本文分析CharSequence及其幾個子類，如String、StringBuilder、StringBuffer、Editable。

1、通過反編譯來比較String和StringBuilder效能

反編譯：通過對可執行程式逆向分析，推匯出他人軟體的結構、思路、演算法、原理，甚至原始碼。

javac將Java原始碼編譯成位元組碼（二進位制檔案），該class檔案由JVM來執行。Java class可以執行在任何支援JVM的硬體平臺和作業系統。

package com.qinuli.demo;
//package語句後面必須帶分號
//函式名必須緊跟返回值型別
//在code目錄執行javac -d .\bin verbose com\qinuli\demo\First.java
//在bin目錄，生成類似等級結構。執行java -verbose com.qinuli.demo.First
public class First{
	public static void main(String[] args){
		System.out.println("Very good!");
	}
}
 

javap用來檢視Java編譯器生成的位元組碼。可以檢視Java程式碼在底層如何執行，判斷是否高效能以及如何提高效能。

package com.qinuli.demo;

public class First{
	public static void main(String[] args){
		System.out.println("Very good!");
		print();
		printProtected();
	}
	private static void print(){
		System.out.println("private method");
	}
	protected static void printProtected(){
		System.out.println("protected method");
	}
}
 

-c除了方法宣告，輸出所有位元組碼指令。

-public只輸出public的屬性和方法，類似protected輸出protected和public，private輸出private、protected和public。

>將內容輸出到指定路徑檔案。

若彈出“編碼GBK的不可對映字元”，則可用用notepad.exe開啟Java檔案並另存為ANSI編碼。

1)具體指令為

javac Hello.java

java Hello

javap -c -private Hello

編譯指令含義：

ldc常量入棧，

astore_1出棧，

aload_1變數入棧

2)具體程式碼為

public class Hello{
	static String s = "hehe";
	public static void main(String[] args){
		String str = "a"+"c";
		String str2 = "b";
		str = str+"Google"+"XX"+str2+"YY"+s;
		//str += "Baidu";
	}
	String field = "grape";
	private void initialize(){
		String ss = "hh"+field;
		ss += "A";
		ss += "B";
		ss += "C";
	}
	private void initialize2(){
		StringBuilder sb = new StringBuilder();
		sb.append("A");
		sb.append("B");
		sb.append("C");
	}
}

4)String執行相加操作，需要先例項化一個StringBuilder，累加完後，再執行toString。因此效能不及直接用StringBuilder

2、StringBuffer使用及String、StringBuffer、StringBuilder的差別

StringBuffer stringBuffer = new StringBuffer("laugh loud");//例項化
stringBuffer.append("good");//追加
stringBuffer.reverse();//翻轉
stringBuffer.delete(0, stringBuffer.length());//刪除
System.out.println(stringBuffer.toString());

2)StringBuffer是執行緒安全的，所以效率弱於StringBuilder

3、String常用方法

String str = "a.b.c.d.e.f";
//取代所有相應字元
Log.println(Log.INFO, TAG, str.replace('.', '_'));
//取代所有相應字串
Log.println(Log.INFO, TAG, str.replace(".", "_"));
//target用regular expression
Log.println(Log.INFO, TAG, str.replaceAll("\\.[b-d]{1}", "_"));
//只取代首個
Log.println(Log.INFO, TAG, str.replaceFirst("\\.", "#"));
//兩個字串相應區域匹配
Log.println(Log.INFO, TAG, "regionMatches:"+str.regionMatches(true, 1, ".b.c", 0, 3));

//String繼承3個介面(Serializable, Comparable<String>, CharSequence)
String str = null;
//全部大寫或小寫，將字串轉換為字元陣列
str.toLowerCase();//String
str.toUpperCase();//String
str.toCharArray();//char[]

//方便給字串排序，可忽略大小寫
str.compareTo("");//String
str.compareToIgnoreCase("");//String

//判斷字串是否包含指定子串
str.contains("");//CharSequence, chars
str.substring(0,1);//String
str.subSequence(0, 1);//CharSequence

//判斷倆字串是否相等
str.equals("");//Object
str.equalsIgnoreCase("");//String
str.contentEquals("");//CharSequence, chars

輸出結果：

2)示例講解String的一些方法

int的二進位制、八進位制、十進位制、十六進位制輸出；wrapper包含的幾個常量；

System.arrayCopy, Arrays.toString, getBytes(), String.valueOf, matches等方法。

public class Client {
	public static void main(String[] args) {
		//補碼錶示
		System.out.println("Integer.MIN_VALUE:"+Integer.toBinaryString(Integer.MIN_VALUE));
		System.out.println("Integer.MAX_VALUE:"+Integer.toBinaryString(Integer.MAX_VALUE));
		System.out.println("Integer.MIN_VALUE:"+Integer.toOctalString(Integer.MIN_VALUE));
		System.out.println("Integer.MIN_VALUE:"+Integer.toHexString(Integer.MIN_VALUE));
		System.out.println("Integer.MAX_VALUE:"+Integer.toString(Integer.MAX_VALUE, 8));
		System.out.println("Integer.MAX_VALUE:"+Integer.toString(Integer.MAX_VALUE));//radix=10
		
//		Float.SIZE;Float.MAX_VALUE;Float.MIN_VALUE;
//		Double.SIZE;Double.MAX_VALUE;Double.MIN_VALUE
//		Character.SIZE;Character.MAX_VALUE;Character.MIN_VALUE
//		Boolean.TYPE;Boolean.TRUE;Boolean.FALSE
//		Byte.MAX_VALUE;Byte.MIN_VALUE;Byte.SIZE
//		Short.MAX_VALUE;Short.MIN_VALUE;Short.SIZE
//		Long.MAX_VALUE;Long.MIN_VALUE;Long.SIZE
		
		System.out.println("陣列拷貝:");
		int[] arr = new int[]{1,2,3,4,5};
		int[] arr2 = new int[]{6,7,8,9,10};
		System.arraycopy(arr, 0, arr2, 2, 2);
		for(int j:arr2){
			System.out.print(j+",");
		}
		
		//列印陣列
		System.out.println(Arrays.toString(new byte[]{-127,-128,127}));
		byte[] multiple = new byte[]{'A','C',-128,-127,-64,127,64};
		System.out.println(Integer.toBinaryString(multiple[0]));//0100,0001
		StringBuilder sb = new StringBuilder();
		for(byte single:multiple){
			sb.append(single);//byte到int轉換
		}
		System.out.println(Arrays.toString(sb.toString().getBytes()));
		
		System.out.println(sb);//向上轉型為Object
		Client client = new Client();
		System.out.println(String.valueOf(client));//相當於toString
		
		String[] filenames = {"1.htm","1.1.htm","1.1.1.htm","1.2.htm","2.htm","2.1.htm",
				"4.htm","10.htm","8.htm","8.html"};
		for(int j=0;j<filenames.length;j++){
			if(filenames[j].matches("[0-9]+\\.htm")){
				System.out.println("篇："+filenames[j]);
			}else if(filenames[j].matches("[0-9]+\\.[0-9]+\\.htm")){
				System.out.println("章："+filenames[j]);
			}else if(filenames[j].matches("[0-9]+\\.[0-9]+\\.[0-9]+\\.htm")){
				System.out.println("節："+filenames[j]);
			}
		}
		for(int j=0;j<filenames.length;j++){
			if(filenames[j].matches("1\\..+\\.htm")){
				//有下級目錄
				break;
			}
		}
	}
}

3）字串拼接

String string = "You son of bitch!";
string = string.concat("-").concat("_");
char[] chars = new char[string.length()];
string.getChars(0, string.length(), chars, 0);//將字串內容賦值到字元陣列
String newString = new String(chars, 0, chars.length);//將字元陣列轉化為字串
Log.d(TAG, "newString-"+newString);

4）字串池

//利用物件池，提高效率，減少記憶體佔用
//new並傳參相當於建立了倆物件，引數存入字串池
String string = new String("bastard");//查詢字串池，若字串池不存在該字串，先存入字串池；存在，直接讀取。
string.intern();//從字串池讀取
Log.d(TAG, "equal: "+(string==string.intern())+"-"+(string.equals(string.intern())));
HashMap<String, String> hashMap = new HashMap<>();//用HashMap實現字串池

4、Editable介面

//EditText是TextView的直接子類，但TextView.getText()返回CharSequence，而EditText.getText()返回Editable
//Editable介面繼承CharSequence, GetChars, Spannable, Appendable
//一個物件，鏈式
Editable editable = editText.getText();//Editable
editable.append('c');//char, CharSequence; Editable
editable.delete(0, 1);//; Editable
editable.insert(0, "c");//CharSequence; Editable

editText.setText("");//CharSequence
editable.clear();
//實現Spannable介面，如SpannableString
editable.clearSpans();
editable.removeSpan(null);
editable.setSpan(null, 0, 1, Spannable.SPAN_EXCLUSIVE_INCLUSIVE);

1) 用SpannableString設定區域性字型顏色和粗斜體

private void setTextAppearance(String f, String s, String t){
SpannableString ss = new SpannableString(f+s+t);
ss.setSpan(new StyleSpan(Typeface.BOLD_ITALIC), 0, f.length(), 
Spannable.SPAN_EXCLUSIVE_INCLUSIVE);
ss.setSpan(new ForegroundColorSpan(Color.BLUE), f.length()+s.length(), 
f.length()+s.length()+t.length(), 
Spannable.SPAN_EXCLUSIVE_INCLUSIVE);
TextView tv = (TextView) findViewById(R.id.txt);
tv.setText(ss);
}
2) 比較getString和getText的不同
//Context.getText
TextView textView = (TextView) findViewById(R.id.txt_getText);
textView.setText(getText(R.string.sometext));
//Context.getString
TextView textView2 = (TextView) findViewById(R.id.txt_getString);
textView2.setText(getString(R.string.sometext));
3) Html.fromHtml
TextView textView3 = (TextView) findViewById(R.id.txt_html);
textView3.setText(Html.fromHtml("<font color='red'>Very good!</font>"));
res/values/strings.xml檔案內容
<string name="sometext">
    <font color="blue">周杰倫</font>張靚穎<b>徐若瑄</b>
</string>