繼上一篇自己動手寫Java String類之後，今晚10點鐘下班，感覺稍早，於是手癢癢就想自己動手來寫StringBulder，畢竟程式設計師啥都講究自己動手才能豐衣足食嘛！StringBuilder與String類的區別是String的變數內容是放在final char[]裡面的，而StringBuilder是放在char[]裡面的，聽說String用到了常量池。以下為自己改版過後的StringBuilder，它實現了Serializable,CharSequence介面。JDK的StringBuilder除了實現這倆介面之外，還繼承了AbstractStringBuilder抽象類，由於AbstractStringBuilder是在java.lang包裡面，許可權是default，學過Java的同學都知道預設的許可權，其他包是無法繼承的，所以MyStringBuilder就不繼承AbstractStringBuilder了。以下為自己動手寫MyStringBuilder。

import java.io.Serializable;
import java.util.Arrays;

public class MyStringBuilder implements Serializable, CharSequence{
    private static final long serialVersionUID = 1L;

    private char[] value;

    private int count;

    public MyStringBuilder() {
        this(16);
    } 

    public MyStringBuilder
 
(CharSequence seq) {
    }
    public MyStringBuilder(int capacity) {
        value = new char[capacity];
    }
    public MyStringBuilder(String str) {
        value = new char[str.length()];
        for(int i = 0; i < value.length; i++) {
            value[i] = str.charAt(i);
        }
        count = str.length();
    } 

    @Override
    public
 
 int length() {
        return count;
    }

    @Override
    public char charAt(int index) {
        //判斷下標是否越界
        if(index < 0 || index > count) {
            throw new StringIndexOutOfBoundsException(index);
        }
        //返回陣列中小標為index的元素
        return value[index];
    }

    @Override
    public CharSequence subSequence(int start, int end) {
        // TODO Auto-generated method stub
        return null;
    }

    public MyStringBuilder append(boolean b){
        if(b) { //b為true
            //1.為value陣列擴容
            value = Arrays.copyOf(value, count+4);
            //.2給value賦值
            value[count++] = 't';
            value[count++] = 'r';
            value[count++] = 'u';
            value[count++] = 'e';
        } else { //b為false
            //1.為value陣列擴容
            value = Arrays.copyOf(value, count+5);
            //.2給value賦值
            value[count++] = 'f';
            value[count++] = 'a';
            value[count++] = 'l';
            value[count++] = 's';
            value[count++] = 'e';
        }

        return this;
    }

    @Override
    public String toString() {
        return new String(value);
    }

    public MyStringBuilder append(char c) {
        //1.給value陣列擴容
        value = Arrays.copyOf(value, count+1);
        //2.給value賦值
        value[count++] = c;
        //返回MyStringBuilder本身,供方法鏈呼叫
        return this;
    }

    public MyStringBuilder append(char[] str) {
        char[] newChar = new char[count+str.length]; //建立一個新陣列，容納新老字元
        System.arraycopy(value, 0, newChar, 0, count);  //將原陣列的值塞進新陣列的前半段
        System.arraycopy(str, 0, newChar, count, str.length);//將要拼接的陣列的值塞進新陣列的後半段
        value = Arrays.copyOf(newChar, newChar.length);  //原來的value陣列擴容
        count = value.length;
        return this;
    }

    public MyStringBuilder append(String string) {
        if(string == null) { //如果物件為空,則返回"null"字串
            return appendNull();
        }
        //1.先對value擴容
        value = Arrays.copyOf(value, count+string.length());
        //2.呼叫String類的.getChars方法,將字元從此字串複製到value字元陣列。
        string.getChars(0, string.length(), value, count); 
        //3.count長度增加
        count += string.length();
        return this;
    }

    private MyStringBuilder appendNull() {
        value = Arrays.copyOf(value, count+4);
        value[count++] = 'n';
        value[count++] = 'u';
        value[count++] = 'l';
        value[count++] = 'l';
        return this;
    }

    public MyStringBuilder append(float f) {
        /**
         *  JDK是呼叫該類來完成轉化的,FloatingDecimal.appendTo(f,this);
         */
        String str = String.valueOf(f);
        return this.append(str);
    }

    public MyStringBuilder append(double d) {
        String str = String.valueOf(d);
        return this.append(str);
    }

    public MyStringBuilder append(Object obj) {
        if(obj == null) {
            return appendNull();
        }
        return append(obj.toString());
    }
    public MyStringBuilder deleteCharAt(int index) {
        if(index < 0 || index > count) {
            throw new StringIndexOutOfBoundsException(index);
        }
        System.arraycopy(value, index+1, value, index, count-index-1);
        count--;
        return this;
    }

    /**
     * 將字元陣列倒置
     * @return value 倒置後的字元陣列
     */
    public MyStringBuilder reverse() {
        int n = count-1;
        int i = 0;
        //JDK使用的倒置演算法比我的效率高，暫時讀不懂它的程式碼
        while(i < count/2) {
            char cj = value[i]; //一邊從頭開始
            char ck = value[n-i];  //一遍從尾開始
            //兩個位置的元素相互交換裡面的內容
            value[i] = ck;
            value[n-i] = cj;
            i++;
        }
        return this;
    }
}

上訴只提供了StringBuilder常用的方法，並沒用完全實現StringBuilder的全部功能，同時apend()的過載方法有很多，這裡只給出了一些比較常見的引數，如:append(String str),append(char[] chars),append(boolean b),append(Object obj);等。當然了，我的字串倒置reverse()估計效率也沒用JDK提供的高，不過功能完成實現出來了就行了！
下面為測試StringBuilder和MyStringBuilder的程式碼:

public class TestMyStringBuilder {

    public static void main(String[] args) {
        char[] chars = new char[4];
        chars[0] = 'J';
        chars[1] = 'a';
        chars[2] = 'v';
        chars[3] = 'a';
        String nullStr = null;
        Student stu = new Student("1","22","男");
        Student stuNull = null;

        StringBuilder sb = new StringBuilder("");
        System.out.println("JDK提供的StringBuilder:");
        System.out.println(sb.append(true).append(' ')
                    .append(false).append(chars).append(nullStr).append("HelloJava")
                    .append(3.141592653f).append(10086.11).append(stu).append(stuNull)
                    .deleteCharAt(10).reverse());
        System.out.println("長度:" + sb.length());
        System.out.println("測試charAt:第10個字元" + sb.charAt(10)+",第20個字元:" + sb.charAt(20));

        MyStringBuilder sb1 = new MyStringBuilder("");
        System.out.println("自己實現的StringBuilder:");
        System.out.println(sb1.append(true).append(' ')
                    .append(false).append(chars).append(nullStr).append("HelloJava")
                    .append(3.141592653f).append(10086.11).append(stu).append(stuNull)
                    .deleteCharAt(10).reverse());
        System.out.println("長度：" + sb1.length());
        System.out.println("測試charAt:第10個字元" + sb1.charAt(10)+",第20個字元:" + sb1.charAt(20));
    }
}

以下為輸出內容：

JDK提供的StringBuilder:
llun]男=xes ,22=ega ,1=di[ tnedutS11.680017295141.3avaJolleHllunavaeslaf eurt
長度:76
測試charAt:第10個字元 ,第20個字元:1
自己實現的StringBuilder:
llun]男=xes ,22=ega ,1=di[ tnedutS11.608017295141.3avaJolleHllunavaeslaf eurtl
長度：76
測試charAt:第10個字元 ,第20個字元:1