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，强烈推荐使用 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;
}
}
复制代码下面是 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。");
}
}
复制代码我分别做了循环 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：github.com/weizhiwen，欢…