java11-6 String类的其它功能

String类的其他功能：

　　替换功能：
　　　　String replace(char old,char new)
　　　　String replace(String old,String new)

　　去除字符串两空格
　　　　String trim()

　　按字典顺序比较两个字符串
　　　　int compareTo(String str) 区分大小写
　　　　int compareToIgnoreCase(String str) 不区分大小写

 public class StringTest3 {

     public static void main(String[] args) {
         String s = "HelloWorld";

         // 字符替代：String replace(char old,char new)
         String s1 = s.replace('o','k');
         System.out.println(s1);//HellkWkrld

         // 字符串替代：String replace(String old,String new)
         String s2 = s.replace("ll", "kk");
         System.out.println(s2);//HekkoWorld

         // 去除字符串两空格
         String s4 = " hello world  ";
         String s5 = s4.trim();
         System.out.println("s4:" + s4 + "---");//左右的空格还保留
         System.out.println("s5:" + s5 + "---");//左右的空格去除了

         //按字典顺序(ASCII码表)比较两个字符串：
         // int compareTo(String str)
         String a1 = "cat";
         String a2 = "dog";
         String a3 = "Cat";
         String a4 = "door";
         String a5 = "cat";
         String a6 = "c";

         int i1 = a1.compareTo(a2);
         System.out.println(i1);
         //-1 c在b的前面一位，ASCII码表，a1的c减去a2的b，等于-1

         System.out.println(a1.compareTo(a3));//32
         //a1的c在ASCII的数值减去a2的C在ASCII的数值。得到32

         System.out.println(a2.compareTo(a4));//-8
         //a2的前2个字母do和a4前2个一样，a2第三个的g减去a4第三个的o，得到-8

         System.out.println(a1.compareTo(a5));//0
         //a1和a5的数组元素完全一样

         System.out.println(a1.compareTo(a6));//2
         //Ctrl+左键点击comperTo看源码
     }

 }

 compareTo的源码，以及“System.out.println(a1.compareTo(a6));//2”的分析

  private final char value[];

     字符串会自动转换为一个字符数组。

 public int compareTo(String anotherString) {
         // this - a1 - "cat"
         // anotherString - a2 - "c"

         int len1 = value.length;
         //this.value.length = a1.toCharArray().length == 3 

         int len2 = anotherString.value.length;
         //anotherString.value.length = a2.toCharArray().length == 1

         int lim = Math.min(len1, len2);
         // lim = Math.min = 3-1 = 2; lim = 2

         char v1[] = value;
         // char v1[] = {'c','a','t'};

         char v2[] = anotherString.value;
         // char v2[] = {'c'};

         int k = 0;
         while (k < lim) {
             char c1 = v1[k]; //c1 = 'c','a','t'
             char c2 = v2[k]; //c2 = 'c'
             if (c1 != c2) {
                 return c1 - c2;
             }
             k++; //当k=1时，k !< lim ,lim = 1; 所以跳出循环
         }
         return len1 - len2; //跳出循环后，到这里，比较两个数组的长度  3-1 = 2
     }

     String a1 = "cat";
     String a6 = "c";
     System.out.println(a1.compareTo(a6));//