爲什麼在Android如果/否則總是其他語句激活

Question

我想要保存timestamp變量中的時間戳，但總是處於活動狀態，並且timestamp變量的值發生變化。

String timestamp = ""; 

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

    if (!timestamp.equals("")) { 

     preferences = PreferenceManager.getDefaultSharedPreferences(getApplicationContext()); 
     timestamp= preferences.getString("timestamp", ""); 
     Toast.makeText(getApplicationContext(), timestamp + "iffff", Toast.LENGTH_LONG).show(); 

    } else { 
     Long tsLong = System.currentTimeMillis()/1000; 
     String ts = tsLong.toString(); 

     preferences = PreferenceManager.getDefaultSharedPreferences(getApplicationContext()); 
     SharedPreferences.Editor editor = preferences.edit(); 
     editor.putString("timestamp", ts); 
     editor.commit(); 
     timestamp = preferences.getString("timestamp", ""); 

     Toast.makeText(getApplicationContext(), timestamp + "elseee", Toast.LENGTH_LONG).show(); 

    } 

}

Answer 1

因爲timestamp是在類中聲明的，所以在調用onCreate之前，它被android破壞並重新創建。所以當你到達onCreate時timestamp總是空的。你應該把代碼檢索if外timestamp，然後做你的測試是這樣的：

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

    preferences = PreferenceManager.getDefaultSharedPreferences(getApplicationContext()); 
    timestamp= preferences.getString("timestamp", ""); 

    if (!timestamp.equals("")) { 
     Toast.makeText(getApplicationContext(), timestamp + "iffff", Toast.LENGTH_LONG).show(); 
    } else { 
     Long tsLong = System.currentTimeMillis()/1000; 
     String ts = tsLong.toString(); 
     SharedPreferences.Editor editor = preferences.edit(); 
     editor.putString("timestamp", ts); 
     editor.commit(); 
     timestamp = preferences.getString("timestamp", ""); 

     Toast.makeText(getApplicationContext(), timestamp + "elseee", Toast.LENGTH_LONG).show(); 

    } 
} 

Answer 2

String timestamp = "";這條語句初始化時間戳「」時間戳所以總是==「」因此，其他部分將會被執行。記住這個函數在這個類中執行一次。所以它總是在執行完之後執行