Optimizations
Conceptually, drawing a straight black line in Java 2D can be thought of as creating a line segment, transforming it according to the current transform, stroking it to create a thin rectangle, querying this shape to compute the pixels being affected, generating the pixels using java.awt.Color.BLACK, and then compositing the results onto the screen.
However, performing this entire sequence of steps for each drawing operation would be very inefficient. Java 2D therefore optimizes common drawing operations so that many of these steps can be skipped. If the paint is a simple solid color, for instance, there is no need to actually command it to generate a list of colors to be painted. Likewise, if the default fully opaque composite is in use, actually asking it to perform the compositing operation is unnecessary and would waste effort.
Java 2D performs the minimum amount of work necessary to make it seem as if it is performing all of these steps for each operation, therefore retaining both great flexibility and high performance.
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