BAEL-9382: Stable Values in Java 25 (#18796)

* Stable Values in Java 25: Sample

* Remove final From Local Variables

---------

Co-authored-by: Josh Cummings <[email protected]>

Author: danielmcnally285

core-java-modules/core-java-25/pom.xml

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-25</artifactId>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>25</source>
+                    <target>25</target>
+                    <compilerArgs>--enable-preview</compilerArgs>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+</project>

core-java-modules/core-java-25/src/test/java/com/baeldung/stablevalues/StableValuesUnitTest.java

@@ -0,0 +1,78 @@
+package com.baeldung.stablevalues;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+
+import org.junit.jupiter.api.BeforeEach
+
+class StableValuesUnitTest {
+
+    private Set<String> cities;
+
+    private String expensiveMethodToDetermineCountry(String city) {
+        switch(city) {
+        case "Berlin":
+            return "Germany";
+        case "London":
+            return "England";
+        case "Madrid":
+            return "Spain";        
+        case "Paris":
+            return "France";
+        default:
+            throw new RuntimeException("Unsupported city");
+        }
+    }
+
+    @Test
+    void givenStableFunctionForCityToCountry_whenValidInputsUsed_thenVerifyFunctionResultsAreExpected() {
+        Function<String, String> cityToCountry = StableValue.function(cities, city -> expensiveMethodToDetermineCountry(city));
+        
+        assertThat(cityToCountry.apply("London")).isEqualTo("England");
+        assertThat(cityToCountry.apply("Madrid")).isEqualTo("Spain");
+        assertThat(cityToCountry.apply("Paris")).isEqualTo("France");
+    }
+
+    @Test
+    void givenStableFunctionForCityToCountry_whenInvalidInputUsed_thenExceptionThrown() {
+        Function<String, String> cityToCountry = StableValue.function(cities, city -> expensiveMethodToDetermineCountry(city));
+        
+        assertThatIllegalArgumentException().isThrownBy(() -> cityToCountry.apply("Berlin"));
+    }
+
+    @Test
+    void givenStableListForFiveTimesTable_thenVerifyElementsAreExpected() {
+        List<Integer> fiveTimesTable = StableValue.list(11, index -> index * 5);
+        
+        assertThat(fiveTimesTable.get(0)).isEqualTo(0);
+        assertThat(fiveTimesTable.get(1)).isEqualTo(5);
+        assertThat(fiveTimesTable.get(2)).isEqualTo(10);
+        assertThat(fiveTimesTable.get(3)).isEqualTo(15);
+        assertThat(fiveTimesTable.get(4)).isEqualTo(20);
+        assertThat(fiveTimesTable.get(5)).isEqualTo(25);
+        assertThat(fiveTimesTable.get(6)).isEqualTo(30);
+        assertThat(fiveTimesTable.get(7)).isEqualTo(35);
+        assertThat(fiveTimesTable.get(8)).isEqualTo(40);
+        assertThat(fiveTimesTable.get(9)).isEqualTo(45);
+        assertThat(fiveTimesTable.get(10)).isEqualTo(50);
+    }
+
+    @Test
+    void givenStableMapForCityToCountry_thenVerifyValuesAreExpected() {
+        Map<String, String> cityToCountry = StableValue.map(cities, city -> expensiveMethodToDetermineCountry(city));
+        
+        assertThat(cityToCountry.get("London")).isEqualTo("England");
+        assertThat(cityToCountry.get("Madrid")).isEqualTo("Spain");
+        assertThat(cityToCountry.get("Paris")).isEqualTo("France");
+    }
+
+    @BeforeEach
+    void init() {
+        cities = Set.of("London", "Madrid", "Paris");
+    }
+}