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

问题

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.)?

回答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")
);

回答2:

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

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

回答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.

回答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.

回答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

回答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

回答7:

You could just do

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

回答8:

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, "?"));

回答9:

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));

回答10:

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

来源：https://stackoverflow.com/questions/20404710/make-string-formats-arg-display-null-valued-arguments-differently-from-nu

标签

java

string

null

string-formatting

string.format