diff --git a/1ano/2semestre/poo/guides/POO-2021-aula03-extra.pdf b/1ano/2semestre/poo/guides/POO-2021-aula03-extra.pdf
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Binary files /dev/null and b/1ano/2semestre/poo/guides/POO-2021-aula03-extra.pdf differ
diff --git a/1ano/2semestre/poo/src/aula03/StringExtras.java b/1ano/2semestre/poo/src/aula03/StringExtras.java
new file mode 100644
index 0000000..d5db692
--- /dev/null
+++ b/1ano/2semestre/poo/src/aula03/StringExtras.java
@@ -0,0 +1,24 @@
+package aula03;
+
+import java.util.Scanner;
+import utils.StringMethods;
+
+public class StringExtras {
+    public static void main(String[] args) {
+        Scanner sin = new Scanner(System.in);
+
+        String str = sin.nextLine();
+
+        System.out.println("Frase convertida para minúsculas: " + str.toLowerCase());
+        System.out.println("Último caracter da frase: " + str.substring(str.length()-1));
+        System.out.println("Os 3 primeiros caracteres: " + str.substring(0, 3));
+        System.out.printf("Número de digitos na frase: %d\n", StringMethods.countDigits(str));
+        System.out.printf("Número de espaços na frase: %d\n", StringMethods.countSpaces(str));
+        System.out.printf("A frase contém apenas letras minúsculas: %b\n", StringMethods.isAllLower(str));
+        System.out.println("Frase sem espaços duplicados: " + StringMethods.removeDuplicatedSpaces(str));
+        System.out.println("A string é um palindromo: " + StringMethods.isPalindrome(str));
+        System.out.println("Acrónimo da string: " + StringMethods.getAcronym(str));
+
+        sin.close();
+    }
+}
diff --git a/1ano/2semestre/poo/src/utils/StringMethods.java b/1ano/2semestre/poo/src/utils/StringMethods.java
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+++ b/1ano/2semestre/poo/src/utils/StringMethods.java
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+package utils;
+
+public class StringMethods {
+    public static int countDigits(String s) {
+        int count = 0;
+        for (char c : s.toCharArray())
+            if (Character.isDigit(c))
+                count++;
+        return count;
+    }
+
+    public static int countSpaces(String s) {
+        int count = 0;
+        for (char c : s.toCharArray())
+            if (c == ' ')
+                count++;
+        return count;
+    }
+
+    public static boolean isAllLower(String s) {
+        for (char c : s.toCharArray())
+            if (Character.isUpperCase(c))
+                return false;
+        return true;
+    }
+
+    public static String removeDuplicatedSpaces(String s) {
+        StringBuilder newStr = new StringBuilder();
+        for (char c : s.toCharArray())
+            if (c != ' ' || !newStr.substring(newStr.length() - 1).equals(" "))
+                newStr.append(c);
+        return newStr.toString();
+    }
+
+    public static boolean isPalindrome(String s) {
+        char[] chars = s.toCharArray();
+        for (int i = 0; i < s.length() / 2; i++)
+            if (chars[i] != chars[chars.length - 1 - i])
+                return false;
+        return true;
+    }
+
+    public static String getAcronym(String s) {
+        String[] words = s.split(" ");
+        StringBuilder acronym = new StringBuilder();
+        for (String word : words)
+            if (word.length() >= 3)
+                acronym.append(word.charAt(0));
+        return acronym.toString().toUpperCase();
+    }
+}