added all the temperature converter static methods

src/main/java/com/thealgorithms/conversions/TemperatureConverter.java

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+package com.thealgorithms.conversions;
+
+/**
+ * A utility class for converting temperatures between
+ * Celsius, Fahrenheit, and Kelvin scales.
+ *
+ * This class provides static methods, so it can be used directly
+ * without creating an instance.
+ */
+public class TemperatureConverter {
+
+    /**
+     * Converts temperature from Celsius to Fahrenheit.
+     *
+     * Formula: (°C × 9/5) + 32 = °F
+     *
+//     * @param celsius temperature in Celsius
+     * @return equivalent temperature in Fahrenheit
+     */
+    static double celsiusToFahrenheit(double celsius) {
+        return (celsius * 9.0) / 5.0 + 32.0;
+    }
+
+    /**
+     * Converts temperature from Celsius to Kelvin.
+     *
+     * Formula: °C + 273.15 = K
+     *
+//     * @param celsius temperature in Celsius
+     * @return equivalent temperature in Kelvin
+     */
+    static double celsiusToKelvin(double celsius) {
+        return celsius + 273.15;
+    }
+
+    /**
+     * Converts temperature from Fahrenheit to Celsius.
+     *
+     * Formula: (°F − 32) × 5/9 = °C
+     *
+//     * @param fahrenheit temperature in Fahrenheit
+     * @return equivalent temperature in Celsius
+     */
+    static double fahrenheitToCelsius(double fahrenheit) {
+        return (fahrenheit - 32.0) * 5.0 / 9.0;
+    }
+
+    /**
+     * Converts temperature from Fahrenheit to Kelvin.
+     *
+     * Formula: (°F − 32) × 5/9 + 273.15 = K
+     *
+//     * @param fahrenheit temperature in Fahrenheit
+     * @return equivalent temperature in Kelvin
+     */
+    static double fahrenheitToKelvin(double fahrenheit) {
+        return (fahrenheit - 32.0) * 5.0 / 9.0 + 273.15;
+    }
+
+    /**
+     * Converts temperature from Kelvin to Celsius.
+     *
+     * Formula: K − 273.15 = °C
+     *
+//     * @param kelvin temperature in Kelvin
+     * @return equivalent temperature in Celsius
+     */
+    static double kelvinToCelsius(double kelvin) {
+        return kelvin - 273.15;
+    }
+
+    /**
+     * Converts temperature from Kelvin to Fahrenheit.
+     *
+     * Formula: (K − 273.15) × 9/5 + 32 = °F
+     *
+//     * @param kelvin temperature in Kelvin
+     * @return equivalent temperature in Fahrenheit
+     */
+    static double kelvinToFahrenheit(double kelvin) {
+        return (kelvin - 273.15) * 9.0 / 5.0 + 32.0;
+    }
+}

src/test/java/com/thealgorithms/conversions/TemperatureConverterTest.java

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+package com.thealgorithms.conversions;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+public class TemperatureConverterTest {
+
+    @Test
+    public void testCelsiusToFahrenheit() {
+        assertEquals(32.0, TemperatureConverter.celsiusToFahrenheit(0), 0.001);
+        assertEquals(212.0, TemperatureConverter.celsiusToFahrenheit(100), 0.001);
+    }
+
+    @Test
+    public void testCelsiusToKelvin() {
+        assertEquals(273.15, TemperatureConverter.celsiusToKelvin(0), 0.001);
+        assertEquals(373.15, TemperatureConverter.celsiusToKelvin(100), 0.001);
+    }
+
+    @Test
+    public void testFahrenheitToCelsius() {
+        assertEquals(0.0, TemperatureConverter.fahrenheitToCelsius(32.0), 0.001);
+        assertEquals(100.0, TemperatureConverter.fahrenheitToCelsius(212.0), 0.001);
+    }
+
+    @Test
+    public void testFahrenheitToKelvin() {
+        assertEquals(273.15, TemperatureConverter.fahrenheitToKelvin(32.0), 0.001);
+        assertEquals(373.15, TemperatureConverter.fahrenheitToKelvin(212.0), 0.001);
+    }
+
+    @Test
+    public void testKelvinToCelsius() {
+        assertEquals(0.0, TemperatureConverter.kelvinToCelsius(273.15), 0.001);
+        assertEquals(100.0, TemperatureConverter.kelvinToCelsius(373.15), 0.001);
+    }
+
+    @Test
+    public void testKelvinToFahrenheit() {
+        assertEquals(32.0, TemperatureConverter.kelvinToFahrenheit(273.15), 0.001);
+        assertEquals(212.0, TemperatureConverter.kelvinToFahrenheit(373.15), 0.001);
+    }
+}