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Design for thread safety

Design tips on when and how to use synchronization, immutable objects, and thread-safe wrappers

By Bill Venners, JavaWorld.com, 08/01/98

Page 6 of 7

RGBColor # 3: Thread safety through immutability

Here's an immutable version of RGBColor:

// In file threads/ex3/RGBColor.java
// Instances of this immutable class
// are thread-safe.
public class RGBColor {
    private final int r;
    private final int g;
    private final int b;
    public RGBColor(int r, int g, int b) {
        checkRGBVals(r, g, b);
        this.r = r;
        this.g = g;
        this.b = b;
    }
    /**
    * returns color in an array of three ints: R, G, and B
    */
    public int[] getColor() {
        int[] retVal = new int[3];
        retVal[0] = r;
        retVal[1] = g;
        retVal[2] = b;
        return retVal;
    }
    public RGBColor invert() {
        RGBColor retVal = new RGBColor(255 - r,
            255 - g, 255 - b);
        return retVal;
    }
    private static void checkRGBVals(int r, int g, int b) {
        if (r < 0 || r > 255 || g < 0 || g > 255 ||
            b < 0 || b > 255) {
            throw new IllegalArgumentException();
        }
    }
}

Note that the setColor() method is simply removed, as it doesn't make sense in an immutable RGBColor object. The getColor() method, which reads the instance variables, is identical to what it has been, except now it doesn't have to be synchronized. The invert() method, which writes to the instance variables, is changed. Instead of inverting the current object's color, this new invert() creates a new RGBColor object that represents the inverse of the object upon which invert() is invoked, and returns a reference to that object.