Merge remote-tracking branch 'upstream/master'

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diff --git a/challenge-147/abigail/java/ch-1.java b/challenge-147/abigail/java/ch-1.java
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+//
+// See https://theweeklychallenge.org/blog/perl-weekly-challenge-147
+//
+
+//
+// Run as: ln ch-1.java ch1.java; javac ch1.java; java ch1
+//
+
+import java.util.*;
+import java.util.ArrayList;
+
+public class ch1 {
+    public static boolean is_prime (int p) {
+        if (p == 2)     {return true;}
+        if (p % 2 == 0) {return false;}
+        for (int i = 3; i * i <= p; i += 2) {
+            if (p % i == 0) {return false;}
+        }
+        return (true);
+    }
+
+    public static void main (String [] args) {
+        ArrayList <Integer> todo = new ArrayList <Integer> ();
+        todo . add (2);
+        todo . add (3);
+        todo . add (5);
+        todo . add (7);
+        for (int p: todo) {
+            System . out . printf ("%d ", p);
+        }
+        int count = 20 - todo . size ();
+        int pow   = 10;
+      main:
+        while (todo . size () > 0) {
+            ArrayList <Integer> new_todo = new ArrayList <Integer> ();
+            for (int d = 1; d <= 9; d ++) {
+                for (int p: todo) {
+                    int candidate = d * pow + p;
+                    if (is_prime (candidate)) {
+                        System . out . printf ("%d ", candidate);
+                        new_todo . add (candidate);
+                        count --;
+                        if (count <= 0) {
+                            break main;
+                        }
+                    }
+                }
+            }
+            pow = pow * 10;
+            todo = new_todo;
+        }
+        System . out . print ("\n");
+    }
+}