请教退出与返回问题，谢谢

package st.st.st;


//主界面
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.KeyEvent;
import android.view.View;
import android.view.Window;
import android.widget.ImageButton;
import android.widget.Toast;

public class St1Activity extends Activity {
    // 定义一个变量，来标识是否退出
    private static boolean isExit = false;
    Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            isExit = false;
        }
    };
   
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
      
        setContentView(R.layout.main);
        initButton();
    }
   
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            exit();
            return false;
        }
        return super.onKeyDown(keyCode, event);
    }

    private void exit() {
        if (!isExit) {
            isExit = true;
            Toast.makeText(getApplicationContext(), "再按一次退出程序",
                    Toast.LENGTH_SHORT).show();
            // 利用handler延迟发送更改状态信息
            mHandler.sendEmptyMessageDelayed(0, 2000);
        } else {
            finish();
            System.exit(0);
        }
    }
   
   
    private void initButton() {
        // TODO Auto-generated method stub
        ImageButton btn1=(ImageButton)findViewById(R.id.imageButton1);
        ImageButton btn2=(ImageButton)findViewById(R.id.imageButton2);
        ImageButton btn3=(ImageButton)findViewById(R.id.imageButton3);
        ImageButton btn4=(ImageButton)findViewById(R.id.imageButton4);
        ImageButton btn5=(ImageButton)findViewById(R.id.imageButton5);
        ImageButton btn6=(ImageButton)findViewById(R.id.imageButton6);
        
        //肥胖测试按钮
        btn1.setOnClickListener(new View.OnClickListener() {
            
            public void onClick(View v) {
                // TODO Auto-generated method stub
                 Intent intent = new Intent();  
                    //设置Intent对象要启动的Activity  
                    intent.setClass(St1Activity.this, St2.class);  
                    //通过Intent对象启动另外一个Activity  
                    startActivity(intent);  
                    St1Activity.this.finish();
                    
            }
        });
        
        //体重记录按钮
        btn2.setOnClickListener(new View.OnClickListener() {
            
            public void onClick(View v) {
                // TODO Auto-generated method stub
                 Intent intent = new Intent();  
                    //设置Intent对象要启动的Activity  
                    intent.setClass(St1Activity.this, St3.class);  
                    //通过Intent对象启动另外一个Activity  
                    startActivity(intent);  
                    St1Activity.this.finish();
                    
            }
        });
        
        //饮食记录按钮
        btn3.setOnClickListener(new View.OnClickListener() {
            
            public void onClick(View v) {
                // TODO Auto-generated method stub
                 Intent intent = new Intent();  
                    //设置Intent对象要启动的Activity  
                    intent.setClass(St1Activity.this, St4.class);  
                    //通过Intent对象启动另外一个Activity  
                    startActivity(intent);  
                    St1Activity.this.finish();
                    
            }
        });
        
        //运动记录按钮
       btn4.setOnClickListener(new View.OnClickListener() {
   
          public void onClick(View v) {
               // TODO Auto-generated method stub
               Intent intent = new Intent();  
                //设置Intent对象要启动的Activity  
                  intent.setClass(St1Activity.this, St5.class);  
                  //通过Intent对象启动另外一个Activity  
                  startActivity(intent);  
                  St1Activity.this.finish();
            
                  }
         });
      //加油提醒按钮
       btn5.setOnClickListener(new View.OnClickListener() {
   
             public void onClick(View v) {
               // TODO Auto-generated method stub
             Intent intent = new Intent();  
             //设置Intent对象要启动的Activity  
             intent.setClass(St1Activity.this, St6.class);  
             //通过Intent对象启动另外一个Activity  
             startActivity(intent);  
             St1Activity.this.finish();
            
     }
  });
       //退出按钮
       btn6.setOnClickListener(new View.OnClickListener() {
            
            public void onClick(View v) {
                // TODO Auto-generated method stub
                finish();
                System.exit(0);
            }
        });
        
    }
}

package st.st.st;
//二级界面
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.KeyEvent;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.Toast;

public class St2 extends Activity {
//肥胖测试
    public RadioGroup group;
    public RadioButton man,woman;
   
   
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.a);
        Button btn1=(Button)findViewById(R.id.button1);//返回主界面按钮
        Button btn2=(Button)findViewById(R.id.button2);//计算按钮
        
        //返回主界面按钮
    btn1.setOnClickListener(new View.OnClickListener() {
            
            public void onClick(View v) {
                // TODO Auto-generated method stub
                // Intent intent = new Intent();  
                    //设置Intent对象要启动的Activity  
                  //  intent.setClass(St2.this,St1Activity.class);  
                    //通过Intent对象启动另外一个Activity  
                   // startActivity(intent);  
                    St2.this.finish();
            }
        });
      //计算按钮事件
    btn2.setOnClickListener(new Button.OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            //取得输入的身高   
            EditText height=(EditText)findViewById(R.id.height);
            float height1=0;
            if(!height.getText().toString().equals("")){
                if(!height.getText().toString().substring(0,1).equals(".")){
             height1=Float.parseFloat(height.getText().toString());
                }
            }
            
            /*取得选择的性别*/
            String sex="";
            group=(RadioGroup)findViewById(R.id.radioGroup);
            man=(RadioButton)findViewById(R.id.radio0);
            woman=(RadioButton)findViewById(R.id.radio1);
            if(man.isChecked()){
                woman.setChecked(false);
                sex="M";
            }
            else{
                woman.setChecked(true);
                sex="F";
            }
        //判断身高是否为空
            
            if(height1<100.0||height1>200.0||height.equals(null)){
                  height.requestFocus();
                  height.setError("取值范围100.0~200.0");
                 }

            else if(height1>=100.0||height1>=200.0){
            Intent intent=new Intent();
            intent.setClass(St2.this, St2_1.class);
            Bundle bundle=new Bundle();
            bundle.putDouble("height", height1);
            bundle.putString("sex", sex);
            /*将Bundle对象assign给Intent*/
            intent.putExtras(bundle);
            /*调用St2_1*/
            startActivity(intent);
              }   
        }
    });
    }
   
    }