StringBuffer不像String隻能通過new來建立對象

String 的值是不可變的，每次對String的操作都會生成新的String對象，不僅效率低，而且耗費大量記憶體空間。

StringBuffer類和String類一樣，也用來表示字元串，但是StringBuffer的内部實作方式和String不同，在進行字元串處理時，不生成新的對象，在記憶體使用上要優于String。

StringBuffer 預設配置設定16位元組長度的緩沖區，當字元串超過該大小時，會自動增加緩沖區長度，而不是生成新的對象。

StringBuffer不像String，隻能通過 new 來建立對象，不支援簡寫方式，例如：

StringBuffer str1 = new StringBuffer(); // 配置設定16個位元組長度的緩沖區

StringBuffer str2 = =new StringBuffer(512); // 配置設定512個位元組長度的緩沖區

// 在緩沖區中存放了字元串，并在後面預留了16個位元組長度的空緩沖區

StringBuffer str3 = new StringBuffer(“www.weixueyuan.net”);

StringBuffer類的主要方法

StringBuffer類中的方法主要偏重于對于字元串的操作，例如追加、插入和删除等，這個也是StringBuffer類和String類的主要差別。實際開發中，如果需要對一個字元串進行頻繁的修改，建議使用 StringBuffer。

1. append() 方法

   append() 方法用于向目前字元串的末尾追加内容，類似于字元串的連接配接。調用該方法以後，StringBuffer對象的内容也發生改變，例如：

   StringBuffer str = new StringBuffer(“biancheng100”);

   str.append(true);

   則對象str的值将變成”biancheng100true”。注意是str指向的内容變了，不是str的指向變了。

字元串的”+“操作實際上也是先建立一個StringBuffer對象，然後調用append()方法将字元串片段拼接起來，最後調用toString()方法轉換為字元串。

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但是，對于長度較小的字元串，”+“操作更加直覺，更具可讀性，有些時候可以稍微犧牲一下效率。

2) deleteCharAt()

deleteCharAt() 方法用來删除指定位置的字元，并将剩餘的字元形成新的字元串。例如：

StringBuffer str = new StringBuffer(“abcdef”);

str. deleteCharAt(3);

該代碼将會删除索引值為3的字元，即”d“字元。

你也可以通過delete()方法一次性删除多個字元，例如：

StringBuffer str = new StringBuffer(“abcdef”);

str.delete(1, 4);

該代碼會删除索引值為1~4之間的字元，包括索引值1，但不包括4。

3) insert() 方法

insert() 用來在指定位置插入字元串，可以認為是append()的更新版。例如：

StringBuffer str = new StringBuffer(“abcdef”);

str.insert(3, “xyz”);

最後str所指向的字元串為 abcdxyzef。

4) setCharAt() 方法

setCharAt() 方法用來修改指定位置的字元。例如：

StringBuffer str = new StringBuffer(“abcdef”);

str.setCharAt(3, ‘z’);

該代碼将把索引值為3的字元修改為 z，最後str所指向的字元串為 abczef。

以上僅僅是部分常用方法的簡單說明，更多方法和解釋請查閱API文檔。

String和StringBuffer的效率對比

為了更加明顯地看出它們的執行效率，下面的代碼，将26個英文字母加了10000次。

public class Demo {

public static void main(String[] args){

String fragment = “abcdefghijklmnopqrstuvwxyz”;

int times = 10000;

// 通過String對象
    long timeStart1 = System.currentTimeMillis();
    String str1 = "";
    for (int i=0; i<times; i++) {
        str1 += fragment;
    }
    long timeEnd1 = System.currentTimeMillis();
    System.out.println("String: " + (timeEnd1 - timeStart1) + "ms");
   
    // 通過StringBuffer
    long timeStart2 = System.currentTimeMillis();
    StringBuffer str2 = new StringBuffer();
    for (int i=0; i<times; i++) {
        str2.append(fragment);
    }
    long timeEnd2 = System.currentTimeMillis();
    System.out.println("StringBuffer: " + (timeEnd2 - timeStart2) + "ms");
}
           

}

運作結果：

String: 5287ms

StringBuffer: 3ms

結論很明顯，StringBuffer的執行效率比String快上千倍，這個差異随着疊加次數的增加越來越明顯，當疊加次數達到30000次的時候，運作結果為：

String: 35923ms

StringBuffer: 8ms

是以，強烈建議在涉及大量字元串操作時使用StringBuffer。

StringBuilder類

StringBuilder類和StringBuffer類功能基本相似，方法也差不多，主要差別在于StringBuffer類的方法是多線程安全的，而StringBuilder不是線程安全的，相比而言，StringBuilder類會略微快一點。

StringBuffer、StringBuilder、String中都實作了CharSequence接口。

CharSequence是一個定義字元串操作的接口，它隻包括length()、charAt(int index)、subSequence(int start, int end) 這幾個API。

StringBuffer、StringBuilder、String對CharSequence接口的實作過程不一樣，如下圖所示：

圖1 對CharSequence接口的實作

可見，String直接實作了CharSequence接口；StringBuilder 和 StringBuffer都是可變的字元序列，它們都繼承于AbstractStringBuilder，實作了CharSequence接口。

總結

線程安全：

StringBuffer：線程安全

StringBuilder：線程不安全

速度：

一般情況下，速度從快到慢為 StringBuilder > StringBuffer > String，當然這是相對的，不是絕對的。

使用環境：

操作少量的資料使用 String；

單線程操作大量資料使用 StringBuilder；

多線程操作大量資料使用 StringBuffer。