Different (unspecified) IAEs could be thrown from method AbstractRegionPainter.decodeGradient():

    java.lang.IllegalArgumentException: Colors and fractions must have equal size
    java.lang.IllegalArgumentException: Fraction values must be in the range 0 to 1: ...
    java.lang.IllegalArgumentException: Keyframe fractions must be increasing: ...
    java.lang.IllegalArgumentException: Start point cannot equalendpoint

They should be specified.
Please see the following code sample which illustrates four exceptions listed above.

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import javax.swing.*;
import java.awt.*;
import static java.awt.Color.*;

public class Test {

    public static void main(String[] args) {
        MyARP arp = new MyARP();
        // uncomment any of calls below to see different types of IAEs
// arp.callDecodeGradient(0.1f, 0.4f, 0.2f, 0.8f, new float[]{0.1f, 0.6f}, new Color[]{BLACK, MAGENTA, BLUE});
// arp.callDecodeGradient(0.1f, 0.4f, 0.2f, 0.8f, new float[]{12.1f, 0.6f}, new Color[]{BLACK, BLUE});
// arp.callDecodeGradient(0.1f, 0.4f, 0.2f, 0.8f, new float[]{.3f, 0.2f}, new Color[]{BLACK, BLUE});
// arp.callDecodeGradient(100.f, Float.MAX_VALUE, 100.f, Float.MAX_VALUE, new float[]{.1f, 0.2f}, new Color[]{BLACK, BLUE});
    }

    static class MyARP extends javax.swing.plaf.nimbus.AbstractRegionPainter {
        protected PaintContext getPaintContext() { return null; }
        protected void doPaint(Graphics2D g, JComponent c,
                               int width, int height,
                               Object[] extendedCacheKeys) { }
        void callDecodeGradient(float x1, float y1, float x2, float y2,
                                       float[] midpoints, java.awt.Color[] colors) {
            decodeGradient(x1, y1, x2, y2, midpoints, colors);
        }
    }
}
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