Java 面试

/**
* 小写转大写
* @param str
* @return
*/
public static String toUpperCase(String str){
return str.toUpperCase();
}
/**
* 字符串反转
* @param str
*/
public static void fanzhuan(String str){
for(int i=str.length()-1;i>=0;i--){
System.out.print(str.charAt(i));
}
}

Java判断一个文件或目录是否存在?

/**
* 文件是否存在
* @param str
*/
public static void cunzai(String str){
File file=new File(str);
if(file.exists()){
System.out.println(true);
}else{
System.out.println(false);
}
}

//饿汉
public class test1(){
private test1(){}
private static final test1 single=new test1();
public static test1 getInstance(){return single;}
}
//懒汉
public class test3(){
private test3(){}
private static test3 single=null;
public synchronized static test3 getInstance(){
if(single==null){
single=new test3();
}
return single;
}
}