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--- java:immutable-objects [2023/04/08 01:38] – odefta
+++ java:immutable-objects [2023/07/04 19:36] (current) – external edit 127.0.0.1
@@ Line 23: / Line 23: @@
 ===== Example of immutable class =====
 <code java>
+import java.util.ArrayList;
+import java.util.List;
 final class Record {
-  private final long id;
+    private final long id;
-  private final String name;
+    private final String name;
-  private final List<String> tokens;
+    private final List<String> tokens;
-  public Record(long id, String name, List<String> tokens) {
+    public Record(long id, String name, List<String> tokens) {
-    this.id = id;
+        this.id = id;
-    this.name = name;
+        this.name = name;
-    this.tokens = tokens;
-  }
-  public long getId() {
+        /*  This works, but:
-    return id;
+         - it will always duplicate elements
-  }
+         - the same copy should be used also in getTokens() method.
+        */
+        //this.tokens = new ArrayList<>(tokens);
-  public String getName() {
+        /*  If tokens is already unmodifiable (for ex. was formed using List.of),
-    return name;
+            then it will not duplicate elements.
-  }
+            Also, it will prevent the tokens to be modified
+            (it will throw java.lang.UnsupportedOperationException).
+         */
+        this.tokens = List.copyOf(tokens);
+    }
-  public List<String> getTokens() {
+    public long getId() {
-    return new ArrayList<>(tokens);
+        return id;
-  }
+    }
-  @Override
+    public String getName() {
-  public String toString() {
+        return name;
-    return "Record{" +
+    }
-        "id=" + id +
-        ", name='" + name + '\'' +
+    /**
-        ", tokens=" + tokens +
+     * No simple setters, return another copy of the object.
-        '}';
+     * @param name
-  }
+     * @return Record
+     */
+    public Record withName(String name) {
+        return new Record(id, name, tokens);
+    }
+    public List<String> getTokens() {
+        //return new ArrayList<>(tokens);
+        return tokens;
+    }
+    @Override
+    public String toString() {
+        return "Record (id = " + id + ", name='" + name + '\'' + ", tokens=" + tokens + ")";
+    }
 }
-</code>
+class TestRecord {
+    public static void main(String[] args) {
+        List<String> tokens = new ArrayList<>();
+        tokens.add("Token 1");
+        tokens.add("Token 2");
+        Record record = new Record(1, "One", tokens);
+        System.out.println(record);
+        tokens.remove(0);
+        System.out.println(record);
+        System.out.println(record.withName("Two"));
+        record.getTokens().add("Token 3");
+        System.out.println(record);
+    }
+}
+</code>
+Output:
+<code>
+Record (id = 1, name='One', tokens=[Token 1, Token 2])
+Record (id = 1, name='One', tokens=[Token 1, Token 2])
+Record (id = 1, name='Two', tokens=[Token 1, Token 2])
+Exception in thread "main" java.lang.UnsupportedOperationException
+	at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:71)
+	at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.add(ImmutableCollections.java:75)
+	at ro.medjava.immutability.TestRecord.main(Record.java:65)
+</code>
+===== Immutable classes in JDK =====
+Some immutable classes from java api:
+  * java.lang.String
+  * java.lang.Integer
+  * java.io.File
+  * java.util.Locale
+  * java.awt.Font
+  * java.net.InetSocketAddress
+<note tip>
+Immutable classes can also be created using builder pattern.
+Builder Pattern is a better option if the immutable class has a lot of attributes and some of them are optional.</note>