public class SingleInstance {
 private SingleInstance() {
 }

 public static SingleInstance getInstance() {
   return SingleInstanceHolder.sInstance;
 }

 private static class SingleInstanceHolder {
   private static SingleInstance sInstance = new SingleInstance();
 }
}

Integer sum = 0;
for(int i=1000; i<5000; i++){
  sum+=i;
}

int result = sum.intValue() + i;
Integer sum = new Integer(result);

@Override public V put(K key, V value) {
    if (key == null) {
      return putValueForNullKey(value);
    }
    //some code here

    // No entry for (non-null) key is present; create one
    modCount++;
    if (size++ > threshold) {
      tab = doubleCapacity();
      index = hash & (tab.length - 1);
    }
    addNewEntry(key, value, hash, index);
    return null;
  }

<activity
  android:name=".MainActivity"
  android:label="@string/app_name"
  android:theme="@style/AppTheme.NoActionBar"
  android:configChanges="orientation"
>

public void onConfigurationChanged(Configuration newConfig) {
  super.onConfigurationChanged(newConfig);
  if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT) {
    setContentView(R.layout.portrait_layout);
  } else if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
    setContentView(R.layout.landscape_layout);
  }
}

Log.i(LOGTAG, "onCreate bundle=" + savedInstanceState);

public void implicitUseStringBuilder(String[] values) {
 String result = "";
 for (int i = 0 ; i < values.length; i ++) {
   result += values[i];
 }
 System.out.println(result);
}

private void printFileByLine(String filePath) {
  try {
    FileInputStream inputStream = new FileInputStream("textfile.txt");
    BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));
    String strLine;
    //Read File Line By Line
    while ((strLine = br.readLine()) != null)  {
      // Print the content on the console
      System.out.println (strLine);
    }
    br.close();
  } catch(FileNotFoundException e) {
    e.printStackTrace();
  } catch (IOException e) {
    e.printStackTrace();
  }
}

private void printFileByLine(String filePath) {
    if (!new File(filePath).exists()) {
      return;
    }
    try {
      FileInputStream inputStream = new FileInputStream("textfile.txt");
      BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));
      String strLine;
      //Read File Line By Line
      while ((strLine = br.readLine()) != null)  {
        // Print the content on the console
        System.out.println (strLine);
      }
      br.close();
    } catch(FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

private void testThread() {
  new Thread() {
    @Override
    public void run() {
      super.run();
      //do some io work
    }
  }.start();
}

public class AppConstants {
  public static final int STATE_OPEN = 0;
  public static final int STATE_CLOSE = 1;
  public static final int STATE_BROKEN = 2;

  @IntDef({STATE_OPEN, STATE_CLOSE, STATE_BROKEN})
  public @interface DoorState {}
}

private void setDoorState(@AppConstants.DoorState int state) {
  //some code
}

<service android:name=".NetworkService"
  android:process=":network"
/>

public class MyApplication extends Application {
  private static final String LOGTAG = "MyApplication";

  @Override
  public void onCreate() {
    String currentProcessName = getCurrentProcessName();
    Log.i(LOGTAG, "onCreate currentProcessName=" + currentProcessName);
    super.onCreate();
    if (getPackageName().equals(currentProcessName)) {
      //init for default process
    } else if (currentProcessName.endsWith(":network")) {
      //init for netowrk process
    }
  }

  private String getCurrentProcessName() {
    String currentProcessName = "";
    int pid = android.os.Process.myPid();
    ActivityManager manager = (ActivityManager) this.getSystemService(Context.ACTIVITY_SERVICE);
    for (ActivityManager.RunningAppProcessInfo processInfo : manager.getRunningAppProcesses()) {
      if (processInfo.pid == pid) {
        currentProcessName = processInfo.processName;
        break;
      }
    }
    return currentProcessName;
  }
}