Throw and throws in Java (#4990)

core-java/src/main/java/com/baeldung/throwsexception/Calculator.java

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+package com.baeldung.throwsexception;
+
+public class Calculator {
+
+    public double divide(double a, double b) {
+        if (b == 0) {
+            throw new DivideByZeroException("Divider cannot be equal to zero!");
+        }
+        return a/b;
+    }
+
+}
+
+
+

core-java/src/main/java/com/baeldung/throwsexception/DataAccessException.java

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+package com.baeldung.throwsexception;
+
+public class DataAccessException extends RuntimeException {
+
+    public DataAccessException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+}

core-java/src/main/java/com/baeldung/throwsexception/DivideByZeroException.java

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+package com.baeldung.throwsexception;
+
+public class DivideByZeroException extends RuntimeException {
+
+    public DivideByZeroException(String message) {
+        super(message);
+    }
+
+}

core-java/src/main/java/com/baeldung/throwsexception/Main.java

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+package com.baeldung.throwsexception;
+
+import com.sun.mail.iap.ConnectionException;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.net.SocketException;
+
+public class Main {
+
+    public static void main(String[] args) throws FileNotFoundException {
+        TryCatch tryCatch = new TryCatch();
+
+        try {
+            tryCatch.execute();
+        } catch (ConnectionException | SocketException ex) {
+            System.out.println("IOException");
+        } catch (Exception ex) {
+            System.out.println("General exception");
+        }
+
+        checkedException();
+        checkedExceptionWithThrows();
+    }
+
+    private static void checkedExceptionWithThrows() throws FileNotFoundException {
+        File file = new File("not_existing_file.txt");
+        FileInputStream stream = new FileInputStream(file);
+    }
+
+    private static void checkedException() {
+        File file = new File("not_existing_file.txt");
+        try {
+            FileInputStream stream = new FileInputStream(file);
+        } catch (FileNotFoundException e) {
+            e.printStackTrace();
+        }
+    }
+
+}

core-java/src/main/java/com/baeldung/throwsexception/PersonRepository.java

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+package com.baeldung.throwsexception;
+
+import javax.annotation.Nullable;
+import java.sql.SQLException;
+import java.util.List;
+
+public class PersonRepository {
+
+    @Nullable
+    public String findNameById(String id) {
+        return id == null ? null : "example-name";
+    }
+
+    public List<String> findAll() throws SQLException {
+        throw new SQLException();
+    }
+
+}

core-java/src/main/java/com/baeldung/throwsexception/SimpleService.java

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+package com.baeldung.throwsexception;
+
+import java.sql.SQLException;
+
+public class SimpleService {
+
+    private PersonRepository personRepository = new PersonRepository();
+
+    public void wrappingException() {
+        try {
+            personRepository.findAll();
+        } catch (SQLException e) {
+            throw new DataAccessException("SQL Exception", e);
+        }
+    }
+
+    public void runtimeNullPointerException() {
+        String a = null;
+        a.length();
+    }
+
+}

core-java/src/main/java/com/baeldung/throwsexception/TryCatch.java

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+package com.baeldung.throwsexception;
+
+import com.sun.mail.iap.ConnectionException;
+
+import java.net.SocketException;
+
+public class TryCatch {
+
+    public void execute() throws SocketException, ConnectionException, Exception  {
+        //code that would throw any of: SocketException, ConnectionException, Exception
+    }
+
+}

core-java/src/test/java/com/baeldung/throwsexception/CalculatorUnitTest.java

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+package com.baeldung.throwsexception;
+
+import org.junit.Test;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CalculatorUnitTest {
+
+    @Test
+    public void whenDividerIsZero_thenDivideByZeroExceptionIsThrown() {
+        Calculator calculator = new Calculator();
+
+        assertThrows(DivideByZeroException.class,
+                () -> calculator.divide(10, 0));
+    }
+
+}

core-java/src/test/java/com/baeldung/throwsexception/PersonRepositoryUnitTest.java

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+package com.baeldung.throwsexception;
+
+import org.junit.Test;
+
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.assertAll;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class PersonRepositoryUnitTest {
+
+    PersonRepository personRepository = new PersonRepository();
+
+    @Test
+    public void whenIdIsNull_thenExceptionIsThrown() throws Exception {
+        assertThrows(Exception.class,
+                () ->
+                        Optional
+                                .ofNullable(personRepository.findNameById(null))
+                                .orElseThrow(Exception::new));
+    }
+
+    @Test
+    public void whenIdIsNonNull_thenNoExceptionIsThrown() throws Exception {
+        assertAll(
+                () ->
+                        Optional
+                                .ofNullable(personRepository.findNameById("id"))
+                                .orElseThrow(Exception::new));
+    }
+
+}

core-java/src/test/java/com/baeldung/throwsexception/SimpleServiceUnitTest.java

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+package com.baeldung.throwsexception;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class SimpleServiceUnitTest {
+
+    SimpleService simpleService = new SimpleService();
+
+    @Test
+    void whenSQLExceptionIsThrown_thenShouldBeRethrownWithWrappedException() {
+        assertThrows(DataAccessException.class,
+                () -> simpleService.wrappingException());
+    }
+
+    @Test
+    void whenCalled_thenNullPointerExceptionIsThrown() {
+        assertThrows(NullPointerException.class,
+                () -> simpleService.runtimeNullPointerException());
+    }
+}