[Solved] java.util.IllegalFormatConversionException

In this post, we will look into how to resolve java.util.IllegalFormatConversionException in Java. IllegalFormatConversionException is an Unchecked exception. It is thrown when the argument corresponding to the format specifier is of an incompatible type. As always, first, we will produce the exception before moving on to the solution.

Read Also: IllegalArgumentException in Java with Examples

[Fixed] java.util.IllegalFormatConversionException

Example 1: Producing the exception by using %d format specifier for double data type

We can easily produce this exception by using integer(%d) format specifier for double data type as shown below:

public class IllegalFormatConversionExceptionExample {
    public static void main(String args[]) {
      // Initializing variables    
      int x=10;
      double y=25;
      /* Below line will throw IllegalFormatConversionException
      as we are trying to print "y" double data type with %d(integer)
      format specifier */      
      System.out.printf("%d, %d ",x,y);
    }
}

Output:
10,
Exception in thread "main" java.util.IllegalFormatConversionException: d != java.lang.Double

Explanation:

The cause of this error is due to the format specifier of incompatible types. %d is used for integer in Java. Variable y is of double data type. In the above example, we were using %d for the double data type. As a result, we were getting java.util.IllegalFormatConversionException.

Solution:

The above runtime exception can be resolved by replacing %d format specifier with %f as shown below:

public class IllegalFormatConversionExceptionExample {
    public static void main(String args[]) {
      // Initializing variables    
      int x=10;
      double y=25;
      /* Below line will not throw IllegalFormatConversionException
      as we are trying to print "y" double data type with %f
      format specifier */      
      System.out.printf("%d, %f ",x,y);
    }
}

Output:
10, 25.000000

Conversions

Example 2: Producing the exception by using %c format specifier for boolean data type

We can easily produce this exception by using character(%c) format specifier for boolean data type as shown below:

public class IllegalFormatConversionExceptionExample2 {
    public static void main(String args[]) {
      // Initializing variables    
      int x=10;
      boolean y = true;
      /* Below line will throw IllegalFormatConversionException
      as we are trying to print "y" boolean data type with %c(character)
      format specifier */      
      System.out.printf("%d, %c ",x,y);
    }
}

Output:
10,
Exception in thread "main" java.util.IllegalFormatConversionException: c != java.lang.Boolean

Explanation:

Just like example 1, the cause of this error is due to the format specifier of incompatible types. %c is used for the character in Java. Variable y is of boolean data type. In the above example, we were using %c for the boolean data type. Hence, we were getting java.util.IllegalFormatConversionException.

Solution:

The above runtime exception can be resolved by replacing %c format specifier with %b as shown below:

public class IllegalFormatConversionExceptionExample2 {
    public static void main(String args[]) {
      // Initializing variables    
      int x=10;
      boolean y = true;
      /* Below line will not throw IllegalFormatConversionException
      as we are trying to print "y" boolean data type with %b(boolean)
      format specifier */
      System.out.printf("%d, %b ",x,y);
    }
}

Output:
10, true

That's all for today. Please mention in the comments in case you are still facing the exception java.util.IllegalFormatConversionException in Java.