[os-to-is] Convert an OutputStream to an InputStream (#13992)

core-java-modules/core-java-io-conversions-2/src/test/java/com/baeldung/outputstreamtoinputstream/ConvertOutputStreamToInputStreamUnitTest.java

@@ -0,0 +1,53 @@
+package com.baeldung.outputstreamtoinputstream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.PipedInputStream;
+import java.io.PipedOutputStream;
+
+import org.junit.jupiter.api.Test;
+
+public class ConvertOutputStreamToInputStreamUnitTest {
+
+    @Test
+    void whenUsingByteArray_thenGetExpectedInputStream() throws IOException {
+        String content = "I'm an important message.";
+        try (ByteArrayOutputStream out = new ByteArrayOutputStream()) {
+            out.write(content.getBytes());
+            try (ByteArrayInputStream in = new ByteArrayInputStream(out.toByteArray())) {
+                String inContent = new String(in.readAllBytes());
+
+                assertEquals(content, inContent);
+            }
+        }
+    }
+
+    @Test
+    void whenUsingPipeStream_thenGetExpectedInputStream() throws IOException {
+        String content = "I'm going through the pipe.";
+
+        ByteArrayOutputStream originOut = new ByteArrayOutputStream();
+        originOut.write(content.getBytes());
+
+        //connect the pipe
+        PipedInputStream in = new PipedInputStream();
+        PipedOutputStream out = new PipedOutputStream(in);
+
+        try (in) {
+            new Thread(() -> {
+                try (out) {
+                    originOut.writeTo(out);
+                } catch (IOException iox) {
+                    // handle IOExceptions
+                }
+            }).start();
+
+            String inContent = new String(in.readAllBytes());
+            assertEquals(content, inContent);
+        }
+    }
+
+}