String、StringBuffer、StringBuilder区别并验证

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String、StringBuffer、StringBuilder的区别

1.String是一个常量，其对象一旦创建完毕就无法改变，当使用“+”拼接字符串时，会生成新的String对象，而不是向原有的String对象追加内容。

　StringBuffer和StringBuilder都是变量，追加内容时不会生成新的对象。

2.String初始化的时候是最快的，StringBuilder其次，StringBuffer最慢

　但是修改内容的时候StringBuilder是最快的，StringBuffer其次，String最慢

3.String是常量，因此线程安全。

　StringBuffer是线程安全的，StringBuilder是非线程安全的

验证

1.String是常量，改变值会生成新的对象；StringBuffer和StringBuilder是变量，改变值不会生成新的对象

 /**
      * 内存比较
      */
     private static void memery() {

         String str1 = "abc";
         System.out.println("String first memery address is " + str1.hashCode());
         str1 += "efg";
         System.out.println("String second memery address is " + str1.hashCode());

         StringBuilder sb1 = new StringBuilder("abc");
         System.out.println("StringBuilder first memery address is " + sb1.hashCode());
         sb1.append("efg");
         System.out.println("StringBuilder second memery address is " + sb1.hashCode());

         StringBuffer sb2 = new StringBuffer("abc");
         System.out.println("StringBuffer first memery address is " + sb2.hashCode());
         sb2.append("efg");
         System.out.println("StringBuffer second memery address is " + sb2.hashCode());

     }

memery

结果：

2.初始化速度：String>StringBuilder>StringBuffer

 /**
      * 初始化性能比较
      */
     private static void initial() {

         long time1 = System.nanoTime();
         String str1 = "abc";
         long time2 = System.nanoTime();
         System.out.println("String time: " + (time2 - time1) + "ns");

         long time3 = System.nanoTime();
         StringBuffer sb1 = new StringBuffer("abc");
         long time4 = System.nanoTime();
         System.out.println("StringBuffer time: " + (time4 - time3) + "ns");

         long time5 = System.nanoTime();
         StringBuilder sb2 = new StringBuilder("abc");
         long time6 = System.nanoTime();
         System.out.println("StringBuilder time: " + (time6 - time5) + "ns");

     }

initial

结果：

3.执行速度（即改变值的时候）：StringBuilder>StringBuffer>String

 /**
      * 性能比较
      */
     private static void performance() {

         long time1 = System.nanoTime();
         String str1 = "";
         for (int i = 0; i < 10; i++) {
             str1 += i;
         }
         long time2 = System.nanoTime();
         System.out.println("the time required for String to execute 1000 times: " + (time2 - time1) + "ns");

         long time3 = System.nanoTime();
         StringBuffer sb1 = new StringBuffer();
         for (int i = 0; i < 10; i++) {
             sb1.append(i);
         }
         long time4 = System.nanoTime();
         System.out.println("the time required for StringBuffer to execute 1000 times: " + (time4 - time3) + "ns");

         long time5 = System.nanoTime();
         StringBuilder sb2 = new StringBuilder();
         for (int i = 0; i < 10; i++) {
             sb2.append(i);
         }
         long time6 = System.nanoTime();
         System.out.println("the time required for StringBuilder to execute 1000 times: " + (time6 - time5) + "ns");

     }

performance

结果：

4.特殊例子：当执行String str = "a" + "b" + "c"时，String的速度是最快的，因为等价于String = "abc"；上面讲过，初始化的时候String是最快的

 /**
      * 特殊例子
      */
     private static void special() {

         long time1 = System.nanoTime();
         String str1 = "abc" + "def" + "ghi" + "jk";
         long time2 = System.nanoTime();
         System.out.println("String time: " + (time2 - time1) + "ns");

         long time3 = System.nanoTime();
         StringBuffer sb1 = new StringBuffer("abc").append("def").append("ghi").append("jk");
         long time4 = System.nanoTime();
         System.out.println("StringBuffer time: " + (time4 - time3) + "ns");

         long time5 = System.nanoTime();
         StringBuilder sb2 = new StringBuilder("abc").append("def").append("ghi").append("jk");
         long time6 = System.nanoTime();
         System.out.println("StringBuilder time: " + (time6 - time5) + "ns");

     }

special

结果：

5.StringBuffer是线程安全的，StringBuilder是非线程安全的

 /**
      * 线程安全性验证
      */
     public static void thread() {

         final StringBuffer sb1 = new StringBuffer();
         final StringBuilder sb2 = new StringBuilder();
         final CountDownLatch latch1 = new CountDownLatch(1000);
         final CountDownLatch latch2 = new CountDownLatch(1000);

         for (int i = 0; i < 1000; i++) {
             new Thread(new Runnable() {
                 public void run() {
                     try {
                         sb1.append("1");
                     } finally {
                         latch1.countDown();
                     }

                 }
             }).start();
         }
         for (int i = 0; i < 1000; i++) {
             new Thread(new Runnable() {
                 public void run() {
                     try {
                         sb2.append("2");
                     } finally {
                         latch2.countDown();
                     }
                 }
             }).start();
         }

         try {
             latch1.await();
             System.out.println(sb1.length());
             latch2.await();
             System.out.println(sb2.length());
         } catch (Exception e) {
             e.printStackTrace();
         }

     }

thread

结果：需多执行几次，StringBuffer长度全部为1000，StringBuilder大部分都是小于1000的

6.完整代码

 package org.java.base;

 import java.util.concurrent.CountDownLatch;

 /**
  * String、StringBuffer、StringBuilder比较
  */
 public class StringAndStringBufferAndStringBuilder {

     public static void main(String[] args) {

         //memery();
         //initial();
         //performance();
         //special();
         thread();

     }

     /**
      * 内存比较
      */
     private static void memery() {

         String str1 = "abc";
         System.out.println("String first memery address is " + str1.hashCode());
         str1 += "efg";
         System.out.println("String second memery address is " + str1.hashCode());

         StringBuilder sb1 = new StringBuilder("abc");
         System.out.println("StringBuilder first memery address is " + sb1.hashCode());
         sb1.append("efg");
         System.out.println("StringBuilder second memery address is " + sb1.hashCode());

         StringBuffer sb2 = new StringBuffer("abc");
         System.out.println("StringBuffer first memery address is " + sb2.hashCode());
         sb2.append("efg");
         System.out.println("StringBuffer second memery address is " + sb2.hashCode());

     }

     /**
      * 初始化性能比较
      */
     private static void initial() {

         long time1 = System.nanoTime();
         String str1 = "abc";
         long time2 = System.nanoTime();
         System.out.println("String time: " + (time2 - time1) + "ns");

         long time3 = System.nanoTime();
         StringBuffer sb1 = new StringBuffer("abc");
         long time4 = System.nanoTime();
         System.out.println("StringBuffer time: " + (time4 - time3) + "ns");

         long time5 = System.nanoTime();
         StringBuilder sb2 = new StringBuilder("abc");
         long time6 = System.nanoTime();
         System.out.println("StringBuilder time: " + (time6 - time5) + "ns");

     }

     /**
      * 性能比较
      */
     private static void performance() {

         long time1 = System.nanoTime();
         String str1 = "";
         for (int i = 0; i < 10; i++) {
             str1 += i;
         }
         long time2 = System.nanoTime();
         System.out.println("the time required for String to execute 1000 times: " + (time2 - time1) + "ns");

         long time3 = System.nanoTime();
         StringBuffer sb1 = new StringBuffer();
         for (int i = 0; i < 10; i++) {
             sb1.append(i);
         }
         long time4 = System.nanoTime();
         System.out.println("the time required for StringBuffer to execute 1000 times: " + (time4 - time3) + "ns");

         long time5 = System.nanoTime();
         StringBuilder sb2 = new StringBuilder();
         for (int i = 0; i < 10; i++) {
             sb2.append(i);
         }
         long time6 = System.nanoTime();
         System.out.println("the time required for StringBuilder to execute 1000 times: " + (time6 - time5) + "ns");

     }

     /**
      * 特殊例子
      */
     private static void special() {

         long time1 = System.nanoTime();
         String str1 = "abc" + "def" + "ghi" + "jk";
         long time2 = System.nanoTime();
         System.out.println("String time: " + (time2 - time1) + "ns");

         long time3 = System.nanoTime();
         StringBuffer sb1 = new StringBuffer("abc").append("def").append("ghi").append("jk");
         long time4 = System.nanoTime();
         System.out.println("StringBuffer time: " + (time4 - time3) + "ns");

         long time5 = System.nanoTime();
         StringBuilder sb2 = new StringBuilder("abc").append("def").append("ghi").append("jk");
         long time6 = System.nanoTime();
         System.out.println("StringBuilder time: " + (time6 - time5) + "ns");

     }

     /**
      * 线程安全性验证
      */
     public static void thread() {

         final StringBuffer sb1 = new StringBuffer();
         final StringBuilder sb2 = new StringBuilder();
         final CountDownLatch latch1 = new CountDownLatch(1000);
         final CountDownLatch latch2 = new CountDownLatch(1000);

         for (int i = 0; i < 1000; i++) {
             new Thread(new Runnable() {
                 public void run() {
                     try {
                         sb1.append("1");
                     } finally {
                         latch1.countDown();
                     }

                 }
             }).start();
         }
         for (int i = 0; i < 1000; i++) {
             new Thread(new Runnable() {
                 public void run() {
                     try {
                         sb2.append("2");
                     } finally {
                         latch2.countDown();
                     }
                 }
             }).start();
         }

         try {
             latch1.await();
             System.out.println(sb1.length());
             latch2.await();
             System.out.println(sb2.length());
         } catch (Exception e) {
             e.printStackTrace();
         }

     }

 }

StringAndStringBufferAndStringBuilder