Eliscpe - 1 year ago 174
Java Question

# Java Swing, draw a line at a specific angle?

I made a JSlider with Java that changes the angle the line needs to be slanted at.

``````angle = new JSlider(SwingConstants.HORIZONTAL, 0, 180, 90);
angle.setSize(300, 50);
angle.setLocation(650, 60);
angle.setPaintTicks(true);
angle.setPaintTrack(true);
angle.setMinorTickSpacing(10);
angle.setMajorTickSpacing(30);
angle.setPaintLabels(true);

``````

I want the code to draw that line that implements the angle from the JSlider.

Here is my code:

``````public void paintComponent(Graphics g){
super.paintComponent(g);

int angle = intAngle;
Graphics2D graphics = (Graphics2D)g;

int startX = getWidth()/2;
int startY = getHeight()/2;
int length = 200;

int endX = startX + length * (int)Math.cos(Math.toRadians(angle));
int endY = startY + length * (int)Math.sin(Math.toRadians(angle));

graphics.drawLine(startX, startY, endX, endY);

}
``````

What is the mathematics behind rotating a line given a value?

Step 1: Extend `JPanel` and override `paintComponent()`. You've mentioned you already do this step, but more info is available here.

Step 2: Get the value of your `JSlider` into your `paintComponent()` method.

Step 3: Add a listener to the `JSlider` that tells your `JPanel` to repaint itself whenever the value changes.

Step 4: Use basic trigonometry to figure out the X and Y coordinates of the line to draw, then draw it. It might look something like this:

``````public void paintComponent(Graphics g){
super.paintComponent(g);
int angle = getSliderValue(); //you have to implement this function

int startX = getWidth()/2;
int startY = getHeight()/2;
int length = 100;

int endX = startX + (int)Math.cos(Math.toRadians(angle)) * length;
int endY = startY + (int)Math.sin(Math.toRadians(angle)) * length;

g.drawLine(startX, startY, endX, endY);
}
``````
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