自己动手实现Java中的StringBuffer类

在网上复习 Java 的基础知识,看到 String 和 StringBuffer 这里,简单的实现了下 StringBuffer 这个类,并对 String,StringBuffer,MyStringBuffer(自己实现的 StringBuffer 类名)做了拼接字符串的性能测试。

String 和 StringBuffer 内部都是通过字符数组实现的,但是 String 内部的字符数组是在第一次赋值时就固定了长度的,不可再更改,而 StringBuffer 内部的字符数组是留有冗余长度的,如果超过初始容量,就动态的增大数组的容量,使其看起来像是一个变长的数组。

在进行大量的字符串拼接时,强烈推荐 StringBuffer 而不是 String,因为 String 拼接字符串,没拼接一次都需要创建一个 String 对象来存放要拼接的字符串。而 StringBuffer 拼接字符串只是在原有字符串对象的基础上扩展字符数组的空间,并不会创建新的 StringBuffer 对象,这点也是 StringBuffer 比 String 速度快的原因。

下面就来简单实现一下 StringBuffer,取名为 MyStringBuffer。

public class MyStringBuffer{
    private int capacity = 16; // 初始容量
    private int length = 0; // 字面值字符串的长度
    private char[] value;
    
    public MyStringBuffer() {
        value = new char[capacity];
    }
    
    public MyStringBuffer(String str) {
        if(str == null) {
            return;
        }
        length = str.length();
        // str字符串的长度大于初始容量
        if(capacity < length) {
            capacity += length;
            value = new char[capacity];
            System.arraycopy(str.toCharArray(), 0, value, 0, length);
        } else {
            value = new char[capacity];
            System.arraycopy(str.toCharArray(), 0, value, 0, length);
        }
    }
    
    public void append(String str) {
        insert(length, str);
    }

    public void append(char c) {
        insert(length, String.valueOf(c));
    }

    public void insert(int pos, String str) {
        if(pos < 0 || pos > length || str == null) {
            return;
        }
        // 容量不够
        while((length + str.length()) > capacity) {
            capacity += length + str.length();
            char[] newValue = new char[capacity];
            System.arraycopy(value, 0, newValue, 0, length);
            value = newValue;
        }
        char[] temp = str.toCharArray();
        // 先移后插
        System.arraycopy(value, pos, value, pos + temp.length, length - pos);
        System.arraycopy(temp, 0, value, pos, temp.length);
        length += temp.length;
    }

    public void insert(int pos, char c) {
        insert(pos, String.valueOf(c));
    }

    public void delete(int start) {
        delete(start, length - 1);
    }

    public void delete(int start, int end) {
        if(start < 0 || start > length || end < 0 || end > (length - 1) || start > length) {
            return;
        }
        System.arraycopy(value, end, value, start, length - end);
        length -= (end - start);
    }

    public void reverse() {
        // 方法一 间接法
        /*char[] temp = new char[value.length];
        for(int i = 0, j = length; i < length; i++, j--) {
            temp[i] = value[j - 1];
        }
        value = temp;*/
        // 方法二 直接法
        for(int i = 0; i < length / 2; i++) {
            char temp = value[i];
            value[i] = value[length - i - 1];
            value[length - i - 1] = temp;
        }
    }

    public int length() {
        return length;
    }
    
    public String toString() {
        char[] realValue = new char[length];
        System.arraycopy(value, 0, realValue, 0, length);
        return new String(realValue);
    }

    public int capacity() {
        return capacity;
    }
}

下面是 MyStringBuffer 的测试类 MyStringBufferTest。

package com.wenshixin.character;

import org.junit.Test;

public class MyStringBufferTest {
    final int TIMES = 1000000; // 循环次数
    final double DIVISOR = 1000000.0; // 除数
    
    @Test
    public void constructorTest() {
        MyStringBuffer myStringBuffer = new MyStringBuffer("Weizhiwen");
        System.out.println(myStringBuffer);
        MyStringBuffer longStringBuffer = new MyStringBuffer("It's a long road, the only key.");
        System.out.println(longStringBuffer);
    }
    
    @Test
    public void lengthAndCapacityTest() {
        MyStringBuffer myStringBuffer = new MyStringBuffer("Weizhiwen");
        System.out.println("length:"+myStringBuffer.length());
        System.out.println("capacity:"+myStringBuffer.capacity());
        MyStringBuffer longStringBuffer = new MyStringBuffer("It's a long road, the only key.");
        System.out.println("length:" + longStringBuffer.length());
        System.out.println("capacity:" + longStringBuffer.capacity());
    }
    
    @Test
    public void reverseTest() {
        MyStringBuffer myStringBuffer = new MyStringBuffer("Weizhiwen");
        myStringBuffer.reverse();
        System.out.println(myStringBuffer);
    }
    
    @Test
    public void insertTest() {
        MyStringBuffer myStringBuffer = new MyStringBuffer("Weiwen");
        myStringBuffer.insert(3, "zhi");
        System.out.println(myStringBuffer);
        myStringBuffer.insert(0, "I'm ");
        System.out.println(myStringBuffer);
        myStringBuffer.insert(myStringBuffer.length(), "!");
        System.out.println(myStringBuffer);
        myStringBuffer.insert(myStringBuffer.length(), "\nIt's a long road, the only key.");
        System.out.println(myStringBuffer);
        myStringBuffer.insert(-1, "nothing");
        System.out.println("低于下限位置插入值,显示:" + myStringBuffer);
        myStringBuffer.insert(myStringBuffer.length() + 2, "nothing");
        System.out.println("高于上限位置插入值,显示:" + myStringBuffer);
    }
    
    @Test
    public void appendTest() {
        MyStringBuffer myStringBuffer = new MyStringBuffer();
        myStringBuffer.append("I'm Weizhiwen");
        System.out.println(myStringBuffer);
        myStringBuffer.append("!");
        System.out.println(myStringBuffer);
    }
    
    @Test
    public void deleteTest() {
        MyStringBuffer myStringBuffer = new MyStringBuffer("I'm not Weizhiwen!");
        myStringBuffer.delete(3, 7);
        System.out.println(myStringBuffer);
    }
    
    @Test
    public void stringPerformenceTest() {
        String string = "1";
        Long startTime = System.currentTimeMillis();
        System.out.println(startTime);
        for(int i = 0; i < TIMES; i++) {
            string += "0";
        }
        Long endTime = System.currentTimeMillis();
        System.out.println(endTime);
        double avgTime = (endTime - startTime) / DIVISOR; 
        System.out.println("String平均一次的拼接时间为:" + avgTime + "毫秒。");
    }
    
    @Test
    public void stringBufferPerformenceTest() {
        StringBuffer stringBuffer = new StringBuffer("1");
        Long startTime = System.currentTimeMillis();
        System.out.println(startTime);
        for(int i = 0; i < TIMES; i++) {
            stringBuffer.append("0");
        }
        Long endTime = System.currentTimeMillis();
        System.out.println(endTime);
        double avgTime = (endTime - startTime) / DIVISOR; 
        System.out.println("StringBuffer平均一次的拼接时间为:" + avgTime + "毫秒。");
    }
    
    @Test
    public void myStringBufferPerformence() {
        MyStringBuffer myStringBuffer = new MyStringBuffer("1");
        Long startTime = System.currentTimeMillis();
        System.out.println(startTime);
        for(int i = 0; i < TIMES; i++) {
            myStringBuffer.append("0");
        }
        Long endTime = System.currentTimeMillis();
        System.out.println(endTime);
        double avgTime = (endTime - startTime) / DIVISOR; 
        System.out.println("MyStringBuffer平均一次的拼接时间为:" + avgTime + "毫秒。");
    }
}

我分别做了循环 1000 次,10,000 次,100,000 次,1000,000 次拼接,结果是平均拼接时间 t(StringBuffer) < t(MyStringBuffer) < t(String)。

通过自己实现 StringBuffer,加深了对 StringBuffer 和 String 者两个类的理解,Java 基础还需加强。

个人 GitHub 地址:https://github.com/weizhiwen,欢迎来访。

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