StringBuffer实现

import java.util.Arrays;

public class MyStringBuffer {
    private int capacity = 16;
    private int length = 0;
    private char[] value;

    
    public MyStringBuffer(){
        value = new char[capacity];
    }

    public MyStringBuffer(String str){
        value = new char[str.length() + capacity];
        length = str.length();
        char[] ss = str.toCharArray();
        for (int i = 0; i < ss.length; i++) {
            value[i] = ss[i];
        }
        capacity = capacity + str.length();
    }

    public int capacity(){
        return value.length;
    }

    @Override
    public String toString() {
        String result = new String(value,0,length);
        return result;
    }

    public int length(){
        return length;
    }

    public void append(String str){
        char[] news = value;
        while(str.length() > capacity - length){
            //扩容
            capacity = (capacity + 1) * 2;
            value = new char[capacity];
        }
        char[] cs = new char[999];
        //原数组为news,从0开始复制全部元素从value数组的0开始替换到value数组
        System.arraycopy(news,0,value,0,news.length);
        char[] ss = str.toCharArray();
        //原数组为ss,从0开始复制全部元素到ss的末尾开始替换
        System.arraycopy(ss,0,value,length,ss.length);
        length += str.length();
    }

    public static void main(String[] args) {
        MyStringBuffer sb = new MyStringBuffer("abcd");
        System.out.println(sb.capacity);
        System.out.println(sb.length);
        System.out.println(sb.toString());
        sb.append("efg");
        System.out.println(sb.capacity);
        System.out.println(sb.length);
        System.out.println(sb.toString());
    }
}