java - How to draw lines in an existing activity? -

in fact, want code bridge constructor game , user must able draw these lines make bridge..

already answered : i've been trying create class allow me draw lines on existing layout, app crashed when try launch activity "game" (by pressing button in main menu). logcat told : "fatal exception : main" , have nullpointerexception.

[edit] : search how lines we've created individual objects (we suppose there bitmaps canvas created), don't know how these bitmaps...

here code of activity "customview" (use draw lines) :

    package com.g70.buildmybridge;  import java.util.linkedlist;  import android.content.context; import android.graphics.bitmap; import android.graphics.canvas; import android.graphics.color; import android.graphics.paint; import android.graphics.path; import android.view.motionevent; import android.view.view;  public class customview extends view {      private paint   mpaint;     private bitmap  mbitmap;     private canvas  mcanvas;     private path    mpath;     private paint   mbitmappaint;      private linkedlist<path> paths = new linkedlist<path>();           public customview (context context) {                super(context);                setfocusable(true);                setfocusableintouchmode(true);                 mpath = new path();                mcanvas = new canvas();                mbitmappaint = new paint();                mpaint = new paint();                mpaint.setcolor(color.black);                mpaint.setstyle(paint.style.stroke);                mpaint.setstrokejoin(paint.join.bevel);                mpaint.setstrokecap(paint.cap.round);                mpaint.setstrokewidth(6);                paths.add(mpath);            }           @override            protected void ondraw(canvas canvas) {              (path p : paths){                     canvas.drawpath(p, mpaint);                 }          }          private float mx, my;         private static final float touch_tolerance = 4;       // à la touche posée         private void touch_start(float x, float y) {                  x=math.round(x/100)*100;                 y=math.round(y/100)*100;                 mpath.reset();                 // deplacement du path et positionnement en x,y                 mpath.moveto(x, y);                 mx = x;                 = y;          }          // la touche levée         private void touch_up(float x, float y) {             float dx=(x-mx);             float dy=(y-my);             if(dy>=0&dy<=50){                 y=my;             }             if(dy>=50){                 y=my+100;             }             if(dx>=0&dx<=50){                 x=mx;             }             if(dx>=50){                 x=mx+100;             }             if(dx<=0&dx>=-50){                 x=mx;             }             if(dy<=0&dy>=-50){                 y=my;             }             if(dx<=-50){                 x=mx-100;             }             if(dy<=-50){                 y=my-100;             }             mpath.lineto(x, y);             // dessin du path sur l'objet paint             mcanvas.drawpath(mpath, mpaint);             // reset du path pour ne pas avoir le chemin précédent             mpath = new path();             paths.add(mpath);             mpath.reset();          }           // lancement des methodes de touch de l'écran tactile du smartphone         @override         public boolean ontouchevent(motionevent event) {             // prend l'axe des x             float x = event.getx();             // prend l'axe des y             float y = event.gety();              switch (event.getaction()) {                 case motionevent.action_down:                     touch_start(x, y);                     invalidate();                     break;                 case motionevent.action_up:                     touch_up(x,y);                     invalidate();                     break;             }             return true;         }          /**public void changecolor(string s){             if(s=="metal"){                 mpaint.setcolor(color.gray);             }             if(s=="wood"){                 mpaint.setcolor(color.yellow);             }         }**/ } 

and here code of game :

package com.g70.buildmybridge;  import android.app.activity; import android.content.intent; import android.os.bundle; import android.widget.button; import android.widget.gridlayout; import android.widget.linearlayout; import android.view.view; import android.view.view.onclicklistener;  public class game extends activity {      protected gridlayout drawlayout;     customview customview;      @override     public void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         setcontentview(r.layout.game1);         drawlayout = (gridlayout) findviewbyid(r.id.game1);         customview = new customview(this);         drawlayout.addview(customview);         customview.requestfocus();           final button exit = (button) findviewbyid(r.id.button4);         exit.setonclicklistener(new onclicklistener() {          @override         public void onclick(view v) {         intent intent = new intent(game.this, mainactivity.class);         startactivity(intent);         }       });          /**final button undo = (button) findviewbyid(r.id.button5);         undo.setonclicklistener(new onclicklistener() {          @override         public void onclick(view v) {             customview.paths.polllast();         }       });**/       }    } 

thank in advance !

here's error in game/oncreate:

mainlayout = (linearlayout) findviewbyid(r.layout.draw); 

here must id, not layout...
so, in /res/layout/game must linearlayout id="@+id/draw"

solution: change above instruction to:

mainlayout = (linearlayout) findviewbyid(r.id.draw); 

note: it's findviewbyid, not findviewbylayout.

[edit]

you never initialize path object in customview class. in constructor, after super line:

mpath = new path(); 

[edit 2]

also these objects need initialized:

private paint mpaint; private bitmap  mbitmap; private canvas  mcanvas; private paint   mbitmappaint; 

you start using new paint object, paint, try use mbitmappaint , mpaint, instead.

[edit 3]

i found post might find interesting: android how draw smooth line following finger

[edit 4]

finally works, want bitmap created objects (each line 1 one !) make them move ... don't find on how them ! edit our code see have done !

that should question... 1 q => 1 a.
makes no sense (and adds confusion) edit question , change different one.
invalidate previous answers...