Java 的 String 物件

String 是常見的 Java 字串物件：

public class StringOperations {
    
    public static void main(String[] args) {
       
        String string2 = "World";
        String string3 = "";
        
        string3 = "Hello".concat(string2);
        System.out.println("String3: " + string3);
        
        System.out.println("Length: " + string3.length());
        
        System.out.println("Sub: " + string3.substring(0, 5));
        
        System.out.println("Upper: " + string3.toUpperCase());
    }
}