feat: add java calculator example

java/BUILD.bazel

@@ -0,0 +1,14 @@
+load("@rules_java//java:defs.bzl", "java_library", "java_test")
+
+java_library(
+    name = "calculator",
+    srcs = ["Calculator.java"],
+    visibility = ["//visibility:public"],
+)
+
+java_test(
+    name = "CalculatorTest",
+    srcs = ["CalculatorTest.java"],
+    test_class = "build.aspect.java.CalculatorTest",
+    deps = [":calculator"],
+)

java/Calculator.java

@@ -0,0 +1,24 @@
+package build.aspect.java;
+
+/** Simple calculator demonstrating Bazel Java build and test. */
+public class Calculator {
+
+  public int add(int a, int b) {
+    return a + b;
+  }
+
+  public int subtract(int a, int b) {
+    return a - b;
+  }
+
+  public int multiply(int a, int b) {
+    return a + b;
+  }
+
+  public double divide(int a, int b) {
+    if (b == 0) {
+      throw new IllegalArgumentException("Cannot divide by zero");
+    }
+    return (double) a / b;
+  }
+}

java/CalculatorTest.java

@@ -0,0 +1,51 @@
+package build.aspect.java;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for Calculator. */
+@RunWith(JUnit4.class)
+public final class CalculatorTest {
+
+  private Calculator calculator;
+
+  @Before
+  public void setUp() {
+    calculator = new Calculator();
+  }
+
+  @Test
+  public void testAdd() {
+    assertEquals(4, calculator.add(2, 2));
+    assertEquals(0, calculator.add(-1, 1));
+    assertEquals(-3, calculator.add(-1, -2));
+  }
+
+  @Test
+  public void testSubtract() {
+    assertEquals(1, calculator.subtract(3, 2));
+    assertEquals(-2, calculator.subtract(0, 2));
+  }
+
+  @Test
+  public void testDivide() {
+    assertEquals(2.0, calculator.divide(6, 3), 0.001);
+    assertEquals(0.5, calculator.divide(1, 2), 0.001);
+  }
+
+  @Test
+  public void testAddWithZero() {
+    assertEquals(5, calculator.add(5, 0));
+    assertEquals(0, calculator.add(0, 0));
+  }
+
+  @Test
+  public void testDivideByZero() {
+    assertThrows(IllegalArgumentException.class, () -> calculator.divide(1, 0));
+  }
+}