这里有一个更有效的方法：

/**
 * Sets the text size for a Paint object so a given string of text will be a
 * given width.
 * 
 * @param paint
 *            the Paint to set the text size for
 * @param desiredWidth
 *            the desired width
 * @param text
 *            the text that should be that width
 */
private static void setTextSizeForWidth(Paint paint, float desiredWidth,
        String text) {

    // Pick a reasonably large value for the test. Larger values produce
    // more accurate results, but may cause problems with hardware
    // acceleration. But there are workarounds for that, too; refer to
    // http://stackoverflow.com/questions/6253528/font-size-too-large-to-fit-in-cache
    final float testTextSize = 48f;

    // Get the bounds of the text, using our testTextSize.
    paint.setTextSize(testTextSize);
    Rect bounds = new Rect();
    paint.getTextBounds(text, 0, text.length(), bounds);

    // Calculate the desired size as a proportion of our testTextSize.
    float desiredTextSize = testTextSize * desiredWidth / bounds.width();

    // Set the paint for that size.
    paint.setTextSize(desiredTextSize);
}

然后，您所需要做的就是settextsizeforwidth(paint,400,str）；(400是问题中的示例宽度）。

为了获得更高的效率，您可以将rect设置为静态类成员，从而避免每次实例化它。但是，这可能会引入并发问题，并且可能会阻碍代码的清晰度。