Added BAEL-3604 code (#9020)

* Added BAEL-3604 code

core-java-modules/core-java-security-2/src/main/java/com/baeldung/checksums/ChecksumUtils.java

@@ -0,0 +1,23 @@
+package com.baeldung.checksums;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.zip.CRC32;
+import java.util.zip.CheckedInputStream;
+import java.util.zip.Checksum;
+
+public class ChecksumUtils {
+
+    public static long getChecksumCRC32(byte[] bytes) {
+        Checksum crc32 = new CRC32();
+        crc32.update(bytes, 0, bytes.length);
+        return crc32.getValue();
+    }
+
+    public static long getChecksumCRC32(InputStream stream, int bufferSize) throws IOException {
+        CheckedInputStream checkedInputStream = new CheckedInputStream(stream, new CRC32());
+        byte[] buffer = new byte[bufferSize];
+        while (checkedInputStream.read(buffer, 0, buffer.length) >= 0) {}
+        return checkedInputStream.getChecksum().getValue();
+    }
+}

core-java-modules/core-java-security-2/src/test/java/com/baeldung/checksums/ChecksumUtilsUnitTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.checksums;
+
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ChecksumUtilsUnitTest {
+
+    byte[] arr;
+
+    @Before
+    void setUp() {
+        arr =  new byte[]{0,10,21,20,35,40,120,56,72,22};
+    }
+
+    @Test
+    void givenByteArray_whenChecksumCreated_checkCorrect() {
+
+        long checksum = ChecksumUtils.getChecksumCRC32(arr);
+
+        assertEquals(3915397664L, checksum);
+    }
+
+    @Test
+    void givenTwoDifferentStrings_whenChecksumCreated_checkCollision() {
+
+        String plumless = "plumless";
+        String buckeroo = "buckeroo";
+
+        long plumlessChecksum = ChecksumUtils.getChecksumCRC32(plumless.getBytes());
+        long buckerooChecksum = ChecksumUtils.getChecksumCRC32(buckeroo.getBytes());
+
+        assertEquals(plumlessChecksum, buckerooChecksum);
+    }
+
+    @Test
+    void givenInputString_whenChecksumCreated_checkCorrect() throws IOException {
+
+        InputStream inputStream = new ByteArrayInputStream(arr);
+        long checksum = ChecksumUtils.getChecksumCRC32(inputStream, 10);
+
+        assertEquals(3915397664L, checksum);
+
+    }
+}