Effective Java 32 Use EnumSet instead of bit fields

Bit fields is used for passing around sets of constants. Such as

// Bit field enumeration constants - OBSOLETE!

public class Text {

public static final int STYLE_BOLD = 1 << 0; // 1

public static final int STYLE_ITALIC = 1 << 1; // 2

public static final int STYLE_UNDERLINE = 1 << 2; // 4

public static final int STYLE_STRIKETHROUGH = 1 << 3; // 8

// Parameter is bitwise OR of zero or more STYLE_ constants

public void applyStyles(int styles) { ... }

}

   

text.applyStyles(STYLE_BOLD | STYLE_ITALIC);

   

Disadvantage of Bit field

1. Harder to interpret a bit field than a simple int enum constant when it's printed as a number.
2. Hard to iterate over all of the elements represented by a bit field.

      

EnumSet - a modern replacement for bit fields

public class Text {

public enum Style { BOLD, ITALIC, UNDERLINE, STRIKETHROUGH }

// Any Set could be passed in, but EnumSet is clearly best

public void applyStyles(Set<Style> styles) { ... }

}

text.applyStyles(EnumSet.of(Style.BOLD, Style.ITALIC) );

   

Advantage of EnumSet

1. Richness, type safety and interoperability of Set implementation.
2. Bulk operations, such as removeAll and retainAll, are implemented using bit-wise arithmetic, just as you'd do manually for bit fields.

   

Disadvantage of EnumSet

It's not immutable before release 1.6.

   

Note

If the underlying enum type has sixty-four or fewer elements—and most do—the entire EnumSet is represented with a single long, so its performance is comparable to that of a bit field.

   

Summary

Because an enumerated type will be used in sets, there is no reason to represent it with bit fields. The EnumSet class combines the conciseness and performance of bit fields with all the many advantages of enum types described in Item 30. You can wrap an EnumSet with Collections.unmodifiable Set, but conciseness and performance will suffer.