* Lower.java

* Upper.java
* Pangram.java
* CharactersSame.java
* CheckAnagrams.java

strings/CharactersSame.java

@@ -0,0 +1,29 @@
+package strings;
+
+public class CharactersSame {
+
+    /**
+     * Driver Code
+     */
+    public static void main(String[] args) {
+        assert isAllCharactersSame("");
+        assert !isAllCharactersSame("aab");
+        assert isAllCharactersSame("aaa");
+        assert isAllCharactersSame("11111");
+    }
+
+    /**
+     * check if all the characters of a string are same
+     *
+     * @param s the string to check
+     * @return {@code true} if all characters of a string are same, otherwise {@code false}
+     */
+    public static boolean isAllCharactersSame(String s) {
+        for (int i = 1, length = s.length(); i < length; ++i) {
+            if (s.charAt(i) != s.charAt(0)) {
+                return false;
+            }
+        }
+        return true;
+    }
+}

strings/CheckAnagrams.java

@@ -0,0 +1,32 @@
+package strings;
+
+import java.util.Arrays;
+
+/**
+ * Two strings are anagrams if they are made of the same letters
+ * arranged differently (ignoring the case).
+ */
+public class CheckAnagrams {
+    public static void main(String[] args) {
+        assert isAnagrams("Silent", "Listen");
+        assert isAnagrams("This is a string", "Is this a string");
+        assert !isAnagrams("There", "Their");
+    }
+
+    /**
+     * Check if two strings are anagrams or not
+     *
+     * @param s1 the first string
+     * @param s2 the second string
+     * @return {@code true} if two string are anagrams, otherwise {@code false}
+     */
+    public static boolean isAnagrams(String s1, String s2) {
+        s1 = s1.toLowerCase();
+        s2 = s2.toLowerCase();
+        char[] values1 = s1.toCharArray();
+        char[] values2 = s2.toCharArray();
+        Arrays.sort(values1);
+        Arrays.sort(values2);
+        return new String(values1).equals(new String(values2));
+    }
+}

strings/Lower.java

@@ -0,0 +1,30 @@
+package strings;
+
+public class Lower {
+
+    /**
+     * Driver Code
+     */
+    public static void main(String[] args) {
+        String[] strings = {"ABC", "ABC123", "abcABC", "abc123ABC"};
+        for (String s : strings) {
+            assert toLowerCase(s).equals(s.toLowerCase());
+        }
+    }
+
+    /**
+     * Converts all of the characters in this {@code String} to lower case
+     *
+     * @param s the string to convert
+     * @return the {@code String}, converted to lowercase.
+     */
+    public static String toLowerCase(String s) {
+        char[] values = s.toCharArray();
+        for (int i = 0; i < values.length; ++i) {
+            if (Character.isLetter(values[i]) && Character.isUpperCase(values[i])) {
+                values[i] = Character.toLowerCase(values[i]);
+            }
+        }
+        return new String(values);
+    }
+}

strings/Pangram.java

@@ -0,0 +1,39 @@
+package strings;
+
+/**
+ * Wikipedia: https://en.wikipedia.org/wiki/Pangram
+ */
+public class Pangram {
+
+    /**
+     * Driver Code
+     */
+    public static void main(String[] args) {
+        assert isPangram("The quick brown fox jumps over the lazy dog");
+        assert !isPangram("The quick brown fox jumps over the azy dog"); /* not exists l character */
+    }
+
+    /**
+     * Check if a string is a pangram string or not
+     *
+     * @param s string to check
+     * @return {@code true} if given string is pangram, otherwise {@code false}
+     */
+    public static boolean isPangram(String s) {
+        boolean[] marked = new boolean[26]; /* by default all letters don't exists */
+        char[] values = s.toCharArray();
+        for (char value : values) {
+            if (Character.isLetter(value)) {
+                int index = Character.isUpperCase(value) ? value - 'A' : value - 'a';
+                marked[index] = true; /* mark current character exists */
+            }
+        }
+
+        for (boolean b : marked) {
+            if (!b) {
+                return false;
+            }
+        }
+        return true;
+    }
+}

strings/Upper.java

@@ -0,0 +1,30 @@
+package strings;
+
+public class Upper {
+
+    /**
+     * Driver Code
+     */
+    public static void main(String[] args) {
+        String[] strings = {"ABC", "ABC123", "abcABC", "abc123ABC"};
+        for (String s : strings) {
+            assert toUpperCase(s).equals(s.toUpperCase());
+        }
+    }
+
+    /**
+     * Converts all of the characters in this {@code String} to upper case
+     *
+     * @param s the string to convert
+     * @return the {@code String}, converted to uppercase.
+     */
+    public static String toUpperCase(String s) {
+        char[] values = s.toCharArray();
+        for (int i = 0; i < values.length; ++i) {
+            if (Character.isLetter(values[i]) && Character.isLowerCase(values[i])) {
+                values[i] = Character.toUpperCase(values[i]);
+            }
+        }
+        return new String(values);
+    }
+}