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));//2