Posted on

Knowing info of current activity

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public class BaseActivity extends Activity {
	private final String TAG = getClass().getSimpleName();
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		Log.d("BaseActivity",  TAG);
	}
}

Finish all activities

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public class ActivityCollector {
	public static List<Activity> activities = new ArrayList<Activity>();
	public static void addActivity(Activity activity) {
		activities.add(activity);
	}

	public static void removeActivity(Activity activity) {
		activities.remove(activity);
	}
	public static void finishAll() {
		for (Activity activity : activities) {
			if (!activity.isFinishing()) {
				activity.finish();
			}
		}
	}
}

public class BaseActivity extends Activity {
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		Log.d(TAG, "Msg");
		ActivityCollector.addActivity(this);
	}
	@Override
	protected void onDestroy() {
		super.onDestroy();
		ActivityCollector.removeActivity(this);
	}
}


public class ThirdActivity extends BaseActivity {
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		Log.d("ThirdActivity", "Task id is " + getTaskId());
		requestWindowFeature(Window.FEATURE_NO_TITLE);
		setContentView(R.layout.third_layout);
		Button button = (Button) findViewById(R.id.button);
		button3.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				ActivityCollector.finishAll();
			}
		});
	}
}

Start a activity

Old school

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Intent intent = new Intent(FirstActivity.this, SecondActivity.class);
intent.putExtra("param1", "data1");
intent.putExtra("param2", "data2");
startActivity(intent);

Do this

Define a method in the target activity

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public class SecondActivity extends BaseActivity {
	public static void actionStart(Context context, String data1, String data2) {
		Intent intent = new Intent(context, SecondActivity.class);
		intent.putExtra("param1", data1);
		intent.putExtra("param2", data2);
		context.startActivity(intent);
	}
	……
}

Then

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button.setOnClickListener(new OnClickListener() {
	@Override
	public void onClick(View v) {
		SecondActivity.actionStart(FirstActivity.this, "data1", "data2");
	}
});

Posted on

推荐实现方式[面试向]

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public class Singleton {

    // Private constructor suppresses   
    private Singleton() {
    }

    private static class LazyHolder {
        static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return LazyHolder.INSTANCE;
    }
}

JVM 在类的初始化阶段（即在 Class 被加载后，且被线程使用之前），会执行类的初始化。在执行类的初始化期间，JVM 会去获取一个锁。这个锁可以同步多个线程对同一个类的初始化。相比其他实现方案（如 double-checked locking 等），该技术方案的实现代码较为简洁，并且在所有版本的编译器中都是可行的。

Heads-up

有一种叫做 双重检查锁 (double-checked locking)

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public class Singleton {
    private static volatile Singleton INSTANCE = null;

    // Private constructor suppresses 
    // default public constructor
    private Singleton() {
    }

    //thread safe and performance  promote 
    public static Singleton getInstance() {
        if (INSTANCE == null) {
            synchronized (Singleton.class) {
                //when more than two threads run into the first null check same time, to avoid instanced more than one time, it needs to be checked again.
                if (INSTANCE == null) {
                    INSTANCE = new Singleton();
                }
            }
        }
        return INSTANCE;
    }
}

此种方法只能用在 JDK5 及以后版本 (注意 INSTANCE 被声明为 volatile)，之前的版本使用 “双重检查锁” 会发生非预期行为.