java - JButton array ActionListener value -

for(gbc.gridy = 3; gbc.gridy > 0; gbc.gridy--)         for(gbc.gridx = 0; gbc.gridx < 3;gbc.gridx++)           {             grid[btnnum] = new jbutton("" + (btnnum+1));             grid[btnnum].setpreferredsize(new dimension(75,75));             frame.add(grid[btnnum], gbc);             grid[btnnum].addactionlistener(this);             grid[btnnum].addkeylistener(this);             btnnum++;           } 

i have array of buttons displayed in 3x3 grid , each 1 has action listener.

public void actionperformed(actionevent e){         string output = "";         if(e.getsource() == grid[0]){             system.out.println("button 1");         } } 

is not correct?

complete example in context:

public class buttongrid implements actionlistener, keylistener{      jframe frame=new jframe(); //creates frame     jbutton[] grid; //names grid of buttons         public buttongrid(){ //constructor             frame.settitle("mpc");             frame.setlayout(new gridbaglayout()); //set layout             jbutton[] grid=new jbutton[12]; //allocate size of grid              gridbagconstraints gbc = new gridbagconstraints();             gbc.gridx = 0;             gbc.gridy = 0;             int btnnum = 0;              grid[9] = new jbutton("0");             grid[9].setpreferredsize(new dimension(75,75));             grid[10] = new jbutton("-");             grid[10].setpreferredsize(new dimension(75,75));             grid[11] = new jbutton("=");             grid[11].setpreferredsize(new dimension(75,75));             frame.add(grid[9], gbc);             gbc.gridx++;             frame.add(grid[10], gbc);             gbc.gridx++;             frame.add(grid[11], gbc);              for(gbc.gridy = 3; gbc.gridy > 0; gbc.gridy--)                 for(gbc.gridx = 0; gbc.gridx < 3;gbc.gridx++){                     grid[btnnum] = new jbutton("" + (btnnum+1));                     grid[btnnum].setpreferredsize(new dimension(75,75));                     frame.add(grid[btnnum], gbc);                     grid[btnnum].addactionlistener(this);                     grid[btnnum].addkeylistener(this);                     btnnum++;                 }             frame.setdefaultcloseoperation(jframe.exit_on_close);             frame.pack(); //sets appropriate size frame             frame.setvisible(true); //makes frame visible  }  public static void main(string[] args) {          new buttongrid();//makes new buttongrid 2 parameters       }  public void actionperformed(actionevent e){         string output = "";         if(e.getsource() == grid[0]){             system.out.println("button one");             //playsound(abc.kick);         }} 

your grid variable null in actionlistener. bet -- you're shadowing variable.

solution: make sure initialize grid field, not local grid variable

public class buttongrid implements actionlistener, keylistener{      jframe frame=new jframe();      jbutton[] grid; // grid field remains null      public buttongrid(){              frame.settitle("mpc");             frame.setlayout(new gridbaglayout());              jbutton[] grid=new jbutton[12]; //  ****** local variable ****** 

at indicated line create shadowed variable. means because redeclare grid within constructor, initialize local variable , not field declared in class leaving field null. solution: don't re-declare grid. change to:

public class buttongrid implements actionlistener, keylistener{      jframe frame = new jframe();      jbutton[] grid;       public buttongrid(){             frame.settitle("mpc");             frame.setlayout(new gridbaglayout());              grid = new jbutton[12]; //  ***** initializes field ******