Make String.format("%s", arg) display null-valued arguments differently from "null"

Question

Consider the custom toString() implementation of a bean:

@Override
public String toString() {
    String.format("this is %s", this.someField);
}

This yields this is null if someField is null.

Is there a way to override the default null string representation of null-valued arguments to another text, i.e., ? without calling explicitly replaceAll(...) in the toString method?

Note: The bean inherits from a superclass that could implement Formattable (http://docs.oracle.com/javase/7/docs/api/java/util/Formattable.html) but I just don't seem to understand how to make this work.

EDIT: The snippet is over-simplified for the sake of example but I'm not looking for ternary operator solutions someField==null ? "?" : someField because:

• there can be (potentially) a great many fields involved in toString() so checking all fields is too cumbersome and not fluent.
• other people whom I have little control over (if any) are writing their own subclasses.
• if a method is called and returns null that would either imply calling the method twice or declaring a local variable.

Rather, can anything be done using the Formattable interface or having some custom Formatter (which is final btw.)?

Ungrateful

I would do String.format("this is %s", this.someField==null?"?":this.someField); – Nonfiction

Or with Guava : String.format("this is %s", Objects.firstNotNull(this.someField, "?")); – Nonfiction

I always think fondly of firstNotNull, esp. similar implementations some SQL dialects, but Guava is too heavy a dependence and belongs more to the dependencies a Utils class than that of a pojo, imo. – Ungrateful

@Ungrateful There are some great solutions. Sure you can't choose one to accept? – Rambert

@KimKern Most, if not all, somehow address the issue and ultimately produce a result compatible with the question, albeit at some expense. That being said, I objectively can't assert that I can pick one answer as truly superior, hence: no, at this stage I'm afraid I can't. – Ungrateful

Answer 1

With java 8 you can now use Optional class for this:

import static java.util.Optional.ofNullable;
...
String myString = null;
System.out.printf("myString: %s",
    ofNullable(myString).orElse("Not found")
);

Inlet

To apply for also 'empty' values, it can be extended with 'filter(s -> !s.isEmpty())' as the following: System.out.printf("myString: %s", ofNullable(myString).filter(s -> !s.isEmpty()).orElse("Not found") – Direct

Answer 2

For a Java 7 solution that doesn't require external libraries:

String.format("this is %s", Objects.toString(this.someField, "?"));

Darwin

With Java 17 you could use: "this is %s".formatted("this is %s", Objects.toString(this.someField, "?")) – Detradetract

Answer 3

The nicest solution, in my opinion, is using Guava's Objects method, firstNonNull. The following method will ensure you will print an empty string if someField is ever null.

String.format("this is %s", MoreObjects.firstNonNull(this.someField, ""));

Guava docs.

Rawdon

String.format("this is %s", StringUtils.defaultIfBlank(this.someField, "")); will do the trick if Apache StringUtils happens to be on your classpath. – Reflexive

Answer 4

A bit late on the subject, but this could be a quite clean-looking solution : First, create your own format method...

private static String NULL_STRING = "?";

private static String formatNull(String str, Object... args){
    for(int i = 0; i < args.length; i++){
        if(args[i] == null){
            args[i] = NULL_STRING;
        }
    }

    return String.format(str, args);
}

Then, use it as will...

@Test
public void TestNullFormat(){
    Object ob1 = null;
    Object ob2 = "a test";

    String str = formatNull("this is %s", ob1);
    assertEquals("this is ?", str);

    str = formatNull("this is %s", ob2);
    assertEquals("this is a test", str);
}

This eliminates the need for multiple, hard-to-read, ternary operators.

Thoreau

... but implies a dependency on your formatNull method. – Ungrateful

Of course! Unless the case forementionned is specific to some output strategy class, where the method can be defined privately in the strategy itself. Then there would be no depenedency problem. If it is used at many places in the application, then yes it would need to be put in some library / util class. – Thoreau

Answer 5

If you don't want to use replaceAll(), You can assign a default text(String) for someField.

But if some time this may assign null again. So you can use validation for that case

 this.someField == null ? "defaultText" : this.someField

Am‚lie

Answer 6

To avoid repeating ternary operator you can wrap it in more readable method that will check if your object is null and return some default value if it is true like

static <T> T changeNull(T arg, T defaultValue) {
    return arg == null ? defaultValue : arg;
}

usage

String field = null;
Integer id = null;
System.out.printf("field is %s %n", changeNull(field, ""));
System.out.printf("id is %d %n", changeNull(id, -1));
System.out.printf("id is %s %n", changeNull(field, ""));

output:

field is  
id is -1 
id is  

Ichnology

Answer 7

You could just do

String.format("this is %s", (this.someField==null?"DEFAULT":this.someField));

Distinguishing

Again, this is using the ternary operator. Thanks anyway. – Ungrateful

Answer 8

public static String format(String format, Object... args){
    for (int i=0;i<args.length;i++){
        if (args[i]==null) args[i]="";
    }
    return String.format(format,args);
}

then use the method ，ok

Tengdin

Answer 9

From java 7, you can use Objects.toString(Object o, String nullDefault).

Applied to your example: String.format("this is %s", Objects.toString(this.someField, "?"));

Clementineclementis

Answer 10

To keep the original value of someField (in case null is a valid value), you can use a ternary operator.

String.format("This is %s", (this.someField == null ? "unknown" : this.someField));

Haws