Method to concatenate 2 Strings in Java

Question

I have a method in Java that concatenates 2 Strings. It currently works correctly, but I think it can be written better.

public static String concat(String str1, String str2) {
  String rVal = null;
  if (str1 != null || str2 != null) {
    rVal = "";
    if (str1 != null) {
      rVal += str1;
    }
    if (str2 != null) {
      rVal += str2;
    }      
  }    
  return rVal;
}

Here are some of the requirements:

If both str1 and str2 are null, the method returns null
If either str1 or str2 is null, it will just return the not null String
If str1 and str2 are not null, it will concatenate them
It never adds "null" to the result

Can anyone do this with less code?

If I call str1.concat(str2), it will throw a NullPointerException when str1 is null. — Ryan
– Ryan, Commented Mar 17, 2010 at 13:29
This question really has 7 votes? Really? Just to be able to re-write the same thing with a ternary operator? — GreenieMeanie
– GreenieMeanie, Commented Mar 17, 2010 at 17:18

Michael Borgwardt · Accepted Answer · 2010-03-17 13:48:21Z

15

Using only plain if clauses:

public static String concat(String str1, String str2) {
    if(str1==null) return str2;
    if(str2==null) return str1;
    return str1 + str2;
}

Or, if you have a deep and passionate love for parentheses:

public static String concat(String str1, String str2) {
    if(str1==null)
    { 
        return str2;
    }
    if(str2==null) 
    {
        return str1;
    }
    return str1 + str2;
}

edited Mar 17, 2010 at 13:48

answered Mar 17, 2010 at 13:33

Michael Borgwardt

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2 Comments

Jon Skeet Over a year ago

You don't like nested conditionals - I don't like if statements without braces :)

Michael Borgwardt Over a year ago

@Jon had those at first, but compactness IMO is better in such a simple case.

Jon Skeet · Accepted Answer · 2010-03-17 13:26:38Z

14

Sure:

public static String concat(String str1, String str2) {
  return str1 == null ? str2
      : str2 == null ? str1
      : str1 + str2;
}

Note that this takes care of the "both null" case in the first condition: if str1 is null, then you either want to return null (if str2 is null) or str2 (if str2 is not null) - both of which are handled by just returning str2.

answered Mar 17, 2010 at 13:26

Jon Skeet

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8 Comments

Michael Borgwardt Over a year ago

Nested conditional operators? Seriously?

BalusC Over a year ago

Some can read it, some cannot. It's indeed more confusing for starters. I would rather have put them in a single line with parentheses. Something like return (str1 == null) ? str2 : ((str2 == null) ? str1 : (str1 + str2));

Jon Skeet Over a year ago

@Michael: Absolutely - I find it a very useful pattern when you want to test multiple conditions with a preference.

Jon Skeet Over a year ago

@BalusC: I find the parentheses add confusion whereas the line breaks show the logical structure more clearly. It's a personal preference thing though.

Hardcoded Over a year ago

urks Please add the version without conditional operator for completeness and beauty. Sometimes more code is less confusion!

|

nishu · Accepted Answer · 2010-03-17 14:22:38Z

4

import org.apache.commons.lang.StringUtils;

StringUtils.join([str1, str2]);

Joins the elements of the provided array into a single String containing the provided list of elements.

No separator is added to the joined String. Null objects or empty strings within the array are represented by empty strings.

 StringUtils.join(null)            = null
 StringUtils.join([])              = ""
 StringUtils.join([null])          = ""
 StringUtils.join(["a", "b", "c"]) = "abc"
 StringUtils.join([null, "", "a"]) = "a"

answered Mar 17, 2010 at 14:22

nishu

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1 Comment

Ryan Over a year ago

@Nishu This is close, but a simple null check is needed in case both str1 and str2 are both null.

Striker · Accepted Answer · 2010-03-17 14:48:10Z

0

Everyone seems to have missed condition 1 where if both strings are null it returns null. The simplest version to read (IMO) then becomes:

public static String concat(String str1, String str2) {  
    if(str1==null && str2==null) return null;  
    if(str1==null) return str2;  
    if(str2==null) return str1;  
    return str1 + str2;  
}

answered Mar 17, 2010 at 14:48

Striker

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1 Comment

defectivehalt Over a year ago

Both Jon and Michael's solutions would return null on the first condition (returning the other String, which is null) if both strings are null. Yours just adds an extra unnecessary test condition.

Carl Manaster · Accepted Answer · 2010-03-17 16:34:06Z

public class ConcatTest extends TestCase {

    // 1. If both str1 and str2 are null, the method returns null
    public void testBothNull() throws Exception {
        assertNull(concat(null, null));
    }

    // 2. If either str1 or str2 is null, it will just return the not null
    // String
    public void testOneNull() throws Exception {
        assertEquals("a", concat(null, "a"));
        assertEquals("b", concat("b", null));
    }

    // 3. If str1 and str2 are not null, it will concatenate them
    public void testNonNull() throws Exception {
        assertEquals("ab", concat("a", "b"));
    }

    // 4. It never adds "null" to the result (not really testable)

    public static String concat(String a, String b) {
        if (a == null && b == null)
            return null;
        return denulled(a) + denulled(b);
    }

    private static String denulled(String s) {
        return s == null ? "" : s;
    }

}

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