How to determine whether an array contains a parti

Question

I have a String[] with values like so:

public static final String[] VALUES = new String[] {"AB","BC","CD","AE"};

Given String s, is there a good way of testing whether VALUES contains s?

Answer 1

I am very late to join this discussion, but since my approach in solving this problem, when I faced it a few years ago, was a bit different than the other answers already posted here, I am posting that solution I used at that time, over here, in case anyone finds it usefull: (The contains() method is ArrayUtils.in() in this code.)

ObjectUtils.java

public class ObjectUtils{

/**
 * A null safe method to detect if two objects are equal.
 * @param object1
 * @param object2
 * @return true if either both objects are null, or equal, else returns false.
 */
public static boolean equals(Object object1,Object object2){
    return object1==null?object2==null:object1.equals(object2);
}

}

ArrayUtils.java

public class ArrayUtils{
/**
 * Find the index of of an object is in given array, starting from given inclusive index.
 * @param ts  Array to be searched in.
 * @param t  Object to be searched.
 * @param start  The index from where the search must start. 
 * @return Index of the given object in the array if it is there, else -1. 
 */
public static <T> int indexOf(final T[] ts, final T t, int start){
    for(int i = start; i < ts.length;++i)
        if(ObjectUtils.equals(ts[i],t))
            return i;
    return -1;
}

/**
 * Find the index of of an object is in given array, starting from 0;
 * @param ts  Array to be searched in.
 * @param t  Object to be searched.
 * @return  indexOf(ts,t,0)
 */
public static <T> int indexOf(final T[] ts, final T t){
    return indexOf(ts, t, 0);
}

/**
 * Detect if the given object is in the given array.
 * @param ts  Array to be searched in.
 * @param t  Object to be searched.
 * @return  If indexOf(ts,t) is greater than -1.
 */
public static <T> boolean in(final T[] ts, final T t){
    return indexOf(ts, t) > -1 ;
}

}

As you can see in the code above, that there are other utility methods ObjectUtils.equals() and ArrayUtils.indexOf(), that were used at other places as well.

Answer 2

You can use ArrayUtils.contains from Apache Commons Lang

public static boolean contains(Object[] array, Object objectToFind)

Note that this method returns false if the passed array is null.

There are also methods available for primitive arrays of all kinds.

Example:

String[] fieldsToInclude = { "id", "name", "location" };

if ( ArrayUtils.contains( fieldsToInclude, "id" ) ) {
    // Do some stuff.
}

Answer 3

Actually , if you use HashSet as Tom Hawtin proposed you don`t need to worry about sorting and your speed is the same as with Binary Search on a presorted array, probably even faster.

It all depends on how your code is set up, obviously, but from where I stand, the order would be:

On an UNsorted array:

1. HashSet
2. asList
3. sort & Binary

On a sorted array:

1. HashSet
2. Binary
3. asList

So either way, HashSet ftw

Answer 4

With Java 8 you can create a stream and check if any entries in the stream matches "s":

String[] values = {"AB","BC","CD","AE"};
boolean sInArray = Arrays.stream(values).anyMatch("s"::equals);

Or as a generic method:

public static <T> boolean arrayContains(T[] array, T value) {
    return Arrays.stream(array).anyMatch(value::equals);
}

Answer 5

Instead of using the quick array initialsation syntax to you could just initialise it as a List straight away in a similar manner using the Arrays.asList method e.g.:

public static final List<String> STRINGS = Arrays.asList("firstString", "secondString" ...., "lastString");

Then you can do (like above): STRINGS.contains("the string you want to find");

Answer 6

Just to clear the code up to start with. We have (corrected):

public static final String[] VALUES = new String[] {"AB","BC","CD","AE"};

This is a mutable static which FindBugs will tell you is very naughty. It should be private:

private static final String[] VALUES = new String[] {"AB","BC","CD","AE"};

(Note, you can actually drop the new String[]; bit.)

So, reference arrays are bad, and in particular here we want a set:

private static final Set<String> VALUES = new HashSet<String>(Arrays.asList(
     new String[] {"AB","BC","CD","AE"}
));

(Paranoid people, such as myself, may feel more at ease if this was wrapped in Collections.unmodifiableSet - it could even be made public.)

"Given String s, is there a good way of testing whether VALUES contains s?"

VALUES.contains(s)

O(1).