题目
String、StringBuilder、StringBuffer有什么异同?
解答
相同点:String、StringBuilder、StringBuffer都可以用来存储字符串。
不同点:
1、String与StringBuilder、StringBuffer的不同点主要在于,String对象创建之后,是不可改变的,平时我们对同一个String变量赋值,实际上是创建了个新的对象。而后两者是可变的,这意思就是说,他们可以在同一内存地址上更改它的值,而无需创建新的对象。
2、StringBuilder与StringBuffer的不同点在于,StringBuffer是线性安全的,但是它比StringBuilder要慢。
参考代码
StringBufferThreadSafeTest
package me.huangzijian; public class StringBufferThreadSafeTest { public static void main(String[] args) { //测试StringBuffer线程安全部分
String s = "123456789";
StringBuffer stringBuffer = new StringBuffer(s);
StringBuilder stringBuilder = new StringBuilder(s); SbfReverseThread sbfRT1 = new SbfReverseThread(stringBuffer);
SbfReverseThread sbfRT2 = new SbfReverseThread(stringBuffer);
SbdReverseThread sbdRT1 = new SbdReverseThread(stringBuilder);
SbdReverseThread sbdRT2 = new SbdReverseThread(stringBuilder); new Thread(sbfRT1).start();
new Thread(sbfRT2).start();
new Thread(sbdRT1).start();
new Thread(sbdRT2).start(); //测试StringBuffer比StringBuilder慢部分,注意不要同时运行两个循环,一次运行一个,这样才精确
long startTime2 = System.currentTimeMillis();
for (int i = 0; i <= 10000000; i++) {
stringBuffer.reverse();
}
long endTime2 = System.currentTimeMillis();
System.out.println("StringBuffer循环1000次所需时间:" + (endTime2 - startTime2) + "ms"); long startTime1 = System.currentTimeMillis();
for (int i = 0; i <= 10000000; i++) {
stringBuilder.reverse();
}
long endTime1 = System.currentTimeMillis();
System.out.println("StringBuilder循环1000次所需时间:" + (endTime1 - startTime1) + "ms");
}
}
SbfReverseThread
package me.huangzijian; public class SbfReverseThread implements Runnable { StringBuffer stringBuffer; public SbfReverseThread(StringBuffer stringBuffer) {
this.stringBuffer = stringBuffer;
} @Override
public void run() {
for (int i = 0; i <= 1000; i++) {
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
stringBuffer.reverse();
System.out.println("StringBuffer:" + stringBuffer);
}
}
}
SbdReverseThread
package me.huangzijian; public class SbdReverseThread implements Runnable { StringBuilder stringBuilder; public SbdReverseThread(StringBuilder stringBuilder) {
this.stringBuilder = stringBuilder;
} @Override
public void run() {
for (int i = 0; i <= 1000; i++) {
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
stringBuilder.reverse();
System.out.println("StringBuilder:" + stringBuilder);
}
}
}
从结果可以看出:
1.StringBuilder对于字符串123456789,两个线程运行到一段时间,就开始混乱,不再是123456789或者987654321了,而StringBuffer无论重复多少次,依然是123456789或者987654321。
2.我机器上,StringBuffer字符串反转运行10000000,用时370ms足有,而StringBuilder字符串反转运行10000000,只需要150ms左右,可见StringBuilder比StringBuffer快。