How to select a line

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So I'm trying to figure out how to implement a method of selecting lines or edges in a drawing area but my math is a bit lacking. This is what I got so far:

  • A collection of lines, each line has two end points (one to start and one to end the line)
  • The lines are drawn correctly on a canvas
  • Mouse clicks events are received when clicking the canvas, so I can get the x and y coordinate of the mouse pointer

I know I can iterate through the list of lines, but I have no idea how to construct an algorithm to select a line by a given coordinate (i.e. the mouse click). Anyone got any ideas or point me to the right direction?

// import java.awt.Point

public Line selectLine(Point mousePoint) {
    for (Line l : getLines()) {
        Point start = l.getStart();
        Point end = l.getEnd();
        if (canSelect(start, end, mousePoint)) {
            return l; // found line!
    return null; // could not find line at mousePoint

public boolean canSelect(Point start, Point end, Point selectAt) {
    // How do I do this?
    return false;

Best way to do this is to use the intersects method of the line. Like another user mentioned, you need to have a buffer area around where they clicked. So create a rectangle centered around your mouse coordinate, then test that rectangle for intersection with your line. Here's some code that should work (don't have a compiler or anything, but should be easily modifiable)

// Width and height of rectangular region around mouse
// pointer to use for hit detection on lines
private static final int HIT_BOX_SIZE = 2;

public void mousePressed(MouseEvent e) {
    int x = e.getX();
    int y = e.getY();

    Line2D clickedLine = getClickedLine(x, y);

* Returns the first line in the collection of lines that
* is close enough to where the user clicked, or null if
* no such line exists

public Line2D getClickedLine(int x, int y) {
int boxX = x - HIT_BOX_SIZE / 2;
int boxY = y - HIT_BOX_SIZE / 2;

int width = HIT_BOX_SIZE;
int height = HIT_BOX_SIZE;

for (Line2D line : getLines()) {
    if (line.intersects(boxX, boxY, width, height) {
        return line;
return null;


42+ Text-Editing Keyboard Shortcuts That Work Almost Everywhere, You can copy, select, or delete entire words or paragraphs with just a few key Shift+Up or Down Arrow Keys – Select lines one at a time. Select an entire line of text by holding down the "Shift" key and pressing "End", if you are at the beginning of the line, or "Home" if you are at the end of the line.

If you use the 2D api then this is already taken care of.

You can use Line2D.Double class to represent the lines. The Line2D.Double class has a contains() method that tells you if a Point is onthe line or not.

Is there a keyboard shortcut to select the entire current line in , Press Home then Shift + End . End then Shift + Home can also be used. If you need to select the line from the very beginning instead of the first indented  Select the line however you prefer (e.g., home, shift, end). Click Stop Recording (under Tools/Macros). Choose Save TemporaryMacro (under Tools/Macros). Then choose Tools/Customize/Keyboard and assign a shortcut to the macro.

Well, first off, since a mathematical line has no width it's going to be very difficult for a user to click exactly ON the line. As such, your best bet is to come up with some reasonable buffer (like 1 or 2 pixels or if your line graphically has a width use that) and calculate the distance from the point of the mouse click to the line. If the distance falls within your buffer then select the line. If you fall within that buffer for multiple lines, select the one that came closest.

Line maths here:

Shortest distance between a point and a line segment

[PDF] How to Select Text Using Keyboard Shortcuts, Select an entire line of text by holding down the "Shift" key and pressing "End", if you are at the beginning of the line, or "Home" if you are at the end of the line. 42+ Text-Editing Keyboard Shortcuts That Work Almost Everywhere. Working With Words. We’re used to the arrow, Backspace, and Delete keys working with a single character at a time. However, we can add the Ctrl Moving the Cursor. Selecting Text. Editing. Formatting.

Sorry, mathematics are still required... This is from java.awt.geom.Line2D:

public boolean contains(double x, double y)

Tests if a specified coordinate is inside the boundary of this Line2D. This method is required to implement the Shape interface, but in the case of Line2D objects it always returns false since a line contains no area.

Specified by: contains in interface Shape

Parameters: x - the X coordinate of the specified point to be tested y - the Y coordinate of the specified point to be tested

Returns: false because a Line2D contains no area.

Since: 1.2

I recommend Tojis answer

Select text - Word - Office Support, You can also select a specific word, line of text, or one or more paragraphs. Place your cursor in front of the first letter of the word, sentence, or paragraphs you  To select a line of text, place your cursor at the start of the line, and press Shift + down arrow. To select a paragraph, place your cursor at the start of the paragraph, and press Ctrl + Shift + down arrow.

Keyboard and Mouse Shortcuts to Select and Edit Text, If the lines in the rectangular selection are of irregular length, hold down the Alt key and press the down arrow key until you've reached the longest line. Then press  Selecting Vertices, Edges, and Faces in Blender. Right-click any vertex to select it. Select and deselect multiple vertices by Shift+right-clicking them. Select large groups of vertices by using the Border Select tool (B), Circle Select (C), or Lasso Select (Ctrl+left-click+drag). In Border and

How to Select a Line in a Word Document, One of the most common tasks people perform in Microsoft Office Word 2010 is selecting a line. Whether you want to copy the line or edit it, you must first select it​  If you want to highlight a whole line of text, move your cursor to the start of the line, hold the Shift key, and then press the down arrow. You may also use the shortcut key combination Shift+End. You may also use the shortcut key combination Shift+End.

Is there any shortcut to select the current line in Visual Studio , If you want to copy a line, simply place cursor somewhere in that line and hit CTRL + C. To cut an entire line CTRL + X. @Sean found what I  Windows shortcut. This shortcut will select the entire row of the current selection. If the current selection spans multiple rows, all rows that intersect the selection will be selected. 222 Excel Shortcuts for Windows and Mac.

  • Or just see if ptLineDist() is less than a certain threshold. You'll get less strange results that way for larger "buffers sizes".
  • Nice solution - I hadn't heard of the ptLineDist function before.
  • Wow, didn't know about the 2D api. That api will take care of a bunch of other things I had problems with before… if I just knew about it before. (>_<);
  • .. are you sure? As I read from the API, contains() always returns false because Line2D has no area
  • I think you can still use the intersect() method if you use a small rectangle as a selection area
  • Thats right. By definition, contains() does not work with Line2D but deciding if the line intersects a very small rectangle around the click will work.
  • Is there any way to make the selection area as thick as the stroke width?