Android为什么使用bindService

public class MainActivity extends AppCompatActivity {

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

        Intent intent = new Intent(this, TestService.class);
        startService(intent);
    }
    //点击按钮 调用服务里面的方法
    public void click(View v) {
        //自己new 对象 脱离了谷歌框架 脱离了环境 没有上下文
        TestService testService = new TestService();
        testService.methodService();
    }
}

public class TestService extends Service {
    public TestService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
//        throw new UnsupportedOperationException("Not yet implemented");
    return null;
    }
    //在服务里面定义的方法
    public void methodService() {
        Toast.makeText(getApplicationContext(), "haha", Toast.LENGTH_SHORT).show();
    }
}

点击按钮报错，

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)