[put-vs-computeIfAbsent] Difference Between putIfAbsent and computeIfAbsent in Java Map (#14977)

core-java-modules/core-java-collections-maps-7/src/test/java/com/baeldung/map/PutIfAbsentVsComputeIfAbsentUnitTest.java

@@ -0,0 +1,98 @@
+package com.baeldung.map;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.Mockito.*;
+
+class Magic {
+    public String nullFunc() {
+        return null;
+    }
+
+    public String strFunc(String input) {
+        return input + ": A nice string";
+    }
+}
+
+public class PutIfAbsentVsComputeIfAbsentUnitTest {
+
+    private static final Map<String, String> MY_MAP = new HashMap<>();
+    private Magic magic = new Magic();
+
+    @BeforeEach
+    void resetTheMap() {
+        MY_MAP.clear();
+        MY_MAP.put("Key A", "value A");
+        MY_MAP.put("Key B", "value B");
+        MY_MAP.put("Key C", "value C");
+        MY_MAP.put("Key Null", null);
+    }
+
+    @Test
+    void whenCallingPutIfAbsentWithAbsentKey_thenGetNull() {
+        String putResult = MY_MAP.putIfAbsent("new key1", magic.nullFunc());
+        assertNull(putResult);
+
+        putResult = MY_MAP.putIfAbsent("new key2", magic.strFunc("new key2"));
+        assertNull(putResult);
+
+        putResult = MY_MAP.putIfAbsent("Key Null", magic.strFunc("Key Null"));
+        assertNull(putResult);
+    }
+
+    @Test
+    void whenCallingComputeIfAbsentWithAbsentKey_thenGetExpectedResult() {
+        String computeResult = MY_MAP.computeIfAbsent("new key1", k -> magic.nullFunc());
+        assertNull(computeResult);
+
+        computeResult = MY_MAP.computeIfAbsent("new key2", k -> magic.strFunc(k));
+        assertEquals("new key2: A nice string", computeResult);
+
+        computeResult = MY_MAP.computeIfAbsent("Key Null", k -> magic.strFunc(k));
+        assertEquals("Key Null: A nice string", computeResult);
+    }
+
+    @Test
+    void whenCallingPutIfAbsentWithAbsentKey_thenNullIsPut() {
+        assertEquals(4, MY_MAP.size()); // initial: 4 entries
+        MY_MAP.putIfAbsent("new key", magic.nullFunc());
+        assertEquals(5, MY_MAP.size());
+        assertTrue(MY_MAP.containsKey("new key")); // new entry has been added to the map
+        assertNull(MY_MAP.get("new key"));
+    }
+
+    @Test
+    void whenCallingComputeIfAbsentWithAbsentKey_thenNullIsNotPut() {
+        assertEquals(4, MY_MAP.size()); // initial: 4 entries
+        MY_MAP.computeIfAbsent("new key", k -> magic.nullFunc());
+        assertEquals(4, MY_MAP.size());
+        assertFalse(MY_MAP.containsKey("new key")); // <- no new entry added
+    }
+
+    @Test
+    void whenCallingPutIfAbsent_thenFunctionIsAlwaysCalled() {
+        Magic spyMagic = spy(magic);
+        MY_MAP.putIfAbsent("Key A", spyMagic.strFunc("Key A"));
+        verify(spyMagic, times(1)).strFunc(anyString());
+
+        MY_MAP.putIfAbsent("new key", spyMagic.strFunc("new key"));
+        verify(spyMagic, times(2)).strFunc(anyString());
+    }
+
+    @Test
+    void whenCallingComputeIfAbsent_thenFunctionIsCalledOnDemand() {
+        Magic spyMagic = spy(magic);
+        MY_MAP.computeIfAbsent("Key A", k -> spyMagic.strFunc(k));
+        verify(spyMagic, never()).strFunc(anyString());
+
+        MY_MAP.computeIfAbsent("new key", k -> spyMagic.strFunc(k));
+        verify(spyMagic, times(1)).strFunc(anyString());
+    }
+
+}