Checked collection methods (#17218)

* Initial commit for checked collection methods

* Removed public

* Fixed test as per review

* Fixed test as per review

* Clean imports

* Fixed test name

Author: sagaofsilence

core-java-modules/core-java-collections-6/src/main/java/com/baeldung/checkedcollections/DataProcessor.java

@@ -0,0 +1,19 @@
+package com.baeldung.checkedcollections;
+
+import java.util.Collection;
+
+class DataProcessor {
+
+    public boolean checkPrefix(Collection<?> data) {
+        boolean result = true;
+        if (data != null) {
+            for (Object item : data) {
+                if (item != null && !((String) item).startsWith("DATA_")) {
+                    result = false;
+                    break;
+                }
+            }
+        }
+        return result;
+    }
+}

core-java-modules/core-java-collections-6/src/test/java/com/baeldung/checkedcollections/DataProcessorUnitTest.java

@@ -0,0 +1,41 @@
+package com.baeldung.checkedcollections;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+
+import org.junit.jupiter.api.Test;
+
+class DataProcessorUnitTest {
+
+    @Test
+    void givenGenericCollection_whenInvalidTypeDataAdded_thenFailsAfterInvocation() {
+        Collection data = new ArrayList<>();
+        data.add("DATA_ONE");
+        data.add("DATA_TWO");
+        data.add(3); // should have failed here
+
+        DataProcessor dataProcessor = new DataProcessor();
+
+        assertThrows(ClassCastException.class, () -> dataProcessor.checkPrefix(data)); // but fails here
+    }
+
+    @Test
+    void givenGenericCollection_whenInvalidTypeDataAdded_thenFailsAfterAdding() {
+        Collection data = Collections.checkedCollection(new ArrayList<>(), String.class);
+        data.add("DATA_ONE");
+        data.add("DATA_TWO");
+
+        assertThrows(ClassCastException.class, () -> {
+            data.add(3); // fails here
+        });
+
+        DataProcessor dataProcessor = new DataProcessor();
+        final boolean result = dataProcessor.checkPrefix(data);
+        assertTrue(result);
+
+    }
+}