接口

public interface IStringBuffer {
	public void append(String str); //追加字符串
    public void append(char c);  //追加字符
    public void insert(int pos,char b); //指定位置插入字符
    public void insert(int pos,String b); //指定位置插入字符串
    public void delete(int start); //从开始位置删除剩下的
    public void delete(int start,int end); //从开始位置删除结束位置-1
    public void reverse(); //反转
    public int length(); //返回长度
}

实现

package how2j;

public class MyStringBuffer implements IStringBuffer {
	// 容量
	int capacity = 16;
	// 长度
	int length = 0;
	// 字符数组
	char[] value;

	// 无参构造方法，初始化 value
	public MyStringBuffer() {
		value = new char[capacity];
	}

	// 有参构造方法
	public MyStringBuffer(String str) {
		this();
		if (str == null)
			return;

		// 扩容
		ensureCapacity(str.length());

		System.arraycopy(str.toCharArray(), 0, value, 0, str.length());

		length = str.length();

	}

	// 扩容函数
	public void ensureCapacity(int minCapacity) {
		while (minCapacity > capacity) {
			capacity = minCapacity * 2;
			char[] newValue = new char[capacity];
			System.arraycopy(value, 0, newValue, 0, length);
			value = newValue;
		}
	}

	@Override
	public void append(String str) {
		insert(length, str);
	}

	@Override
	public void append(char c) {
		append(String.valueOf(c));
	}

	@Override
	public void insert(int pos, char b) {
		insert(pos, String.valueOf(b));
	}

	// pu
	@Override
	public void insert(int pos, String b) {

		// 边界判断
		if (pos < 0 || pos > length || b == null)
			return;

		// 扩容
		ensureCapacity(b.length() + length);

		char[] cs = b.toCharArray();

		// 把要插入的位置腾出来
		System.arraycopy(value, pos, value, pos + cs.length, length - pos);

		// 插入数据
		System.arraycopy(cs, 0, value, pos, cs.length);

		length += cs.length;
	}

	public void delete(int start) {
		delete(start, length);
	}

	public void delete(int start, int end) {

		// 边界判断
		if (start < 0 || start > length || end < 0 || end > length || start >= end)
			return;

		// 用后面的数据把要删除部分覆盖
		System.arraycopy(value, end, value, start, length - end);

		length -= end - start;
	}

	public void reverse() {
		for (int i = 0; i < length / 2; i++) {
			char tmp = value[i];
			value[i] = value[length - i - 1];
			value[length - i - 1] = tmp;
		}

	}

	public int length() {
		return length;
	}

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

	public static void main(String[] args) {
		MyStringBuffer sb = new MyStringBuffer("there light");
		System.out.println(sb);
		sb.insert(1, "let ");
		System.out.println(sb);

		sb.insert(4, "be ");
		System.out.println(sb);
		sb.insert(0, "God Say:");
		System.out.println(sb);
		sb.append("!");
		System.out.println(sb);
		sb.append('?');
		System.out.println(sb);
		sb.reverse();
		System.out.println(sb);

		sb.reverse();
		System.out.println(sb);

		sb.delete(0, 4);
		System.out.println(sb);
		sb.delete(4);
		System.out.println(sb);

	}
}