测试返回字符串是否为 Java 中的 null 的最佳方法

Question

I have a function that concatenate a set of strings like this:

StringBuffer sb = new StringBuffer();

sb.append(fct1());
sb.append(fct2());
sb.append(fct3());

Where fct1(), fct2() and fct3() should return a String. The problem is that I must test the returned values like this:

sb.append(fct1() == null ? "" : fct1());

because I get an exception if the value is null.

The problem is that I have many instructions like this and, above all, I can't modify these functions that return the strings(fct1, fct2 and fct3).

Is there a solution that will "sanitize" automatically my strings?

Thank you.

PS: I created a function that can do it:

public String testNullity(String aString){
aString == null ? "" : aString;
}

so that I can call it like this:

sb.append(testNullity(fct1));
sb.append(testNullity(fct2));
...

Answer 1

Another alternative might be

 public class SafeStringBuilder  {
    private StringBuilder builder = new StringBuilder();

    public SafeStringBuilder append(String s) {
        if (s != null) 
            builder.append(s);
        return this;
    }
 }

Answer 2

If you don't mind introducing a dependency, use Guava 's Joiner instead of StringBuffer :

Joiner j = Joiner.on("").skipNulls();
StringBuilder sb = new StringBuilder();
j.appendTo(sb, fct1());
j.appendTo(sb, fct2());
j.appendTo(sb, fct3());
String result = sb.toString();

// or even just
Joiner j = Joiner.on("").skipNulls();
String result = j.join(fct1(), fct2(), fct3());

---

NB In general, unless you need StringBuffer 's thread safety, you should use StringBuilder instead. Same API, better performance.

Answer 3

There is nothing whatsoever you can do to make this solution simpler except shortening the method name. (There might be a solution using aspect-oriented programming, but on the whole I don't really consider that simpler.)

Answer 4

Unfortunately your solution with testNullity() is the best you can get with Java (consider better naming though). In fact, there is already a method that does that: StringUtils.html#defaultString .

Answer 5

You can create your own wrapper around StringBuffer :

class MyStringBuffer {
    StringBuffer _sb = new StringBuffer();
    public boolean append(String s) {
        _sb.append(s==null ? "" : s);
        return s == null;
    }
    public String toString() { return _sb.toString(); }
}

Answer 6

There is no such function in the standard API, though some methods (which do other things) have it built in.

For example, System.getProperty() has a variant which takes a default value, and if it can't find the given property, it will not return null , but the given default value. You might think of providing your fct* methods with such a "default" argument, if it makes sence.

I think C# has a ?? operator which does about what you want (you would call sb.append(fct2()?? "") ), but I suppose Java will not add any new operators soon.

A better variant of your checking function would be this:

public void appendIfNotNull(StringBuffer sb, String s) {
    if(s != null) {
       sb.append(s);
    }
}

This avoids the superfluous append call with an empty string if there is nothing to append.