Use interface references to collections Page

Use interface references to Collections

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Use Java interface references to Java Collections - http://www.javapractices.com/topic/TopicAction.do?Id=26

In general, references to Java objects should be as generic as possible. The user of such a reference will be protected from possible changes to the underlying implementation Java class|class. The ripple effects of such a change are limited to the single line of code which creates the object, and will not propagate any further.

In the case of Java collections|collections, this means habitually referring to collection objects using Java List | List, Java Map | Map, Java Set | Set, Java Queue | Queue, and Java Deque | Deque interface references.

Note as well that all of these except Map can be referred to using the even more generic Java Collection | Collection.

Example

import java.util.*;

public class Nucleus {

public static void main (String... args) {
Nucleus lithium = new Nucleus (3,4);
//note the generic Map reference is used here, not LinkedHashMap
Map quarks = lithium.getQuarkSummary();
log("Number of up quarks in lithium nucleus: " + quarks.get("Up"));
log("Number of down quarks in lithium nucleus: " + quarks.get("Down"));
}

public Nucleus(int numProtons, int numNeutrons) {
this.numProtons = numProtons;
this.numNeutrons = numNeutrons;
}

/** Note this get method is final. */
public final int getNumProtons() {
return numProtons;
}

/** Note this get method is final. */
public final int getNumNeutrons() {
return numNeutrons;
}

/**
* This method returns a Map which summarizes how many quarks of each
* flavour are in the nucleus.
*
* @return a generic Map reference, instead of a LinkedHashMap; the
* user will be protected from the detail of what implementation of Map
* has been selected here.
*/
public final Map getQuarkSummary() {
LinkedHashMap result = new LinkedHashMap<>();
int numUp =
numProtons * UP_QUARKS_PER_PROTON +
numNeutrons * UP_QUARKS_PER_NEUTRON
;
int numDown =
numProtons * DOWN_QUARKS_PER_PROTON +
numNeutrons * DOWN_QUARKS_PER_NEUTRON
;
//this makes use of auto-boxing of ints into Integers:
result.put("Up", numUp);
result.put("Down", numDown);
return result;
}

//PRIVATE
private final int numProtons;
private final int numNeutrons;

private static final int UP_QUARKS_PER_PROTON = 2;
private static final int DOWN_QUARKS_PER_PROTON = 1;

private static final int UP_QUARKS_PER_NEUTRON = 1;
private static final int DOWN_QUARKS_PER_NEUTRON = 2;

private static void log(String aMessage){
System.out.println(aMessage);