String、StringBuffer、StringBuilder三者之间的区别

吧啦吧啦，今天在公司做算法题的时候，不仅就想写下了

String是不可变类，所以任何对String的操作都将引发新的String对象的生成。但是StringBuffer是可变类，任何对StringBuffer所指代的字符串改变都不会产生新的对象。

新引入的StingBuilder类不是线程安全，但其在单线程中的性能比StringBuffer高。

下面是一点小例子

import java.util.ArrayList;import java.util.Iterator;import java.util.List;/** * 从JDK1.5中,有了StringBuilder。 */public class DifferenceStringBufferAndStringBuilder {    private static final String base = "String";    private static final int count = 3000000;    public static void main(String[] args) {        stringTest();        stringBufferTest();        stringBuilderTest();        addToStringBuilder();        addToStringBuffer();    }    /**     * string执行性能测试     */    public static void stringTest() {        long begin, end;        begin = System.currentTimeMillis();        String test = new String(base);        // 在这里为什么要缩150，因为其实时间是很长的        for (int i = 0; i < count / 150; i++) {            test = test + "add";        }        end = System.currentTimeMillis();        System.out.println((end - begin) + "millis has elapsed when used String");    }    /**     * stringBuffer     */    public static void stringBufferTest() {        long begin, end;        begin = System.currentTimeMillis();        StringBuffer stringBuffer = new StringBuffer(base);        for (int i = 0; i < count; i++) {            stringBuffer.append("add");        }        end = System.currentTimeMillis();        System.out.println((end - begin) + "millis has elapsed when used StringBuffer");    }    /**     * stingBuilder     */    public static void stringBuilderTest() {        long begin, end;        begin = System.currentTimeMillis();        StringBuilder stringBuilder = new StringBuilder(base);        for (int i = 0; i < count; i++) {            stringBuilder.append("add");        }        end = System.currentTimeMillis();        System.out.println((end - begin) + "mills has elapsed when used StringBuilder");    }    /**     *转换为StringBuilder     */    public static String appendItemsToStringBuilder(List list){        StringBuilder stringBuilder = new StringBuilder();        for (Iterator i = list.iterator();i.hasNext();){            stringBuilder.append(i.next()).append("");        }        return stringBuilder.toString();    }    public static void addToStringBuilder(){        List list = new ArrayList();        list.add("l");        list.add("y");        list.add("z");        System.out.println(DifferenceStringBufferAndStringBuilder.appendItemsToStringBuilder(list));    }    public static String appendItemsToStringBuffer(List list){        StringBuffer stringBuffer = new StringBuffer();        for (Iterator i = list.iterator();i.hasNext();){            stringBuffer.append(i.next()).append("");        }        return stringBuffer.toString();    }    public static void addToStringBuffer(){        List list = new ArrayList();        list.add("l");        list.add("y");        list.add("z");        System.out.println(DifferenceStringBufferAndStringBuilder.appendItemsToStringBuffer(list));    }}

最后输出的是

1127millis has elapsed when used String86millis has elapsed when used StringBuffer35mills has elapsed when used StringBuilderlyzlyz

所以根据结果来看，采用String对象时，哪怕是次数是其他对象的1/150,执行时间上也比其他对象高很多，而采用StringBuffer对象和采用StringBuilder对象也有明显的差距。所以如果是在单线程下运行，就不必考虑到线程同步的问题，优先采用StringBuilder类，当然，如果是要保证线程安全的话，就要考虑到StringBuffer了。

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除了对多线程的支持不一样的话，其实这两个类没啥区别的，上面不就很好的说明了嘛。