为啥 prefix不等于 java 呢
// 这里有个要注意的
String v1 = "123456";
String v2 = "123456";
System.out.println(v1 == v2);
System.out.println(v1 == "123456");
System.out.println(v2 == "123456");
System.out.println(v1.equals("123456"));这样就可以
String fileName = "HelloWorld.java";
int index = fileName.indexOf('.');
String prefix = fileName.substring(index+1);
System.out.println(prefix);
System.out.println(prefix == "java"); //为啥他就是 false 了 很神奇啊
