首先来说说String,它不是关键字，它是java中的一个比较特殊的类，final类，是不允许被继承的。java中用String不一定会创建对象，但是通过new关键字一定就创建对象，String通过new关键字创建的对象是不可改变的。这也就是它运行速度较慢的原因。想要具体了解的同学可以看下面这篇文章，这里不详细说。
https://www.cnblogs.com/xiaoxi/p/6036701.html
现在来谈谈这3者的运行速度吧，这三者的运行速度大小应该是
StringBuilder>StringBuffer>String
通过下面代码演示可以证明
首先是String
String str="hello";
System.out.println(str);
long starttime=System.currentTimeMillis();
str+="world";
System.out.println(str);
for(int i=0;i<1000;i++){
str+="ld";
}
System.out.println(str);
long lasttime=System.currentTimeMillis();
int a=(int) (lasttime-starttime);
System.out.println("结果="+a);
}运行完这串代码，结果如下
hello
helloworld
helloworldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldld
结果=6我们可以发现这串代码运行完的时间为6，在看同样的代码执行，但是通过其他2个类来调用结果就会不一样。
StringBuffer str1=new StringBuffer("hello");
System.out.println(str1);
long starttime1=System.currentTimeMillis();
str1.append("world");
System.out.println(str);
for(int i=0;i<1000;i++){
str1.append("ld");
}
System.out.println(str1);
long lasttime1=System.currentTimeMillis();
int a1=(int) (lasttime1-starttime1);
System.out.println("结果="+a1);hello
helloworldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldld
helloworldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldldld
结果=3同样用StringBuilder执行这串代码肯定也是比String快的，原因就是上文提到的在java中用String时每次增加字符串数据都会重新开个新的储存空间来存储，而另外2个StringBuilder和StringBuffer存储的时候是一个可变的空间来进行存储，这样就相当的节约时间。那么StringBulider和StringBuffer的区别在哪呢？
StringBuffer是线程安全的，在多线程的应用程序中使用，而StringBuffer是非线程安全的，在单线程的应用程序中使用。
总结下就是，String适合处理少量的字符串操作，而StringBuffer和StringBulider适合大量的字符串操作。