public void actionPerformed(ActionEvent event){
if(event.getSource()==writeButton) write();
else if (event.getSource()==clearButton) clear();
}
private void write(){
mainField.setText("hoge");
}
private void clear(){
mainField.setText("");
}

public void mouseEntered(MouseEvent e) { mouseIn = true; stateField.setText("In");}
public void mouseExited(MouseEvent e) { mouseIn = false; stateField.setText("Out");}
public void mouseClicked(MouseEvent e) {}
public void mousePressed(MouseEvent e) {}
public void mouseReleased(MouseEvent e) {}

public void keyPressed(KeyEvent e){
if(mouseIn && e.getKeyCode() == KeyEvent.VK_ENTER) write();
}
public void keyReleased(KeyEvent e) {}
public void keyTyped(KeyEvent e){}

public static void main(String[] args){
JFrame frame = new KeyTest();
}
}