double amount =200.0;
Locale locale = new Locale("en", "US");      
NumberFormat currencyFormatter = NumberFormat.getCurrencyInstance(locale);
System.out.println(currencyFormatter.format(amount));

or

double amount =200.0;
System.out.println(NumberFormat.getCurrencyInstance(new Locale("en", "US"))
        .format(amount));

The best way to display currency

Output

$200.00

Note: Locale constructors have been deprecated. See Obtaining a Locale for other options.

So, since Locale constructors are deprecated, we can use Locale.Builder() to construct a Locale object.

    double amount =200.0;
    Locale locale = new Locale.Builder().setLanguage("en").setRegion("US").build();
    NumberFormat currencyFormatter = NumberFormat.getCurrencyInstance(locale);
    System.out.println(currencyFormatter.format(amount));

or

    double amount =200.0;
    System.out.println(NumberFormat.getCurrencyInstance(new Locale.Builder().setLanguage("en").setRegion("US").build()).format(amount));

Output

$200.00

If you don't want to use sign use this method

double amount = 200;
DecimalFormat twoPlaces = new DecimalFormat("0.00");
System.out.println(twoPlaces.format(amount));

200.00

This also can be use (With the thousand separator )

double amount = 2000000;    
System.out.println(String.format("%,.2f", amount));          

2,000,000.00

I would add to answer from les2 https://stackoverflow.com/a/7828512/1536382 that I believe the number of fraction digits is not taken from the currency, it must be set manually, otherwise if client (NumberFormat) has JAPAN locale and Currency is EURO or en_US, then the amount is displayed 'a la' Yen', without fraction digits, but this is not as expected since in euro decimals are relevant, also for Japanese ;-).

So les2 example could be improved adding format.setMaximumFractionDigits(usd.getDefaultFractionDigits());, that in that particular case of the example is not relevant but it becomes relevant using a number with decimals and Locale.JAPAN as locale for NumberFormat.

    java.util.Currency usd = java.util.Currency.getInstance("USD");
    java.text.NumberFormat format = java.text.NumberFormat.getCurrencyInstance(
          java.util.Locale.JAPAN);
    format.setCurrency(usd);
    System.out.println(format.format(23.23));
    format.setMaximumFractionDigits(usd.getDefaultFractionDigits());
    System.out.println(format.format(23.23));

will output:

USD23
USD23.23

In NumberFormat code something similar is done for the initial/default currency of the format, calling method DecimalFormat#adjustForCurrencyDefaultFractionDigits. This operation is not done when the currency is changed afterwards with NumberFormat.setCurrency