如何在一个活动中使用java类？

如何在一个活动中使用 Java 类，我的意思是将该活动的不同组件分布在一堆 Java 类中。我对 Android 有点陌生，这是我迄今为止尝试过的：

MainActivity.java

package com.example.alex.myapplication;



 import android.support.v7.app.ActionBarActivity;
 import android.os.Bundle;
 import android.view.Menu;
 import android.view.MenuItem;



 public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
     new Something(this);


}





@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
  }

一些东西.java

import android.view.View;
import android.widget.EditText;
import android.widget.Button;
import android.app.Activity;




    public class Something  {

  private Activity activity;
  private Button add,subtract,multiply,devide;
  private EditText editA, editB, editC;
  private double doubleA,doubleB,doubleC;





      public Something(Activity a){

              activity=a;
              click();

       }


       public void click(){
           editA = (EditText) activity.findViewById(R.id.editText);
           editB = (EditText) activity.findViewById(R.id.editText2);
           editC = (EditText) activity.findViewById(R.id.editText3);
          doubleA =Double.parseDouble(editA.getText().toString());
          doubleB =Double.parseDouble(editB.getText().toString());


              add = (Button) activity.findViewById(R.id.add);
              subtract = (Button) activity.findViewById(R.id.subtract);
              multiply = (Button) activity.findViewById(R.id.multiply);
              devide = (Button) activity.findViewById(R.id.devide);

               add.setOnClickListener(new View.OnClickListener() {
                   @Override
                   public void onClick(View v) {

                     doubleC = doubleA+doubleB;
                     String s = "" + doubleC;
                      editC.setText(s);


                   }
               });

               subtract.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {

                   doubleC = doubleA-doubleB;
                   String s = "" + doubleC;
                   editC.setText(s);


               }
           });

               multiply.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {

                   doubleC = doubleA*doubleB;
                   String s = "" + doubleC;
                   editC.setText(s);


               }
           });



           devide.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {

                   doubleC = doubleA/doubleB;
                   String s = "" + doubleC;
                   editC.setText(s);


               }
           });



       }



       }

所以我不确定为什么我的侦听器没有在我的按钮上工作，所以我尝试将活动传递给将侦听器添加到按钮的类，但实际上这不起作用，现在我的应用程序甚至无法启动模拟器。我想做的就是让“MainActivity”处理“Gui”并让“Something”类处理侦听器，但无论我做什么，我似乎都无法使它们相互通信以形成一个活动。

LogCat

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
        at android.app.Activity.findViewById(Activity.java:2071)
        at com.example.alex.myapplication.Something.click(Something.java:32)
        at com.example.alex.myapplication.MainActivity.onCreate(MainActivity.java:19)
        at android.app.Activity.performCreate(Activity.java:5937)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
        at android.app.ActivityThread.performLaunchActivity
(ActivityThread.java:2251)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
        at android.app.ActivityThread.access$800(ActivityThread.java:144)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at   com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run           (ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

就像之前发布的带有 1 个 java 类的 XD 一样，按下按钮后再次调用它，例如，您可以有一个不可见的按钮、单选按钮，无论您想要什么不可见，只需单击一下，它就会变得可见并对您有用，我在这里：

首先控制活动的变量将要执行的操作

String num ="";

然后有你的按钮，我用了其中的两个，其他的是不可见的

Button bn1;
Button bn2;
Button bn3;
Button bn4;
bn3.setVisibility(View.INVISIBLE);
bn4.setVisibility(View.INVISIBLE);

然后按钮代码，取决于您想要多少个按钮

Button.setOnClickListener(new Button onclickListener(){
        public void onClick(){
            //get a default variable in this case String num
    Intent intent = new Intent(MainActivity.this, MainActivity.class); num="cero"; intent.putExtra("po", num);
                        CodigoPeticion=2; startActivityForResult (intent,CodigoPeticion); finish(); break;
                }
            }
        });

这个获取字符串编号：

Bundle extras = getIntent().getExtras();
        if (extras!= null) {
            num =extras.getString("po");
        }

最后但并非最不重要的是，这个根据字符串做一些事情：

    if (num.matches("cero")){
                //do something, enable more buttons, disable radiobuttons,
bn3.setVisibility(View.VISIBLE);
            }else if(num.matches("one")){//this string is from another button
                //do something else in the same activity, as you spected enable radiobuttons, show a image, etc
bn4.setVisibility(View.VISIBLE);
            }else{
                //some textview with a specific title
                TextView.setText("Something's Wrong");
            }

不要忘记 bn3 和 bn4 听众！

希望对您有帮助，再见！