diff --git a/core-java-modules/core-java-io-4/src/test/java/com/baeldung/mockinginputstream/GeneratingInputStream.java b/core-java-modules/core-java-io-4/src/test/java/com/baeldung/mockinginputstream/GeneratingInputStream.java
new file mode 100644
index 0000000000..5fd0c3cf03
--- /dev/null
+++ b/core-java-modules/core-java-io-4/src/test/java/com/baeldung/mockinginputstream/GeneratingInputStream.java
@@ -0,0 +1,23 @@
+package com.baeldung.mockinginputstream;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public class GeneratingInputStream extends InputStream {
+    private final int desiredSize;
+    private int actualSize = 0;
+    private final byte[] seed;
+
+    public GeneratingInputStream(int desiredSize, String seed) {
+        this.desiredSize = desiredSize;
+        this.seed = seed.getBytes();
+    }
+
+    @Override
+    public int read() {
+        if (actualSize >= desiredSize) {
+            return -1;
+        }
+        return seed[actualSize++ % seed.length];
+    }
+}
diff --git a/core-java-modules/core-java-io-4/src/test/java/com/baeldung/mockinginputstream/MockingInputStreamUnitTest.java b/core-java-modules/core-java-io-4/src/test/java/com/baeldung/mockinginputstream/MockingInputStreamUnitTest.java
new file mode 100644
index 0000000000..4b29a778b0
--- /dev/null
+++ b/core-java-modules/core-java-io-4/src/test/java/com/baeldung/mockinginputstream/MockingInputStreamUnitTest.java
@@ -0,0 +1,59 @@
+package com.baeldung.mockinginputstream;
+
+import org.junit.Test;
+
+import java.io.ByteArrayInputStream;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+public class MockingInputStreamUnitTest {
+    @Test
+    public void givenSimpleImplementation_shouldProcessInputStream() throws IOException {
+        int byteCount = processInputStream(new InputStream() {
+            private final byte[] msg = "Hello World".getBytes();
+            private int index = 0;
+            @Override
+            public int read() {
+                if (index >= msg.length) {
+                    return -1;
+                }
+                return msg[index++];
+            }
+        });
+        assertThat(byteCount).isEqualTo(11);
+    }
+
+    @Test
+    public void givenByteArrayInputStream_shouldProcessInputStream() throws IOException {
+        String msg = "Hello World";
+        int bytesCount = processInputStream(new ByteArrayInputStream(msg.getBytes()));
+        assertThat(bytesCount).isEqualTo(11);
+    }
+
+    @Test
+    public void givenFileInputStream_shouldProcessInputStream() throws IOException {
+        InputStream inputStream = MockingInputStreamUnitTest.class.getResourceAsStream("/mockinginputstreams/msg.txt");
+        int bytesCount = processInputStream(inputStream);
+        assertThat(bytesCount).isEqualTo(11);
+        inputStream.close();
+    }
+
+    @Test
+    public void givenGeneratingInputStream_shouldProcessInputStream() throws IOException {
+        InputStream inputStream = new GeneratingInputStream(10_000, "Hello World");
+        int bytesCount = processInputStream(inputStream);
+        assertThat(bytesCount).isEqualTo(10_000);
+        inputStream.close();
+    }
+
+    int processInputStream(InputStream inputStream) throws IOException {
+        int count = 0;
+        while(inputStream.read() != -1) {
+            count++;
+        }
+        return count;
+    }
+}
diff --git a/core-java-modules/core-java-io-4/src/test/resources/mockinginputstreams/msg.txt b/core-java-modules/core-java-io-4/src/test/resources/mockinginputstreams/msg.txt
new file mode 100644
index 0000000000..5e1c309dae
--- /dev/null
+++ b/core-java-modules/core-java-io-4/src/test/resources/mockinginputstreams/msg.txt
@@ -0,0 +1 @@
+Hello World
\ No newline at end of file