = new LengthComparator();
/**
* This is a private class implementing the comparator.
*/
private static class MaxComparator implements Comparator {
/**
* @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 {
/**
* @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();
}
}
</java5>
3. Comparador de Tabelas de Gatos
(a publicar)
Exemplo de Aplicação
<java5>
/**
* Sample uses.
*/
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)
}
}
</java5>[[category:PO Exemplos]