There is a slight difference between these two formats. The "#.##" means it will print the number with maximum two decimal places whereas "#.00" means it will always display two decimal places and if the decimal places are less than two, it will replace them with zeros. see the example below with output.

public static final DecimalFormat df1 = new DecimalFormat( "#.##" );
public static final DecimalFormat df2 = new DecimalFormat( "#.00" );

System.out.println(df1.format(7.80));
System.out.println(df1.format(85));
System.out.println(df1.format(85.786));

System.out.println(df2.format(7.80));
System.out.println(df2.format(85));
System.out.println(df2.format(85.786));

And the output will be

7.8
85
85.79

7.80
85.00
85.79

One of the way would be using NumberFormat.

NumberFormat formatter = new DecimalFormat("#0.00");     
System.out.println(formatter.format(4.0));

Output:

4.00

Yes, one of the changes was updating Locale Data to Unicode CLDR v33 (see https://www.oracle.com/java/technologies/javase/jdk-11-relnote.html)

This change is relevant for locale-specific formatting, which impacts the usage of the utils like DecimalFormat or SimpleDateFormat.

So if your code depends on locale-specific formatting, you should double check it after switching to Java 11.