Java N-Tuple implementation

Question

I just made a Java n-tuple which is type-safe.
I\'m using some unconventional methods to achieve type-safety (I just made it for fun).

Can someone can give some

Answer 1

Kudos on learning by doing. Here are suggestions of "opportunities" for improvement:

1. Only one kind of Tuple can ever exist (once Typelock is set). This hurts reusability and scalability in programs wanting to use multiple types of Tuples unless you resort to cut-n-paste reuse (BirthdayTuple, DimensionsTuple, StreetAddressTuple, ...). Consider a TupleFactory class that accepts the target types and creates a tuple builder object to generate tuples.

2. The validity of "null" as a value in a Tuple isn't documented. I think before Typelock is set, null is allowed; but after Typelock is set, code will generate a NullPointerException - this is inconsistent. If they are not allowed, the constructor should catch it and disallow it (regardless of Typelock). If they are allowed, then the code overall (constructor, equals, hashcode, etc) needs modification to allow for it.

3. Decide whether Tuples are intended to be immutable value objects. Based on its lack of setter methods, I'd guess so. If so, then be careful of "adopting" the incoming array - lastTuple=this.arr. Even though its a var arg constructor, the constructor could be called with an array directly. The class adopts the array (keeps a reference to it) and the values in the array could be altered outside the class afterward. I'd do a shallow copy of the array, but also document the potential issue with Tuples with non-immutable values (that could be changed outside the Tuple).

4. Your equals method lacks the null check (if (obj == null) return false) and the class check (either obj instanceof Tuple or this.getClass().equals(object.getClass())). The equals idiom is well documented.

5. There's no way to view the values of a Tuple except through toString. This protects the values and the overall immutability of , but I think it limits the usefulness of the class.

6. While I realize its just an example, I wouldn't expect to use this class for something like birthdays/dates. In solution domains with fixed object types, real classes (like Date) are so much better. I would imagine this class to be useful in specific domains where tuples are first class objects.

Edit Been thinking about this. Here's my take on some code (on github + tests):

===
Tuple.java
===
package com.stackoverflow.tuple;

/**
 * Tuple are immutable objects.  Tuples should contain only immutable objects or
 * objects that won't be modified while part of a tuple.
 */
public interface Tuple {

    public TupleType getType();
    public int size();
    public <T> T getNthValue(int i);

}


===
TupleType.java
===
package com.stackoverflow.tuple;

/**
 * Represents a type of tuple.  Used to define a type of tuple and then
 * create tuples of that type.
 */
public interface TupleType {

    public int size();

    public Class<?> getNthType(int i);

    /**
     * Tuple are immutable objects.  Tuples should contain only immutable objects or
     * objects that won't be modified while part of a tuple.
     *
     * @param values
     * @return Tuple with the given values
     * @throws IllegalArgumentException if the wrong # of arguments or incompatible tuple values are provided
     */
    public Tuple createTuple(Object... values);

    public class DefaultFactory {
        public static TupleType create(final Class<?>... types) {
            return new TupleTypeImpl(types);
        }
    }

}


===
TupleImpl.java (not visible outside package)
===
package com.stackoverflow.tuple;

import java.util.Arrays;

class TupleImpl implements Tuple {

    private final TupleType type;
    private final Object[] values;

    TupleImpl(TupleType type, Object[] values) {
        this.type = type;
        if (values == null || values.length == 0) {
            this.values = new Object[0];
        } else {
            this.values = new Object[values.length];
            System.arraycopy(values, 0, this.values, 0, values.length);
        }
    }

    @Override
    public TupleType getType() {
        return type;
    }

    @Override
    public int size() {
        return values.length;
    }

    @SuppressWarnings("unchecked")
    @Override
    public <T> T getNthValue(int i) {
        return (T) values[i];
    }

    @Override
    public boolean equals(Object object) {
        if (object == null)   return false;
        if (this == object)   return true;

        if (! (object instanceof Tuple))   return false;

        final Tuple other = (Tuple) object;
        if (other.size() != size())   return false;

        final int size = size();
        for (int i = 0; i < size; i++) {
            final Object thisNthValue = getNthValue(i);
            final Object otherNthValue = other.getNthValue(i);
            if ((thisNthValue == null && otherNthValue != null) ||
                    (thisNthValue != null && ! thisNthValue.equals(otherNthValue))) {
                return false;
            }
        }

        return true;
    }

    @Override
    public int hashCode() {
        int hash = 17;
        for (Object value : values) {
            if (value != null) {
                hash = hash * 37 + value.hashCode();
            }
        }
        return hash;
    }

    @Override
    public String toString() {
        return Arrays.toString(values);
    }
}


===
TupleTypeImpl.java (not visible outside package)
===
package com.stackoverflow.tuple;

class TupleTypeImpl implements TupleType {

    final Class<?>[] types;

    TupleTypeImpl(Class<?>[] types) {
        this.types = (types != null ? types : new Class<?>[0]);
    }

    public int size() {
        return types.length;
    }

    //WRONG
    //public <T> Class<T> getNthType(int i)

    //RIGHT - thanks Emil
    public Class<?> getNthType(int i) {
        return types[i];
    }

    public Tuple createTuple(Object... values) {
        if ((values == null && types.length == 0) ||
                (values != null && values.length != types.length)) {
            throw new IllegalArgumentException(
                    "Expected "+types.length+" values, not "+
                    (values == null ? "(null)" : values.length) + " values");
        }

        if (values != null) {
            for (int i = 0; i < types.length; i++) {
                final Class<?> nthType = types[i];
                final Object nthValue = values[i];
                if (nthValue != null && ! nthType.isAssignableFrom(nthValue.getClass())) {
                    throw new IllegalArgumentException(
                            "Expected value #"+i+" ('"+
                            nthValue+"') of new Tuple to be "+
                            nthType+", not " +
                            (nthValue != null ? nthValue.getClass() : "(null type)"));
                }
            }
        }

        return new TupleImpl(this, values);
    }
}


===
TupleExample.java
===
package com.stackoverflow.tupleexample;

import com.stackoverflow.tuple.Tuple;
import com.stackoverflow.tuple.TupleType;

public class TupleExample {

    public static void main(String[] args) {

        // This code probably should be part of a suite of unit tests
        // instead of part of this a sample program

        final TupleType tripletTupleType =
            TupleType.DefaultFactory.create(
                    Number.class,
                    String.class,
                    Character.class);

        final Tuple t1 = tripletTupleType.createTuple(1, "one", 'a');
        final Tuple t2 = tripletTupleType.createTuple(2l, "two", 'b');
        final Tuple t3 = tripletTupleType.createTuple(3f, "three", 'c');
        final Tuple tnull = tripletTupleType.createTuple(null, "(null)", null);
        System.out.println("t1 = " + t1);
        System.out.println("t2 = " + t2);
        System.out.println("t3 = " + t3);
        System.out.println("tnull = " + tnull);

        final TupleType emptyTupleType =
            TupleType.DefaultFactory.create();

        final Tuple tempty = emptyTupleType.createTuple();
        System.out.println("\ntempty = " + tempty);

        // Should cause an error
        System.out.println("\nCreating tuple with wrong types: ");
        try {
            final Tuple terror = tripletTupleType.createTuple(1, 2, 3);
            System.out.println("Creating this tuple should have failed: "+terror);
        } catch (IllegalArgumentException ex) {
            ex.printStackTrace(System.out);
        }

        // Should cause an error
        System.out.println("\nCreating tuple with wrong # of arguments: ");
        try {
            final Tuple terror = emptyTupleType.createTuple(1);
            System.out.println("Creating this tuple should have failed: "+terror);
        } catch (IllegalArgumentException ex) {
            ex.printStackTrace(System.out);
        }

        // Should cause an error
        System.out.println("\nGetting value as wrong type: ");
        try {
            final Tuple t9 = tripletTupleType.createTuple(9, "nine", 'i');
            final String verror = t9.getNthValue(0);
            System.out.println("Getting this value should have failed: "+verror);
        } catch (ClassCastException ex) {
            ex.printStackTrace(System.out);
        }

    }

}

===
Sample Run
===
t1 = [1, one, a]
t2 = [2, two, b]
t3 = [3.0, three, c]
tnull = [null, (null), null]

tempty = []

Creating tuple with wrong types: 
java.lang.IllegalArgumentException: Expected value #1 ('2') of new Tuple to be class java.lang.String, not class java.lang.Integer
    at com.stackoverflow.tuple.TupleTypeImpl.createTuple(TupleTypeImpl.java:32)
    at com.stackoverflow.tupleexample.TupleExample.main(TupleExample.java:37)

Creating tuple with wrong # of arguments: 
java.lang.IllegalArgumentException: Expected 0 values, not 1 values
    at com.stackoverflow.tuple.TupleTypeImpl.createTuple(TupleTypeImpl.java:22)
    at com.stackoverflow.tupleexample.TupleExample.main(TupleExample.java:46)

Getting value as wrong type: 
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
    at com.stackoverflow.tupleexample.TupleExample.main(TupleExample.java:58)

Answer 2

Here's a truly awful n-tuple implementation that uses generics to provide compile-time type checks. The main method (provided for demo purposes) shows just how horrendous this would be to use:

interface ITuple { }

/**
 * Typed immutable arbitrary-length tuples implemented as a linked list.
 *
 * @param <A> Type of the first element of the tuple
 * @param <D> Type of the rest of the tuple
 */
public class Tuple<A, D extends ITuple> implements ITuple {

    /** Final element of a tuple, or the single no-element tuple. */
    public static final TupleVoid END = new TupleVoid();

    /** First element of tuple. */
    public final A car;
    /** Remainder of tuple. */
    public final D cdr;

    public Tuple(A car, D cdr) {
        this.car = car;
        this.cdr = cdr;
    }

    private static class TupleVoid implements ITuple { private TupleVoid() {} }

    // Demo time!
    public static void main(String[] args) {
        Tuple<String, Tuple<Integer, Tuple<String, TupleVoid>>> triple =
                new Tuple<String, Tuple<Integer, Tuple<String, TupleVoid>>>("one",
                        new Tuple<Integer, Tuple<String, TupleVoid>>(2,
                                new Tuple<String, TupleVoid>("three",
                                        END)));
        System.out.println(triple.car + "/" + triple.cdr.car + "/" + triple.cdr.cdr.car);
        //: one/2/three
    }
}

Answer 3

If you're really interested in writing type-safe containers, look into generics:

public class Tuple<T> {
  private final T[] arr;
  public Tuple (T... contents) {
    arr = contents;  //not sure if this compiles??
  }

  // etc

  public static final void main(String[] args) {
    Tuple<String> stringTuple = new Tuple<String>("Hello", "World!");
    Tuple<Integer> intTuple = new Tuple<Integer>(2010,9,4);
  }
}