New Feature Examples, Corrections & Rewrites

Author: BruceEckel

README.md

@@ -8,6 +8,7 @@ includes tests to verify that the code in the book is correct.
 
 > NOTE: Do not attempt to use JDK 16 or greater with gradle. 
 > This produces a `BUG!` message from Gradle, which is broken for those versions.
+> To test new Java features in JDK 16 or 17 you must compile and run those examples by hand.
 
 ## Contents
 

collectiontopics/CanonicalMapping.java

@@ -4,49 +4,29 @@
 // Visit http://OnJava8.com for more book information.
 // Demonstrates WeakHashMap
 import java.util.*;
-
-class Element {
-  private String ident;
-  Element(String id) { ident = id; }
-  @Override public String toString() { return ident; }
-  @Override public int hashCode() {
-    return Objects.hashCode(ident);
-  }
-  @Override public boolean equals(Object r) {
-    return r instanceof Element &&
-      Objects.equals(ident, ((Element)r).ident);
-  }
-  @SuppressWarnings("deprecation")
-  @Override protected void finalize() {
-    System.out.println("Finalizing " +
-      getClass().getSimpleName() + " " + ident);
-  }
-}
-
-class Key extends Element {
-  Key(String id) { super(id); }
-}
-
-class Value extends Element {
-  Value(String id) { super(id); }
-}
+import java.util.stream.*;
 
 public class CanonicalMapping {
+  static void showKeys(Map<String, String> m) {
+    // Display sorted keys
+    List<String> keys = new ArrayList<>(m.keySet());
+    Collections.sort(keys);
+    System.out.println(keys);
+  }
   public static void main(String[] args) {
-    int size = 1000;
-    // Or, choose size via the command line:
-    if(args.length > 0)
-      size = Integer.valueOf(args[0]);
-    Key[] keys = new Key[size];
-    WeakHashMap<Key,Value> map =
+    int size = 100;
+    String[] savedKeys = new String[size];
+    WeakHashMap<String,String> map =
       new WeakHashMap<>();
     for(int i = 0; i < size; i++) {
-      Key k = new Key(Integer.toString(i));
-      Value v = new Value(Integer.toString(i));
+      String key = String.format("%03d", i);
+      String value = Integer.toString(i);
       if(i % 3 == 0)
-        keys[i] = k; // Save as "real" references
-      map.put(k, v);
+        savedKeys[i] = key; // Save as "real" references
+      map.put(key, value);
     }
+    showKeys(map);
     System.gc();
+    showKeys(map);
   }
 }

collectiontopics/ListSortSearch.java

@@ -13,9 +13,9 @@ public static void main(String[] args) {
     System.out.println(list);
     Collections.shuffle(list, new Random(47));
     System.out.println("Shuffled: " + list);
-    // Use ListIterator to trim off last elements:
-    ListIterator<String> it = list.listIterator(10);
-    while(it.hasNext()) {
+    ListIterator<String> it =
+      list.listIterator(10);               // [1]
+    while(it.hasNext()) {                  // [2]
       it.next();
       it.remove();
     }

collectiontopics/QueueBehavior.java

@@ -15,7 +15,7 @@ static Stream<String> strings() {
   }
   static void test(int id, Queue<String> queue) {
     System.out.print(id + ": ");
-    strings().map(queue::offer).count();
+    strings().forEach(queue::offer);
     while(queue.peek() != null)
       System.out.print(queue.remove() + " ");
     System.out.println();

concurrent/PSP2.txt

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 0: main
-1: ForkJoinPool.commonPool-worker-2
+1: main
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enumerations/ArrowInSwitch.java

@@ -0,0 +1,42 @@
+// enumerations/ArrowInSwitch.java
+// (c)2021 MindView LLC: see Copyright.txt
+// We make no guarantees that this code is fit for any purpose.
+// Visit http://OnJava8.com for more book information.
+// {NewFeature} Since JDK 14
+import static java.util.stream.IntStream.range;
+
+public class ArrowInSwitch {
+  static void colons(int i) {
+    switch(i) {
+      case 1: System.out.println("one");
+              break;
+      case 2: System.out.println("two");
+              break;
+      case 3: System.out.println("three");
+              break;
+      default: System.out.println("default");
+    }
+  }
+  static void arrows(int i) {
+    switch(i) {
+      case 1 -> System.out.println("one");
+      case 2 -> System.out.println("two");
+      case 3 -> System.out.println("three");
+      default -> System.out.println("default");
+    }
+  }
+  public static void main(String[] args) {
+    range(0, 4).forEach(i -> colons(i));
+    range(0, 4).forEach(i -> arrows(i));
+  }
+}
+/* Output:
+default
+one
+two
+three
+default
+one
+two
+three
+*/