java day20 stringbuffer和stringbuilder的使用

package com.atguigu.java;import org.junit.Test;/*** 关于StringBuffer和StringBuilder的使用** @author shkstart* @create 2019 下午 3:32*/
public class StringBufferBuilderTest {/*对比String、StringBuffer、StringBuilder三者的效率：从高到低排列：StringBuilder > StringBuffer > String*/@Testpublic void test3(){//初始设置long startTime = 0L;long endTime = 0L;String text = "";StringBuffer buffer = new StringBuffer("");StringBuilder builder = new StringBuilder("");//开始对比startTime = System.currentTimeMillis();for (int i = 0; i < 20000; i++) {buffer.append(String.valueOf(i));}endTime = System.currentTimeMillis();System.out.println("StringBuffer的执行时间：" + (endTime - startTime));startTime = System.currentTimeMillis();for (int i = 0; i < 20000; i++) {builder.append(String.valueOf(i));}endTime = System.currentTimeMillis();System.out.println("StringBuilder的执行时间：" + (endTime - startTime));startTime = System.currentTimeMillis();for (int i = 0; i < 20000; i++) {text = text + i;}endTime = System.currentTimeMillis();System.out.println("String的执行时间：" + (endTime - startTime));}/*StringBuffer的常用方法： StringBuffer append(xxx)：提供了很多的append()方法，用于进行字符串拼接 StringBuffer delete(int start,int end)：删除指定位置的内容 StringBuffer replace(int start, int end, String str)：把[start,end)位置替换为str StringBuffer insert(int offset, xxx)：在指定位置插入xxx StringBuffer reverse() ：把当前字符序列逆转 public int indexOf(String str) public String substring(int start,int end):返回一个从start开始到end索引结束的左闭右开区间的子字符串 public int length() public char charAt(int n ) public void setCharAt(int n ,char ch)总结：增：append(xxx)删：delete(int start,int end)改：setCharAt(int n ,char ch) / replace(int start, int end, String str)查：charAt(int n )插：insert(int offset, xxx)长度：length();*遍历：for() + charAt() / toString()*/@Testpublic void test2(){StringBuffer s1 = new StringBuffer("abc");s1.append(1);s1.append('1');System.out.println(s1);
// s1.delete(2,4);
// s1.replace(2,4,"hello");
// s1.insert(2,false);
// s1.reverse();String s2 = s1.substring(1, 3);System.out.println(s1);System.out.println(s1.length());System.out.println(s2);}/*String、StringBuffer、StringBuilder三者的异同？String:不可变的字符序列；底层使用char[]存储StringBuffer:可变的字符序列；线程安全的，效率低；底层使用char[]存储StringBuilder:可变的字符序列；jdk5.0新增的，线程不安全的，效率高；底层使用char[]存储源码分析：String str = new String();//char[] value = new char[0];String str1 = new String("abc");//char[] value = new char[]{'a','b','c'};StringBuffer sb1 = new StringBuffer();//char[] value = new char[16];底层创建了一个长度是16的数组。System.out.println(sb1.length());//sb1.append('a');//value[0] = 'a';sb1.append('b');//value[1] = 'b';StringBuffer sb2 = new StringBuffer("abc");//char[] value = new char["abc".length() + 16];//问题1. System.out.println(sb2.length());//3//问题2. 扩容问题:如果要添加的数据底层数组盛不下了，那就需要扩容底层的数组。默认情况下，扩容为原来容量的2倍 + 2，同时将原有数组中的元素复制到新的数组中。指导意义：开发中建议大家使用：StringBuffer(int capacity) 或 StringBuilder(int capacity)*/@Testpublic void test1(){StringBuffer sb1 = new StringBuffer("abc");sb1.setCharAt(0,'m');System.out.println(sb1);StringBuffer sb2 = new StringBuffer();System.out.println(sb2.length());//0}}