【java笔试系列二】String、StringBuffer和StringBuilder
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一.区别
备注:StringBuilder是JDK1.5以后加入的,没有实现线程安全,因此性能略高。
二.效率测试
public class StringTest { public static String BASEINFO = "Mr.Y"; public static final int COUNT = 2000000; /** * 执行一项String赋值测试 */ public static void doStringTest() { String str = new String(BASEINFO); long starttime = System.currentTimeMillis(); for (int i = 0; i < COUNT / 100; i++) { str = str + "miss"; } long endtime = System.currentTimeMillis(); System.out.println((endtime - starttime) + " millis has costed when used String."); } /** * 执行一项StringBuffer赋值测试 */ public static void doStringBufferTest() { StringBuffer sb = new StringBuffer(BASEINFO); long starttime = System.currentTimeMillis(); for (int i = 0; i < COUNT; i++) { sb = sb.append("miss"); } long endtime = System.currentTimeMillis(); System.out.println((endtime - starttime) + " millis has costed when used StringBuffer."); } /** * 执行一项StringBuilder赋值测试 */ public static void doStringBuilderTest() { StringBuilder sb = new StringBuilder(BASEINFO); long starttime = System.currentTimeMillis(); for (int i = 0; i < COUNT; i++) { sb = sb.append("miss"); } long endtime = System.currentTimeMillis(); System.out.println((endtime - starttime) + " millis has costed when used StringBuilder."); } /** * 测试StringBuffer遍历赋值结果 * * @param mlist */ public static void doStringBufferListTest(List<String> mlist) { StringBuffer sb = new StringBuffer(); long starttime = System.currentTimeMillis(); for (String string : mlist) { sb.append(string); } long endtime = System.currentTimeMillis(); System.out.println(sb.toString() + "buffer cost:" + (endtime - starttime) + " millis"); } /** * 测试StringBuilder迭代赋值结果 * * @param mlist */ public static void doStringBuilderListTest(List<String> mlist) { StringBuilder sb = new StringBuilder(); long starttime = System.currentTimeMillis(); for (Iterator<String> iterator = mlist.iterator(); iterator.hasNext();) { sb.append(iterator.next()); } long endtime = System.currentTimeMillis(); System.out.println(sb.toString() + "builder cost:" + (endtime - starttime) + " millis"); } public static void main(String[] args) { doStringTest(); doStringBufferTest(); doStringBuilderTest(); List<String> list = new ArrayList<String>(); list.add(" I "); list.add(" like "); list.add(" BeiJing "); list.add(" tian "); list.add(" an "); list.add(" men "); list.add(" . "); doStringBufferListTest(list); doStringBuilderListTest(list); } } 运行结果:1521 millis has costed when used String.55 millis has costed when used StringBuffer.22 millis has costed when used StringBuilder. I like BeiJing tian an men . buffer cost:1 millis I like BeiJing tian an men . builder cost:0 millis从上面的结果可以看出,不考虑多线程,采用String对象时,执行时间比其他两个都要高,而采用StringBuffer对象和采用StringBuilder对象的差别也比较明显。由此可见,如果我们的程序是在单线程下运行,或者是不必考虑到线程同步问题,我们应该优先使用StringBuilder类;如果要保证线程安全,自然是StringBuffer。从后面List的测试结果可以看出,除了对多线程的支持不一样外,这两个类的使用方式和结果几乎没有任何差别,
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三. StringBuffer常用方法
StringBuffer s = new StringBuffer(); StringBuffer sb1=new StringBuffer(512);StringBuffer sb2=new StringBuffer(“how are you?”)public StringBuffer append(boolean b) //该方法的作用是追加内容到当前StringBuffer对象的末尾,类似于字符串的连接,调用该方法以后,StringBuffer对象的内容也发生改 变,例如:StringBuffer sb = new StringBuffer(“abc”);sb.append(true); public StringBuffer deleteCharAt(int index) // 该方法的作用是删除指定位置的字符,然后将剩余的内容形成新的字符串。例如:StringBuffer sb = new StringBuffer(“KMing”);sb. deleteCharAt(1);public StringBuffer delete(int start,int end) //该方法的作用是删除指定区间以内的所有字符,包含start,不包含end索引值的区间。例如:StringBuffer sb = new StringBuffer(“TestString”);sb. delete (1,4); public StringBuffer insert(int offset, boolean b) //该方法的作用是在StringBuffer对象中插入内容,然后形成新的字符串。例如:StringBuffer sb = new StringBuffer(“TestString”);sb.insert(4,false); public StringBuffer reverse() //该方法的作用是将StringBuffer对象中的内容反转,然后形成新的字符串。例如:StringBuffer sb = new StringBuffer(“abc”);sb.reverse(); public void setCharAt(int index, char ch) //该方法的作用是修改对象中索引值为index位置的字符为新的字符ch。例如:StringBuffer sb = new StringBuffer(“abc”);sb.setCharAt(1,’D’); public void trimToSize() //该方法的作用是将StringBuffer对象的中存储空间缩小到和字符串长度一样的长度,减少空间的浪费,和String的trim()是一样的作用,不在举例。 //g、length方法 //该方法的作用是获取字符串长度 ,不用再说了吧。 //h、setlength方法 //该方法的作用是设置字符串缓冲区大小。StringBuffer sb=new StringBuffer();sb.setlength(100); StringBuffer sb=new StringBuffer(“string”);int i=sb.capacity(); StringBuffer sb=new StringBuffer();sb.ensureCapacity(32); getChars(int start,int end,char chars[],int charStart); StringBuffer sb = new StringBuffer("I love You");int begin = 0;int end = 5;char[] ch = new char[end-begin];sb.getChars(begin, end, ch, 0);System.out.println(ch);
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参考:http://blog.csdn.net/mad1989/article/details/26389541