Java 试图在Android Studio中运行程序，但出现错误，我的应用程序已停止工作

我在Android Studio Emulator（Nexus_7_API_21）中不断发现一个错误，即每当我尝试运行它时，我的应用程序就停止工作。我启动了LogCat，我认为错误是由于空指针引用引起的，但我不知道问题出在哪里

这是我的.java主文件

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.view.View;
import android.widget.RadioButton;
import android.widget.RelativeLayout;

public class MainActivity extends ActionBarActivity
{
    RelativeLayout bg;
    RadioButton r1;
    RadioButton r2;
    RadioButton r3;
    RadioButton r4;
    Button b;

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

        bg = (RelativeLayout) findViewById(R.id.back);
        r1 = (RadioButton) findViewById(R.id.rButton1);
        r2 = (RadioButton) findViewById(R.id.rButton2);
        r1 = (RadioButton) findViewById(R.id.rButton3);
        r1 = (RadioButton) findViewById(R.id.rButton4);

        r1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
        r2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
        r3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
        r4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
    }

    @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);
    }
}

这是我的XML文件（

activity\u main.XML

）：

这是个问题。您使用的是r1，而不是r3和r4

r3 = (RadioButton) findViewById(R.id.rButton3);
r4 = (RadioButton) findViewById(R.id.rButton4);

r1 = (RadioButton) findViewById(R.id.rButton3);
r1 = (RadioButton) findViewById(R.id.rButton4);

r3 = (RadioButton) findViewById(R.id.rButton3);
r4 = (RadioButton) findViewById(R.id.rButton4);