계산기프로그램을 만들고 있습니다.
제가 convert라는 버튼을 누르면 main에서 main2로 레이아웃을 변경하도록 프로그래밍 하였습니다.
main과 main2의 버튼과 EditText개수가 모두 같아서 id를 전부 똑같이 설정해 줬습니다.
그런데 convert버튼을 누르고나면 main2로 레이아웃이 변경되지만 모든 버튼이 먹통이됩니다.
어떻게 해결해야 할까요?

package hw1.hw1;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.EditText;
import android.widget.Button;


public class hw1Activity extends Activity implements OnClickListener {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        fld=(EditText)findViewById(R.id.cal);
       
        Button button=(Button)findViewById(R.id.num1);
        button.setOnClickListener(this);
        Button button2=(Button)findViewById(R.id.num2);
        button2.setOnClickListener(this);
        Button button3=(Button)findViewById(R.id.num3);
        button3.setOnClickListener(this);
        Button button4=(Button)findViewById(R.id.num4);
        button4.setOnClickListener(this);
        Button button5=(Button)findViewById(R.id.num5);
        button5.setOnClickListener(this);
        Button button6=(Button)findViewById(R.id.num6);
        button6.setOnClickListener(this);
        Button button7=(Button)findViewById(R.id.num7);
        button7.setOnClickListener(this);
        Button button8=(Button)findViewById(R.id.num8);
        button8.setOnClickListener(this);
        Button button9=(Button)findViewById(R.id.num9);
        button9.setOnClickListener(this);
        Button button0=(Button)findViewById(R.id.num0);
        button0.setOnClickListener(this);
        Button buttoncal=(Button)findViewById(R.id.calculate);
        buttoncal.setOnClickListener(this);
        Button buttonclr=(Button)findViewById(R.id.Clr);
        buttonclr.setOnClickListener(this);
        Button buttoncon=(Button)findViewById(R.id.convert);
        buttoncon.setOnClickListener(this);
        Button buttonplus=(Button)findViewById(R.id.plus);
        buttonplus.setOnClickListener(this);
        Button buttonminus=(Button)findViewById(R.id.minus);
        buttonminus.setOnClickListener(this);
        Button div=(Button)findViewById(R.id.div);
        div.setOnClickListener(this);
        Button buttonmul=(Button)findViewById(R.id.mul);
        buttonmul.setOnClickListener(this);
    }
    boolean flag=false;
    int operation=0;
    int first=0;
    int second=0;
    double answer=0;
    String history="";
    String number1="";
    String number2="";
    EditText fld;
    EditText et;
    int len = 0;


    public void onClick(View v){
     if (v.getId()==R.id.num1){
      if(operation==0){
        number1=number1+"1";
        history=history+"1";
      }
      else if(operation!=0){
       number2=number2+"1";
       history=history+"1";
      }
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num2){
          if(operation==0){
            number1=number1+"2";
            history=history+"2";
          }
          else if(operation!=0){
           number2=number2+"2";
           history=history+"2";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num3){
          if(operation==0){
            number1=number1+"3";
            history=history+"3";
          }
          else if(operation!=0){
           number2=number2+"3";
           history=history+"3";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num4){
          if(operation==0){
            number1=number1+"4";
            history=history+"4";
          }
          else if(operation!=0){
           number2=number2+"4";
           history=history+"4";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num5){
          if(operation==0){
            number1=number1+"5";
            history=history+"5";
          }
          else if(operation!=0){
           number2=number2+"5";
           history=history+"5";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num6){
          if(operation==0){
            number1=number1+"6";
            history=history+"6";
          }
          else if(operation!=0){
           number2=number2+"6";
           history=history+"6";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num7){
          if(operation==0){
            number1=number1+"7";
            history=history+"7";
          }
          else if(operation!=0){
           number2=number2+"7";
           history=history+"7";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num8){
          if(operation==0){
            number1=number1+"8";
            history=history+"8";
          }
          else if(operation!=0){
           number2=number2+"8";
           history=history+"8";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num9){
          if(operation==0){
            number1=number1+"9";
            history=history+"9";
          }
          else if(operation!=0){
           number2=number2+"9";
           history=history+"9";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.num0){
          if(operation==0){
            number1=number1+"0";
            history=history+"0";
          }
          else if(operation!=0){
           number2=number2+"0";
           history=history+"0";
          }
          fld.setText(history);
             len = fld.length();
             fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.convert){
      if(flag==false){
      setContentView(R.layout.main2);
      flag=true;
      }
      else if(flag==true){
      setContentView(R.layout.main); 
      flag=false;
      }
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.plus){
      operation=1;
      history+="+";
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.minus){
      operation=2;
      history+="-";
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.mul){
      operation=3;
      history+="*";
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.div){
      operation=4;
      history+="/";
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
     }
     else if (v.getId()==R.id.calculate){
      if(operation==0){      
       answer=Integer.parseInt(number1);
       number1="";
      }
      else if(operation==1){
       answer=Integer.parseInt(number1)+Integer.parseInt(number2);
       history=history+"\n="+answer+"\n";
       number1="";
       number2="";
      }
      else if(operation==2){
       answer=Integer.parseInt(number1)-Integer.parseInt(number2);
       history=history+"\n="+answer+"\n";
       number1="";
       number2="";
      }      
      else if(operation==3){
       answer=Integer.parseInt(number1)*Integer.parseInt(number2);
       history=history+"\n="+answer+"\n";
       number1="";
       number2="";
      }     
      else if(operation==4){
       answer=(Integer.parseInt(number1)+0.0)/(Integer.parseInt(number2)+0.0);
       history=history+"\n="+answer+"\n";
       number1="";
       number2="";
      }
      fld.setText(history);
         len = fld.length();
         fld.setSelection(len - 1);
      operation=0;
     }
     else if (v.getId()==R.id.Clr){
      history="";
      fld.setText(history);
         fld.setSelection(0);
     }
    }
}