Summary
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Make java.awt.MultipleGradientPaint a sealed class

Problem
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java.awt.MultipleGradientPaint has only two current sub-classes, and no others can be created by applications since there are no public or protected constructors, but this is not as clear as it could be.

Solution
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Use the new JDK 17 sealed modifier on the MultipleGradientPaint class and enumerate the allowed extant sub-classes.

Specification
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 java.awt.MultipleGradientPaint

<pre>
-public abstract class MultipleGradientPaint implements Paint 
+public abstract sealed class MultipleGradientPaint implements Paint
+    permits LinearGradientPaint,
+            RadialGradientPaint 
</pre>