Effective Java! Use EnumSet Instead of Bit Fields

Effective Java Review (90 Part Series)

Sometimes when we have an enumerated type that is primarily used in sets developers will set each value as an int or long with each being a different power of two. Something like the following:


public class TextStyles {
  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

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

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This class can then be used using bitwise ORs to combine styles. For example:

text.applyStyles(TextStyles.STYLE_BOLD | TextStyles.STYLE_UNDERLINE);

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We can also perform union and intersection operations with this setup. It also proves to be very space efficient. All of this being said, this system suffers from the same issues that we discussed in the previous chapter. While debugging this system likely suffers from even further confusion as figuring out the current state of the variables requires considering the bits of each value. Another thing that we need to consider is how many enumerated types we will ever need to determine if we need an int or long. If we need to change this type later it is difficult to change the API throughout the system.

There is a better way built into the java.util package in the form of the EnumSet class. This class implements the Set interface with all of it’s richness, type safety, and interoperability. On top of this the implementation is extremely efficient in using a single long internally if the enum holds less than 64 values. Bulk actions on the set are also performed with bitwise operations. This means that the performance is comparable to the bit method described above. This ends up being the best of both worlds.

Let’s see what our above example would look like using a proper enum and EnumSet:

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

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

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and it’s use:

text.apply(EnumSet.of(Style.BOLD, Style.UNDERLINE));

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This leads to a much safe and extensible system. This is one of the rare examples where we don’t really have any real downsides. We get to keep the performance we desire with the richer API of the collections.

Effective Java Review (90 Part Series)

原文链接：Effective Java! Use EnumSet Instead of Bit Fields

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