StringBuider 在什么条件下、如何使用效率更高

 /**
  * 循环内 String 拼接字符串，一次循环后销毁
  */
 public static void useString(){
  for (int i = 0; i < CYCLE_NUM_BIGGER; i++) {
   String str = str1 + i + str2 + i + str3 + i + str4 ;
  }
 }

 /**
  * 循环内 使用 StringBuilder 拼接字符串，一次循环后销毁
  */
 public static void useStringBuilder(){
  for (int i = 0; i < CYCLE_NUM_BIGGER; i++) {
   StringBuilder sb = new StringBuilder();
   String s = sb.append(str1).append(i).append(str2).append(i).append(str3).append(i).append(str4).toString();
  }
 }

 /**
  * 多次循环拼接成一个字符串 用 String
  */
 public static void useStringSpliceOneStr (){
  String str = "";
  for (int i = 0; i < CYCLE_NUM_LOWER; i++) {
   str += str1 + str2 + str3 + str4 + i;
  }
 }

 /**
  * 多次循环拼接成一个字符串 用 StringBuilder
  */
 public static void useStringBuilderSpliceOneStr(){
  StringBuilder sb = new StringBuilder();
  for (int i = 0; i < CYCLE_NUM_LOWER; i++) {
   sb.append(str1).append(str2).append(str3).append(str4).append(i);
  }
 }

 public static int executeSometime(int kind, int num) throws InterruptedException {
  Thread.sleep(2000);
  int sum = 0;
  for (int i = 0; i < num + 5; i++) {
   long begin = System.currentTimeMillis();

   switch (kind){
    case 1:
     useString();
     break;
    case 2:
     useStringBuilder();
     break;
    case 3:
     useStringSpliceOneStr();
     break;
    case 4:
     useStringBuilderSpliceOneStr();
     break;
    default:
     return 0;
   }

   long end = System.currentTimeMillis();

   if(i > 5){
    sum += (end - begin);
   }
  }
  return sum / num;
 }

public class StringTest {
 public static final int CYCLE_NUM_BIGGER = 10_000_000;
 public static final int CYCLE_NUM_LOWER = 10_000;
 public static final String str1 = "张三";
 public static final String str2 = "李四";
 public static final String str3 = "王五";
 public static final String str4 = "赵六";


 public static void main(String[] args) throws InterruptedException {
  int time = 0;
  int num = 5;

  time = executeSometime(1, num);
  System.out.println("String拼接 "+ CYCLE_NUM_BIGGER +" 次，" + num + "次平均时间：" + time + " ms");

  time = executeSometime(2, num);
  System.out.println("StringBuilder拼接 "+ CYCLE_NUM_BIGGER +" 次，" + num + "次平均时间：" + time + " ms");

  time = executeSometime(3, num);
  System.out.println("String拼接单个字符串 "+ CYCLE_NUM_LOWER +" 次，" + num + "次平均时间：" + time + " ms");

  time = executeSometime(4, num);
  System.out.println("StringBuilder拼接单个字符串 "+ CYCLE_NUM_LOWER +" 次，" + num + "次平均时间：" + time + " ms");

 }
} 

 /**
  * 循环内 使用 StringBuilder 拼接字符串，一次循环后销毁
  */
 public static void useStringBuilderOut(){
  StringBuilder sb = new StringBuilder();
  for (int i = 0; i < CYCLE_NUM_BIGGER; i++) {
//   sb.setLength(0);
   sb.delete(0, sb.length());
   String s = sb.append(str1).append(i).append(str2).append(i).append(str3).append(i).append(str4).toString();
  }
 }

 public static void createStringBuider() {
  for (int i = 0; i < CYCLE_NUM_BIGGER; i++) {
   StringBuilder sb = new StringBuilder();
  }
 }

 public static void cleanStringBuider() {
  StringBuilder sb = new StringBuilder();
  for (int i = 0; i < CYCLE_NUM_BIGGER; i++) {
   sb.delete(0, sb.length());
  }
 }