root cause finder

core-java/src/main/java/com/baeldung/exceptions/RootCauseFinder.java

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+package com.baeldung.exceptions;
+
+import java.util.Objects;
+
+/**
+ * Utility class to find root cause exceptions.
+ */
+public class RootCauseFinder {
+
+    public static Throwable findCauseUsingPlainJava(Throwable throwable) {
+        Objects.requireNonNull(throwable);
+        Throwable rootCause = throwable;
+        while (rootCause.getCause() != null) {
+            rootCause = rootCause.getCause();
+        }
+        return rootCause;
+    }
+
+    static class IntParser {
+
+        private IntParser() {
+        }
+
+        public static int parse(String input) throws InvalidNumber {
+            if (input == null || input.isEmpty()) {
+                throw new IllegalArgumentException();
+            }
+
+            try {
+                return new IntParser().stringToInt(input.trim());
+            } catch (NaNException ex) {
+                throw new InvalidNumber(input, ex);
+            }
+        }
+
+        private int stringToInt(String numberAsString) throws NaNException {
+            try {
+                return Integer.valueOf(numberAsString);
+            } catch (NumberFormatException ex) {
+                throw new NaNException(numberAsString, ex);
+            }
+        }
+
+    }
+
+    static class InvalidNumber extends Exception {
+
+        InvalidNumber(String input, Throwable thr) {
+            super("Invalid input for a number: " + input, thr);
+        }
+    }
+
+    static class NaNException extends Exception {
+
+        NaNException(String number, Throwable thr) {
+            super(number + "is not a number", thr);
+        }
+
+    }
+
+}

core-java/src/test/java/com/baeldung/exceptions/RootCauseFinderTest.java

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+package com.baeldung.exceptions;
+
+import com.google.common.base.Throwables;
+import org.apache.commons.lang3.exception.ExceptionUtils;
+import org.junit.jupiter.api.Test;
+
+import static com.baeldung.exceptions.RootCauseFinder.*;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Tests the {@link RootCauseFinder}.
+ */
+public class RootCauseFinderTest {
+
+    @Test
+    public void givenNestedException_whenFindingRootCauseUsingJava_thenRootCauseFound() {
+        try {
+            IntParser.parse("text");
+        } catch (InvalidNumber ex) {
+            assertTrue(findCauseUsingPlainJava(ex) instanceof NumberFormatException);
+        }
+    }
+
+    @Test
+    public void givenNonNestedException_whenFindingRootCauseUsingJava_thenRootCauseFound() {
+        try {
+            IntParser.parse(null);
+        } catch (Exception ex) {
+            assertTrue(findCauseUsingPlainJava(ex) instanceof IllegalArgumentException);
+        }
+    }
+
+    @Test
+    public void givenNestedException_whenFindingRootCauseUsingApacheCommons_thenRootCauseFound() {
+        try {
+            IntParser.parse("text");
+        } catch (InvalidNumber ex) {
+            assertTrue(ExceptionUtils.getRootCause(ex) instanceof NumberFormatException);
+        }
+    }
+
+    @Test
+    public void givenNestedException_whenFindingRootCauseUsingGuava_thenRootCauseFound() {
+        try {
+            IntParser.parse("text");
+        } catch (InvalidNumber ex) {
+            assertTrue(Throwables.getRootCause(ex) instanceof NumberFormatException);
+        }
+    }
+
+}