Android App - Logic, String & Math

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以下是開發Android 過程中的一些有關"字串"及"數學運算"的技巧, 記錄在此主要是方便本人日後翻閱, 也與大家分享
(1) Textview 的文字處理

如 text1 是 Textview, 並不是String, 而是 CharSequence, 以面是一些可執行的程式

text1.setText("ABC")
text1.setText(String.valueOf(time));  //int time
text1.getText()
text1.getText().toString()   // CharSequence 變為String
text1.getText().toString().substring(0,1)  //取左邊起第1及2 個字元
text1.length()

要把文字right justify (靠右)，加入以下到main.xml
<TextView>
   ...
   android:gravity="center_vertical|right"
   ...
</TextView>

(2) primitive data types:

• byte :  A byte variable is an 8-bit signed integer between -128 and 127. Often used for arrays.
• short : A short variable is a 16-bit signed integer between -32,768 and 32,767. Again, often used for arrays.
• int : An int variable is a 32-bit signed integer between -2,147,483,648 and 2,147,483,647. This is the most commonly used “number” variable.
• long : A long variable is a 64-bit signed integer between -9,223,372,036,854,775,808 and 9,223,372,036,854,775,807. Used when the int data type isn’t big enough.
• float : A float variable is a single precision 32-bit floating point number.
• double : A double variable is a double-precision 64-bit floating point number. Use this data type for decimal values.
• boolean : A boolean variable has only two possible values: true and false. Use this data type for conditional statements.
• char : A char variable is a single 16-bit Unicode character.

(3) 數字變字串
     最簡單的方法
     int j=100;
     String x = "" + j;     
         或者
     String testString = Integer.toString(j);


(4) 字串變數字

Convert a String to an int

call the static method parseInt() on the Integer class. Below is an example:

String string = "123";  

int value = Integer.parseInt(string)

     Convert a String to a long
      call the static method parseLong() on the Long class. Below is an example:
     String string = "123";

     long value = Long.parseLong(string);  

     Convert a String to a float
    call the static method parseFloat() on the Float class. Below is an example:
    String string = "123.4";

    float value = Float.parseFloat(string);  

Convert a String to a double

     call the static method parseDouble() on the Double class. Below is an example:
    String string = "123.4";
    double value = Double.parseDouble(string);

     Convert a byte array to String
    byte[] byteArray = new byte[] {87,79,87,46,46,46};            String value = new String(byteArray);

or

    String value = new String(byteArray,"UTF-8");

(5) 數字轉換
     Float 轉 Double , 指定小數位Double USRMB;
      Float USRMBfloat = 3.145656456;                     String tempfloat2 = String.format("%.2f",USRMBfloat);
      USRMB = Double.parseDouble(tempfloat2);

     Double 轉  Float
     Double USRMB = 3.145656456;
     Float USRMBfloat;   
     USRMBfloat = USRMB.floatValue());

(6) 只讓Apps 直向顯示
可以在 AndroidManifest.xml 上加入android:screenOrientation="portrait" 即可, 例如
       <activity
            android:name=".NumberActivity"
            android:label="@string/app_name"
            android:screenOrientation="portrait">

(7) 字串處理

找出文字的長度
String test="I am happy";
int i;
i = test.length();

加入 CR/LF
String test;
test = "I am happy \nYou are happy too";

concat()
把兩個字串加在一起. 你亦可用 '+' 號把兩字串加在一起

contains()
檢查字串中是否包特定字串
if (line.contains("you")==true)  
{  
.......
}  

把句子中顯示指定字串的次數找出來
輸入句子及字串內容, 執行findsubstringoccurrence 便會把多字次數的結果回傳
public int findsubstringoccurrence(String fullstring, String findStr){   
     int lastIndex = 0;
     int count =0;
     while(lastIndex != -1){
        lastIndex = fullstring.indexOf(findStr,lastIndex);
        if (lastIndex != -1) {
            count ++;
            lastIndex+=findStr.length(); }
         }
     return count;
  }

replace(), replaceFirst(), replaceAll()
更換String 中的部份內容，replace() 已可以把句子中所有要變換的字串都一次處理
例句
String1 = String1.replace("cry", "happy");
注意 :單執行String1.replace("cry", "happy"); 是不成功的, 一定把結果放入一個新variable 內

split()
把一個長字串拆散為數個較短字串
例句
someWords = "Red Orange Yellow Green Blue Indigo";  
String[] aColors = someWords.split(" ");  
// aColors[0] contain "Red"
// aColors[1] contain Orange"
// aColors[2] contain Yellow"

以上 splits 方法在低版本 Android SKD 不支持, 可改用 StringTokenizer
StringTokenizer
StringTokenizer tokens = new StringTokenizer(someWords , " ");
String first = tokens.nextToken();
String second = tokens.nextToken();//小心如有tokens 為 NULL (零長度) StrngTokenizer 會令程式死亡自己關閉

substring()
抽出一個字串中的部份字串String test;String substr=mysourcestring.subString(startIndex,endIndex);eg.
test = "How are you?";
test = test.substring(1, 3); 從左起取1,2位字母 , 即 "ow"

toUpperCase()， toLowerCase()
改變字體的大小草
例句
String strUpperCaseVersion = strVowels.toUpperCase();  
String strLowerCaseVersion = strVowels.toLowerCase();

trim()
移走字串前後空白
例句
String notrimword = strsource.trim();

endsWith()
字串尾的字元 (尤其合中文字)
例句
Sting i="快樂";
int k=i.length(); //k=2
if ((k>1) && i.endsWith("樂")) {i = i.substring(0, k-1);}  // i 變為 "快"


取 String s 的第一位的 character, 放入chartemp
char chartemp = String.valueOf(s).charAt(0)   

charSquence 轉 String
CharSequence in = "some string";String cs = in.toString();


String 轉 charSquenceString
a = "test";
CharSequence c = a;

(8) 判斷
switch (item){
    case 1:

       break;
    case 2:

       break;
}


if (i==3) {xxxxxx;}
else if (i==4) {xxxxxx;}

(9) 在程式內改變 Activity 的 Title
setTitle("This is a new title");

(10) 比較字串
不要採用 "==" 去比較兩個字串是否一樣，因為 == 是用來比較 object references 而並非內容, 請採用
editText1.toString().equals("match string")或
String str1;
Str1.equals("match string")

(11) 把EditText 中的文字變為整數
在 XML 檔內加入 android:inputType="number" , 強制容許數字輸入.
文字導出數字
Int number = Integer.parseInt(myEditText.getText().toString()))

(12) 把數字變為文字放到EditText
myEditText.setText(String.valueOf(number));