Java-字符串大小写转换

package com.tj;
 
public class MyClass implements Cloneable {
    public static void main(String[] args) {
        String string = "Hello World!";
 
        String upper = string.toUpperCase();
        System.out.println(upper);
 
        String lower = string.toLowerCase();
        System.out.println(lower);
 
    }
}