BAEL-6352: Check if the First Letter of a String is a Number

core-java-modules/core-java-string-operations-5/src/main/java/com/baeldung/firstchardigit/FirstCharDigit.java

@@ -0,0 +1,62 @@
+package com.baeldung.firstchardigit;
+
+import java.util.regex.Pattern;
+
+import com.google.common.base.CharMatcher;
+
+public class FirstCharDigit {
+
+    public static boolean checkUsingCharAtMethod(String str) {
+        if (str == null || str.length() == 0) {
+            return false;
+        }
+
+        char c = str.charAt(0);
+        return c >= '0' && c <= '9';
+    }
+
+    public static boolean checkUsingIsDigitMethod(String str) {
+        if (str == null || str.length() == 0) {
+            return false;
+        }
+
+        return Character.isDigit(str.charAt(0));
+    }
+
+    public static boolean checkUsingPatternClass(String str) {
+        if (str == null || str.length() == 0) {
+            return false;
+        }
+
+        return Pattern.compile("^[0-9].*")
+          .matcher(str)
+          .matches();
+    }
+
+    public static boolean checkUsingMatchesMethod(String str) {
+        if (str == null || str.length() == 0) {
+            return false;
+        }
+
+        return str.matches("^[0-9].*");
+    }
+
+    public static boolean checkUsingCharMatcherInRangeMethod(String str) {
+        if (str == null || str.length() == 0) {
+            return false;
+        }
+
+        return CharMatcher.inRange('0', '9')
+          .matches(str.charAt(0));
+    }
+
+    public static boolean checkUsingCharMatcherForPredicateMethod(String str) {
+        if (str == null || str.length() == 0) {
+            return false;
+        }
+
+        return CharMatcher.forPredicate(Character::isDigit)
+          .matches(str.charAt(0));
+    }
+
+}

core-java-modules/core-java-string-operations-5/src/test/java/com/baeldung/firstchardigit/FirstCharDigitUnitTest.java

@@ -0,0 +1,58 @@
+package com.baeldung.firstchardigit;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class FirstCharDigitUnitTest {
+
+    @Test
+    void givenString_whenUsingCharAtMethod_thenSuccess() {
+        assertTrue(FirstCharDigit.checkUsingCharAtMethod("12 years"));
+        assertFalse(FirstCharDigit.checkUsingCharAtMethod("years"));
+        assertFalse(FirstCharDigit.checkUsingCharAtMethod(""));
+        assertFalse(FirstCharDigit.checkUsingCharAtMethod(null));
+    }
+
+    @Test
+    void givenString_whenUsingIsDigitMethod_thenSuccess() {
+        assertTrue(FirstCharDigit.checkUsingIsDigitMethod("10 cm"));
+        assertFalse(FirstCharDigit.checkUsingIsDigitMethod("cm"));
+        assertFalse(FirstCharDigit.checkUsingIsDigitMethod(""));
+        assertFalse(FirstCharDigit.checkUsingIsDigitMethod(null));
+    }
+
+    @Test
+    void givenString_whenUsingPatternClass_thenSuccess() {
+        assertTrue(FirstCharDigit.checkUsingPatternClass("1 kg"));
+        assertFalse(FirstCharDigit.checkUsingPatternClass("kg"));
+        assertFalse(FirstCharDigit.checkUsingPatternClass(""));
+        assertFalse(FirstCharDigit.checkUsingPatternClass(null));
+    }
+
+    @Test
+    void givenString_whenUsingMatchesMethod_thenSuccess() {
+        assertTrue(FirstCharDigit.checkUsingMatchesMethod("123"));
+        assertFalse(FirstCharDigit.checkUsingMatchesMethod("ABC"));
+        assertFalse(FirstCharDigit.checkUsingMatchesMethod(""));
+        assertFalse(FirstCharDigit.checkUsingMatchesMethod(null));
+    }
+
+    @Test
+    void givenString_whenUsingCharMatcherInRangeMethod_thenSuccess() {
+        assertTrue(FirstCharDigit.checkUsingCharMatcherInRangeMethod("2023"));
+        assertFalse(FirstCharDigit.checkUsingCharMatcherInRangeMethod("abc"));
+        assertFalse(FirstCharDigit.checkUsingCharMatcherInRangeMethod(""));
+        assertFalse(FirstCharDigit.checkUsingCharMatcherInRangeMethod(null));
+    }
+
+    @Test
+    void givenString_whenUsingCharMatcherForPredicateMethod_thenSuccess() {
+        assertTrue(FirstCharDigit.checkUsingCharMatcherForPredicateMethod("100"));
+        assertFalse(FirstCharDigit.checkUsingCharMatcherForPredicateMethod("abdo"));
+        assertFalse(FirstCharDigit.checkUsingCharMatcherForPredicateMethod(""));
+        assertFalse(FirstCharDigit.checkUsingCharMatcherForPredicateMethod(null));
+    }
+
+}