自己动手实现了一个StringBuffer，算是个玩具代码，各种边界问题都没有做检查，但是对于理解StringBuffer来说，足够了。

package character;import java.util.Arrays;public class IStringBuffer {    private char[] value;    private int count;    //default no-arg constructor    public MyStringBuffer() {        value = new char[16];    }    //constructor with capacity    public MyStringBuffer(int capacity) {        value = new char[capacity];    }    //constructor with string    public MyStringBuffer(String str) {        value = new char[str.length() + 16];        append(str);    }    public static void main(String[] args) {        MyStringBuffer msb = new MyStringBuffer("the");        msb.append(" redpig is writting java programs for fun");        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());        msb.append('!');        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());        msb.insert(23, "funny ");        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());        msb.insert(23, ' ');        msb.insert(23, 'a');        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());        msb.delete(23, 31);        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());        msb.delete(36);        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());        msb.reverse();        System.out.println("msb is: " + msb);        System.out.println("length is: " + msb.length());        System.out.println("capacity is: " + msb.capacity());    }    //追加字符串    public void append(String str) {        int newCapacity;        if ((str.length() + count) - value.length > 0) {            newCapacity = (value.length << 1) + 2;            if (newCapacity - (str.length() + count) < 0)                newCapacity = str.length() + count;            value = Arrays.copyOf(value, newCapacity);        }        str.getChars(0, str.length(), value, count);        count += str.length();    }    //追加字符    public void append(char c) {        this.append(String.valueOf(c));    }    public void insert(int pos, String str) {        if ((str.length() + count) - value.length > 0) {            int newCapacity;            newCapacity = (value.length << 1) + 2;            if (newCapacity - (str.length() + count) < 0)                newCapacity = str.length() + count;            value = Arrays.copyOf(value, newCapacity);        }        System.arraycopy(value, pos, value, pos + str.length(), count - pos);        str.getChars(0, str.length(), value, pos);        count += str.length();    }    public void insert(int pos, char c) {        this.insert(pos, String.valueOf(c));    }    public void delete(int start, int end) {        System.arraycopy(value, end, value, start, capacity() - end);        count -= (end - start);    }    public void delete(int start) {        this.delete(start, count);    }    public void reverse() {        int n = count - 1;        for (int j = (n - 1) >> 1; j >= 0; j--) {            int k = n - j;            char cj = value[j];            char ck = value[k];            value[j] = ck;            value[k] = cj;        }    }    public int length() {        return count;    }    public int capacity() {        return value.length;    }    public String toString() {        return String.valueOf(value);    }}